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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES ELECROMAGNETIC THEORY FORMULA SHEET CONTENTS PART-A (CORE) 1. Electrostatics…………………………………………………………………..... (5-17) 1.1 Coulomb’s Law and Superposition Principle…….................................................. (5) 1.1.1 Electric field 1.2 Gauss’s law ……..................................................................................................... (6) 1.2.1 Field lines and Electric flux 1.2.2 Applications 1.3 Electric Potential……............................................................................................. (7) 1.3.1 Curl of Electric field 1.3.2 Potential of localized charges 1.4 Laplace’s and Poisson equations…….................................................................... (8) 1.5 Electrostatic boundary condition…….................................................................... (8) 1.6 Work and Energy in electrostatics…….................................................................. (9) 1.6.1 The energy of point charge distribution 1.6.2 Energy of continuous charge distribution 1.7 Basic properties Conductors ……........................................................................ (10) 1.8 Multiple Expansions……..................................................................................... (11) 1.8.1 The Electric Potential and Field of a Dipole 1.8.2 Approximate potential at large distances 1.9 Polarization……................................................................................................. (13) 1.9.1 The Field of a polarized Object (Bound Charges) 1.10 The Electric Displacement............................................................................. (13-14) 1.10.1 Gauss Law in the Presence of Dielectrics 1.10.2 Linear Dielectrics (Susceptibility, Permittivity, Dielectric Constant) 1.10.3 Boundary Condition 1.10.4 Energy in dielectric system

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.12 Image problems.................................................................................... (15-17) 1.12.1 The Classic Image problem 1.12.2 Induced Surface Charge 1.12.3 Force and Energy 1.12.4 Other Image Problem 2. Magnetostatic.......................................................................................... (18-24) 2.1 Magnetic force on current element............................................................. (18) 2.1.1 Current in a wire 2.1.2 Surface current density 2.1.3 Volume current density 2.2 Continuity equation................................................................................... (19) 2.3 Biot-Savart law.......................................................................................... (19) 2.3.1 Magnetic field due to wire 2.3.2 Magnetic field due to Solenoid and Toroid 2.4 Ampere's Law........................................................................................... (20) 2.5 Magnetic Vector Potential........................................................................ (20) 2.6 Magnetostatic boundary condition........................................................... (21) 2.7 Multiple Expansion of Vector Potential................................................... (22) 2.8 Magnetisation........................................................................................... (23) 2.8.1 The Field of a magnetized Object (Bound Currents) 2.9 The Auxiliary field H.......................................................................... (23-24) 2.9.1 Ampere’s Law in in presence of magnetic materials 2.9.2 Magnetic Susceptibility and Permeability 2.9.3 Boundary Condition 3. Dynamics of charged particles in static and uniform electromagnetic fields 3.1 Charged particle in static electric field………………………..……. (25-26) 3.1.1 Charged particle enters in the direction of field (Linear motion) 3.1.2 Charged particle enters in the direction perpendicular to field (Parabolic motion) Head office fiziks, H.No. 23, G.F, Jia Sarai, Near IIT, Hauz Khas, New Delhi-16 Phone: 011-26865455/+91-9871145498 Website: www.physicsbyfiziks.com Email: [email protected]

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3.2 Charged particle in static magnetic field.......................................................... (27) 3.2.1 Charged particle enters in the direction perpendicular to field (Circular motion) 3.2.2 Charged particle enters in the direction making an angle with the field (Helical motion) 3.3 Charged particle in uniform electric and magnetic field (Cycloid motion).... (28)

4. Electromagnetic induction…………………………………………………... (29) 4.1 Faraday’s Law 4.1.1 Lenz’s Law 4.1.2 Inductance 4.1.3 Energy stored in the field

5. Maxwell's equations…………………………………………………..…... (30) 5.1 Maxwell’s equation in free space 5.1.1 Electrodynamics before Maxwell’s 5.1.2 How Maxwell fixed Ampere’s Law 5.1.3 Paradox of charging capacitor 5.1.4 Maxwell’s equation in free space 5.2 Maxwell’s equation in linear isotropic media …………………………... (30) 5.3 Boundary conditions on the fields at interfaces ……………………….... (31)

6. Electromagnetic waves in free space…………………………….….... (31-34) 6.1 Poynting Theorem…………………………………………………..….... (31) 6.2 Waves in one dimension (Sinusoidal waves) ……………….................... (32) 6.2.1 The wave equation 6.2.2 Terminology 6.2.3 Complex notation 6.2.4 Polarization Head office fiziks, H.No. 23, G.F, Jia Sarai, Near IIT, Hauz Khas, New Delhi-16 Phone: 011-26865455/+91-9871145498 Website: www.physicsbyfiziks.com Email: [email protected]

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.3 Electromagnetic waves in vacuum……………….................................... (35) 6.3.1 The wave equation for E and B 6.3.2 Monochromatic plane waves 6.3.3 Energy and momentum in Electromagnetic wave 6.4 Electromagnetic waves in matter………………..................................... (38) 6.5 Electromagnetic waves in conductors……………….............................. (39)

7. Applications of Electromagnetic waves………................................ (41-43) 7.1 Reflection and Refraction………………................................................ (41) 7.1.1 Normal incidence 7.1.2 Oblique incidence 7.1.3 Fresnel’s relation (Parallel and Perpendicular Polarization)

PART ‘B’ ADVANCED

8. Potential and Field formulation for Time Varying Fields……........ (44-45) 8.1 Scalar and vector potentials..................................................................... (44) 8.2 Gauge transformation.............................................................................. (44) 8.3 Coulomb and Lorentz gauge.................................................................... (45) 9. Waveguides……................................................................................... (45-49) 9.1 Rectangular Waveguides......................................................................... (45) 9.2.1 TE Waves in Rectangular Waveguide 9.2.2 TM Waves in Rectangular Waveguide 10. Radiation- from moving charges ....................................................... (50) 10.1 Radiation 10.1.1 Power radiated by a point charge

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1. Electrostatics The electric field at any point due to stationary source charges is called as electrostatic field. 1.1 Coulomb’s Law and Superposition Principle The electric force on a test charge Q due to a single point charge q, which is at rest and a distance R apart is given by Coulomb’s law

Q

1 Qq F . 4 0 R 2 R

R

The constant 0 is called the permittivity of free space. q

In mks units, 0 8.85 10

12

C2 N .m 2

1.1.1 Electric field If we have many point charges q1 , q2 ,...... at distances R1 , R2 ,...... from test charge Q, then according to the principle of superposition the total force on Q is q2Q 1 q1Q F F1 F2 ............ 2 R1 2 R2 ........ , 4 0 R1 R2

F QE

where E P

1 4 0

n

qi

R i 1

2 i

Ri

E is called the electric field of the source charges. Physically E P is the force per unit charge that would be exerted on a test charge placed at P. If charge is distributed continuously over some region, then

E r

1 4 0

1 ˆ Rdq . 2 R line

The electric field of a line charge is dq dl

E r

1 4 0

( r ) ˆ Rdl where is charge per unit length. R2 line

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES For surface charge dq da

E r

1 4 0

( r ) ˆ Rda where is charge per unit area. 2 R surface

For a volume charge dq d

E r

1 4 0

( r ) ˆ Rd where is charge per unit volume. 2 R volume

1.2 Gauss’s law 1.2.1 Field lines and Electric flux

Consider that a point charge q is situated at the origin: E

1 q 4 0 r 2

r

This field is represented by the field line as shown in figure below. The magnitude of the field is indicated by the density of the field lines: it's strong near the center

E

where the field line are close together, and weak farther out, where they are relatively far apart. The field strength (E) is proportional to the number of field lines per unit area (area perpendicular to the lines).

The flux of E through a surface S, E E .d a is a measure of the “number of field S

lines” passing through S. A charge outside the surface would contribute nothing to the total flux, since its field lines go in one side and out the other. It follows, then, that for any closed surface,

1

E .d a

Qenc

0

where Qenc is the total charge enclosed within the surface. This is Gauss’s law in integral form.

Gauss’s law in differential form:

.E

1 0

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.2.2 Applications of Gauss’s Law Gauss's law is always true, but it is not always useful. Gauss's law is useful for only three kinds of symmetry: 1. Spherical symmetry. Make your Gaussian surface a concentric sphere. 2. Cylindrical symmetry. Make your Gaussian surface a coaxial cylinder. 3. Plane symmetry. Make your Gaussian surface a “pillbox,” which extends equally above and below the surface. Gaussian surface r

Gaussian pillbox

R

Gaussian surface

1.3 Electric Potential 1.3.1 Curl of Electric field

Then integral around a closed path is zero i.e.

E .d l 0

This line integral is independent of path. It depends on two end points. Applying stokes theorem, we get E 0 . The electric field is not just any vector but only those vector whose curl is zero. P

So, we can define a function V r E .d l

where is some standard reference point. V then depends only on the point r. It is called the electric potential. Evidently, the potential difference between two points a and b is b

V b V a E .d l

E V

a

Potential obeys the superposition principle.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.3.2 Potential of localized charges Potential of a point charge q is V

1 q where R is the distance from the charge. 4 0 R n

1 4 0

R

For continuous volume charge distribution V ( r )

1 4 0

The

charges

The potential of a collection of point charge is V

potential

and V ( r )

of

line

and

surface

i 1

qi

.

i

(r ) d R are

V (r )

1 (r ) dl 4 0 R

1 ( r ) da . 4 0 R

1.4 Laplace’s and Poisson equations Since E V and .E 2V . 0 0 This is known as Poisson's equation. In regions where there is no charge, so that 0 , Poisson's equation reduces to Laplace's equation, 2V 0 . 1.5 Electrostatic boundary condition The boundary between two medium is a thin sheet of surface charge . above below

|| || and Eabove Ebelow E above E below nˆ 0 0

where nˆ is unit vector perpendicular to the surface, pointing upward. Vabove Vbelow

where

Vabove Vbelow n n 0

V V nˆ denotes the normal derivative of V (that is the rate of change in the n

direction perpendicular to the surface.)

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.6 Work and Energy in electrostatics The work done in moving a test charge Q in an external field E , from point a to b is b W F dl Q E dl Q V b V a b

a

a

q1

a

q2

If a and b r

W Q V r V QV r since V 0

Q

qi

b

In this sense potential is potential energy (the work it takes to create the system) per unit charge (just as the field is the force per unit charge).

1.6.1 The energy of point charge distribution q3

When the first charge q1 is placed, no work has been done. When q2 is placed work done W2 q2V1 where V1

is

the

potential

due

to q1

R13

so,

q 1 W2 q2 1 . 4 0 R12

Similarly W3

when

third

charge q3

is

placed

r1

q1

r3

R23 R12

r 2

q2

q q 1 q3 1 2 . 4 0 R13 R23

The work necessary to assemble the first three charges is W

In general, W

1 4 0

n

n

i 1 j 1 j i

qi q j Rij

1 8 0

n

n

i 1 j 1 j i

qi q j Rij

1 q1q2 q1q3 q2 q3 . 4 0 R12 R13 R23

1 n qiV ri , 2 i 1

where V(ri) is the potential at point ri (the position of qi) due to all other charges.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.6.2 Energy of continuous charge distribution For a volume charge density W

1 Vd and 2

W

0 2

E 2 d

all space

(i) Energy of a uniformly charged spherical shell of total charge q and radius R is W

q2 . 80 R

(ii) Energy stored in a uniformly charged solid sphere of radius R and charge q is W

1 3q 2 . 40 5R

1.7 Basic properties Conductors 1. E 0 inside a conductor. 2. 0 inside a conductor. 3. Any net charge resides on the surface. 4. A conductor is an equipotential. 5. E is perpendicular to the surface, just outside a conductor. Because the field inside a conductor is zero, boundary E above E below nˆ requires that the field immediately outside is E nˆ . 0 0 In terms of potential equation

condition

Vabove Vbelow V yields 0 n n n 0

These equations enable us to calculate the surface charge on a conductor, if we can determine E or V . 1 2 Force per unit area on the conductor is f nˆ . 2 0 This amounts to an outwards electrostatic pressure on the surface, tending to draw the conductor into the field, regardless the sign of . Expressing the pressure in terms of the field just outside the surface,

P

0 2 E . 2

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.8 Multiple Expansions 1.8.1 The Electric Potential and Field of a Dipole If we choose coordinates so that p (dipole moment) lies at the

z

origin and points in the z direction, then potential at (r , ) is: rˆ. p p cos Vdip r , . 2 4 o r 4 o r 2

E dip r ,

p

r

y

1 ˆ ˆ 3 p r r p 4 0 r 3

x

Note:

(a) When a dipole is placed in a uniform electric field ( E ), net force on the dipole is zero where p qd . and it experiences a torque p E (b) In non-uniform field, dipoles have net force F p E and torque p E .

(c) Energy of an ideal dipole p in an electric field E is U p.E .

(d) Interaction energy of two dipoles separated by a distance r is

U

1 p1 p 2 3 p1 rˆ p 2 rˆ . 4 0 r 3

1.8.2 Approximate potential at large distances Approximate potential at large distances due to arbitrary localized charge distribution

d ' r '

V r

R

P r

1 1 1 1 r ' d ' 2 r ' cos ' r ' d ' 2 4 0 r r r

' 2

r

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1 3 2 ' ' ' 2 cos 2 r d ...

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1 The first term (n = 0) is the monopole contribution (it goes like ). The second term r

(n = 1) is the dipole term (it goes like

1 ). The third term is quadrupole; the fourth r2

octopole and so on. The lowest nonzero term in the expansion provides the approximate potential at large r and the successive terms tell us how to improve the approximation if greater precision is required. The monopole and dipole terms Ordinarily, the multipole expansion is dominated (at large r) by the monopole term:

Vmon r

1 Q . 4 0 r

where Q d is the total charge of the configuration. If the total charge is zero, the dominant term in the potential will be the dipole (unless, of course, it also vanishes): 1 1 1 1 1 rˆ. p ' ' ' ' ' ' ' Vdip r r cos r d rˆ. r r d , 4 0 r 2 4 0 r 2 4 0 r 2 ' ' ' where dipole moment p r r d .

The dipole moment is determined by the geometry (size, shape and density) of the charge distribute. The dipole moment of a collection of point charge is n ' p qi ri . i 1

Note: Ordinarily, the dipole moment does change when we shift the origin, but there is an important exception: If the total charge is zero, then the dipole moment is independent of the choice of origin.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.9 Polarization

(Polarization) P dipole moment per unit volume

1.9.1 The Field of a polarized Object (Bound Charges) Suppose we have a piece of polarized material with polarization vector P containing a lot of microscopic

1 4 0

S

p

dipoles lined up. Then V r

R

1 1 1 P.d a ' '.P d ' . R 4 0 V R

The first term looks like the potential of a surface bound charge b P.nˆ (where nˆ is

the normal unit vector). The second term looks like the potential of a volume bound charge b .P .

1 Thus V r 4 0

S

b 1 b da ' d ' , this means the potential (and hence also the R 4 0 V R

field) of a polarized object is the same as that produced by a volume charge density b .P plus a surface charge density b P.nˆ . 1.10 The Electric Displacement 1.10.1 Gauss Law in the Presence of Dielectrics Within the dielectric, the total charge density can be written as b f where b is volume bound charge f free charge density. .D f where D 0 E P is known as the electric displacement. .D f Thus Gauss’ law reads, Or, in integral form D.d a Q fenc , where Q fenc denotes the total free charge enclosed in the volume.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.10.2 Linear Dielectrics (Susceptibility, Permittivity, Dielectric Constant) For any substances, the polarization is proportional to the field provided is not too strong: P E P 0 e E (Materials that obey this relation are called linear dielectrics) The constant of proportionality, e is called the electric susceptibility of the medium. The value of e depends on the microscopic structure of the substance and also on external conditions such as temperature. In linear media we have D 0 E P 0 E 0 e E 0 E (1 e ) E , where 0 (1 e ) This new constant is called the permittivity of the material. Also r

(1 e ) is called relative permittivity or dielectric constant, of the material. 0

1.10.3 Boundary Condition on D The boundary between two medium is a thin sheet of free surface charge f . ||

||

||

||

Dabove Dbelow f and D above D below P above P below

1.10.4 Energy in dielectric system W

1 D E d . 2 all space

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.12 Image problems 1.12.1 The Classic Image problem Suppose a point charge q is held a distance d above an infinite grounded conducting plane. We can find out what is the potential in the region above the plane. z z q q d d

y V 0

y d

q x x Forget about the actual problem; we are going to study a complete different situation. The new problem consists of two point charges +q at (0, 0, d) and –q at (0, 0,-d) and no conducting plane. For this configuration we can easily write down the potential: 1 q q V x, y, z 2 4 0 x 2 y 2 z d 2 x 2 y2 z d

(The denominators represent the distances from (x, y, z) to the charges +q and –q, respectively.) It follows that 1. V 0 when z 0 and 2. V 0 for x 2 y 2 z 2 d 2 , and the only charge in the region z 0 is the point charge +q at (0, 0, d). Thus the second configuration produces exactly the same potential as the first configuration, in the upper region z 0 .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.12.2 Induced Surface Charge The surface charge density induced on the conductor surface is 0

V z

z 0

qd

x, y

2 x 2 y 2 d 2

3

2

As expected, the induced charge is negative (assuming q is positive) and greatest at x=y=0. Q da q

The total induced charge 1.12.3 Force and Energy

The charge q is attracted towards the plane, because of the negative induced surface

1 q2 charge. The force: F zˆ . 4 0 2d 2 One can determine the energy by calculating the work required to bring q in from infinity. d

d 1 q2 1 q2 1 q2 W F .dl dz 4 0 4 z 2 4 0 4 z 4 0 4d d

1.12.4 Other Image Problem The method just described is not limited to a single point charge; any stationary charge distribution near a grounded conducting plane can be treated in the same way, by

introducing its mirror image.

R R'

r

R

a

q

V 0

b

q'

q a

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES Let us examine the completely different configuration, consisting of the point charge q together with another point charge q '

2 R q placed at a distance b R to the right of a a

the centre of sphere. No conductor, now-just two point charges. The potential of this configuration is V(r, )

V(r, )

1 q q' 40 R R

q 1 1 2 2 2 2 40 r a 2ra cos R (ra / R) 2ra cos

Clearly when r = R, V 0 Force The force on q, due to the sphere, is the same as the force of the image charge q, thus:

F

1 q 2 Ra 40 (a 2 R 2 )2

Energy To bring q in from infinity to a, we do work W

1 q 2R 40 2(a 2 R 2 )

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 2. MAGNETOSTATICS The magnetic field at any point due to steady current is called as magnetostatic field. 2.1 Magnetic force on current element

The magnetic force on a charge Q, moving with velocity v in a magnetic field B is,

F mag Q v B . This is known as Lorentz force law. In the presence of both electric and

magnetic fields, the net force on Q would be: F Q E v B 2.1.1 Current in a wire A line charge λ traveling down a wire at a speed v constitutes a current I v .

v t v

Magnetic force on a segment of current carrying wire is, F mag I dl B

P

2.1.2 Surface current density

When charge flows over a surface, we describe it by the surface current K . K

d dl

is the current per unit widthFlow

perpendicular to flow. dl Also K v where is surface charge density and v is its velocity. Magnetic force on surface current F mag K B da

K

2.1.3 Volume current density d J is the current per unit area-perpendicular to da da flow. Also J v where is volume charge density and v is its velocity. Magnetic force on volume current F mag J B d .Current crossing a surface S is J d a

J

Flow

S

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 2.2 Continuity equation General continuity equation J

t

This is the precise mathematical statements of local charge conservation. Thus for magnetostatic fields

0 and hence the continuity equation becomes: t J 0.

2.3 Biot-Savart law (magnetic field of steady line current) The magnetic field of a steady line current is given by 0 I Rˆ 0 dl ' Rˆ B r dl I 4 R 2 4 R 2

where 0 4 10 7

r

I

R

N ( permeability of free space) A2

dI '

For surface and volume current Biot - Savart law becomes: 0 K ( r ') Rˆ 0 J ( r ') Rˆ ( r ) da ' and (r ) d ' . 4 R2 4 R 2 2.3.1 Magnetic field due to wire

Let us find the magnetic field a distance d from a long straight wire carrying a steady current I. I B 0 (sin 2 sin 1 )ˆ 4d For Infinite wire: 1

1

I and 2 B 0 ˆ 2 2 2d

2

Note:

I

1. Force (per unit length) of attraction between two

Wire segment

long, parallel wires a distance d apart, carrying currents I1 and I2 in same direction are: f

0 I1 I 2 . 2 d

2. If currents are in opposite direction they will repel with same magnitude.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3. The magnetic field a distance d above the center of a circular loop of radius R, which

I R2 carries a steady current I is B 0 zˆ . 2 (R 2 d 2 )3/2 I At the center of the circle B(0) 0 zˆ 2R 2.3.2 Magnetic field due to Solenoid and Toroid The magnetic field of a very long solenoid, consisting of n closely wound turns per unit length of a cylinder of radius R and carrying a steady current I is: 0 nI zˆ inside the solenoid B outside the solenoid. 0 ^ 0 NI Magnetic field due to toroid is B 2 r 0

for points inside the coil for points outside the coil

where N is the total number of turns. 2.4 Ampere's Law In general we can write

B dl I where I 0 enc enc J .d a is the total current enclosed

by the amperian loop. B 0 J

Right hand Rule If the fingers of your right hand indicate the direction of integration around the boundary, then your thumb defines the direction of a positive current. 2.5 Magnetic Vector Potential A : Since B 0 B A For magnetostatic fields, A 0 and 2 A 0 J 0 J r ' If J goes to zero at infinity, A r d ' for volume current. 4 R 0 I 1 0 K For line and surface currents, A r dl '; A r da ' 4 R 4 R

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2.6 Magnetostatic boundary condition (Boundary is sheet of current, K ) Just as the electric field suffers a discontinuity at a surface charge, so the magnetic field is discontinuous at a surface current. Only this time it is the tangential component that changes.

B.d a 0

Since

Babove Bbelow

For tangential components || || r B.dl 0 I enc Babove B K B is parallel to surface but to K below 0 || || B is parallel to surface and along K B.dl 0 I enc Babove Bbelow Thus the component of B that is parallel to the surface but perpendicular to the current

is discontinuous in the amount 0 K . A similar amperian loop running parallel to the current reveals that the parallel component is continuous. The result can be summarized in a single formula:

B above B below 0 K nˆ , where nˆ is a unit vector perpendicular to the surface, pointing “upward”. Like the scalar potential in electrostatics, the vector potential is continuous across, an boundary: Aabove Abelow For . A 0 guarantees that the normal component is continuous, and A B , in the form A . dl B.d a line

S

Aabove Abelow But the derivative of A inherits the discontinuity of B : 0 K n n

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 2.7 Multiple Expansion of Vector Potential

1 We can always write the potential in the form of a power series in , where r is the r distance to the point in question. Thus we can always write

0 I 4

1 1 1 1 2 3 2 r d l ' r 2 r ' cos ' d l ' r 3 r ' 2 cos ' 2 d l ' ......... z First term, monopole dl 0 (no magnetic Ar

monopole)

0 m rˆ Second term, dipole Adip r 4 r 2 where

m

is

the

magnetic

dipole

moment:

m

m I d a I A where A is area vector Thus A dip

r

y

m sin ˆ r 0 4 r 2

x

Hence

B dip r A

1 0m ˆ 0 3 3 m rˆ rˆ m ˆ 2 cos r sin 4 r 4 r 3

Note:

(a) When a magnetic dipole is placed in a uniform magnetic field ( B ), net force on the dipole is zero and it experiences a torque m B . (b) In non-uniform field, dipoles have net force F m B and torque m B .

(c) Energy of an ideal dipole m in an magnetic field B is U m.B . (d) Interaction energy of two dipoles separated by a distance r is

U

1 m1 m2 3 m1 rˆ m2 rˆ . 4 0 r 3 .

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2.8 Magnetisation M

Magnetization M is magnetic dipole moment per unit volume. 2.8.1 The Field of a magnetized Object (Bound Currents)

Consider a piece of magnetized material with magnetization M . Then the vector potential is given by 1 A r 0 M r d ' 0 4 R 4 v

1 d a . M r R

This means the potential(and hence also the field) of a magnetized object is the same as would be produced by a volume current J b M throughout the material, plus a surface current K b M nˆ , on the boundary. We first determine these bound currents, and then find the field they produce. 2.9 The Auxiliary field ( H ) 2.9.1 Ampere’s Law in in presence of magnetic materials In a magnetized material the total current can be written as J J b J f where J b is B bound current and J f is free current. Thus H J f where H M 0 In integral form H dl I fenc where I fenc is the total free current passing through the

amperian loop. H plays a role in magnetostatic analogous to D in electrostatic: Just as D allowed us to write Gauss's law in terms of the free charge alone, H permits us to express Ampere's law in terms of the free current alone- and free current is what we control directly. Note:

When we have to find B or H in a problem involving magnetic materials, first look for symmetry. If the problem exhibits cylindrical, plane, solenoid, or toroidal symmetry, then we can get H directly from the equation H dl I fenc .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 2.9.2 Magnetic Susceptibility and Permeability For most substances magnetization is proportional to the field H , M m H , where m is magnetic susceptibility of the material.

B 0 H M 0 1 m H B H

where

0 0 r 0 1 m ,

is

permeability of material.

2.9.3 Boundary Condition ( H ) The boundary between two medium is a thin sheet of free surface current K f . The Ampere’s law states that H above H below K f nˆ . And

H above H below M above M below

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3. Dynamics of charged particles in static and uniform electromagnetic fields (The Lorentz Force Law)

The magnetic force on a charge Q, moving with velocity v in a magnetic field B is,

F mag Q v B

This is known as Lorentz force law. In the presence of both electric and magnetic fields, the net force on Q would be: F Q E v B

3.1 Charged particle in static electric field 3.1.1 Charged particle enters in the direction of field (Linear motion) Q

2

1

E

The force on the charge Q in electric field E is F Q E . Acceleration of the charge particle in the direction of the electric field is a If r is the position vector at any time t then Let at t=0, r r0 C1 r0 r If initially u 0, r0 0 r

QE 2 t ut r0 2m

ut

F QE . m m QE 2 t 2m 2

r0

1

QE 2 QE t and v t. 2m m

The energy acquired by the charged particle in moving from point 1 to 2 is 2

2

2

W F .d l m a.d l m 1

1

1

2 1 dv .vdt m v.d v W mv 22 v12 2 dt 1

If the potential difference between points 1 to 2 is V then W QV

1 m v 22 v12 2

If the particle starts from rest i.e v1 0 and final velocity is v then

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W QV

1 2 mv v 2

2QV m

1 2 1 Q2E2 2 QE 2 t QE t QEr Kinetic energy of the particle K .E. mv m. 2 2 2 2m m 3.1.2 Charged particle enters in the direction perpendicular to field (Parabolic motion) Let us consider a charge particle enters in an electric field region with velocity v x at t=0. The electric field is in the y-direction and the field region has length l. After traversing a distance l it strikes a point P on a screen which is placed at a distance L from the field region.

L vx

P

y2 y1

Ey

y

x

l

QE y 1 Thus y a y t 2 2 2m QE y l y1 2m v x

x vx

2

and which represents parabolic path.

2

and y 2 L tan

QE y l Thus distance of point P from the center of the screen is, y1 y 2 2m v x

Angle of deviation in the field region, tan

2

L tan

dy QE y x dx mv x2

Angle of deviation in the field free region, tan

QE y mv x2

l

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3.2 Charged particle in static magnetic field

The magnetic force on a charge Q, moving with velocity v in a magnetic field B is,

F mag Q v B

This is known as Lorentz force law. 3.2.1 Charged particle enters in the direction perpendicular to field (Circular motion) If a charge particle enters in a magnetic field at angle of 90o, then motion will be circular with the magnetic force providing the centripetal acceleration.

y

2

For uniform circular motion: QvB m

v mv R R QB

v

R

where R is the radius of the circle and m is the mass of the charge

particle.

z

Momentum of the charged particle p QBR Kinetic energy (KE) Time period T

F

Q

x

B

p 2 Q2 B2 R2 2m 2m

2R 2m v QB

3.2.2 Charged particle enters in the direction making an angle with the field (Helical motion) If the charge particle enters in a magnetic field making an angle θ, then motion will be helical. v v sin

and

v|| v cos ,

mv and the radius of helix is R QB .

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B

||

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3.3 Charged particle in uniform electric and magnetic field (Cycloid motion) F Q E v B Q E zˆ Bz yˆ By zˆ z ma m y yˆ z zˆ

E y z , z y B QB where (cylotron frequency ) m

E

Solving above differential equations, we get

o

E E y t t sin t , z t 1 cos t B B E 2 2 y Rt z R R 2 where R B This is the formula for a circle, of radius R, x

whose center is

0, Rt , R

travels in the

a

c

b

B

y-direction at constant speed, v R

E B

The curve generated in this way is called a cycloid.

d l Q v B vdt 0 .

Magnetic forces do not work because v B is perpendicular to v , so dWmag F mag

Magnetic forces may alter the direction in which a particle moves, but they can not speed up or slow down it.

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y

fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 4. Electromagnetic induction 4.1 Faraday’s Law v

v

I

I

B

I

B

B

b

a

changing magnetic field

c

Experiment 1: He pulled a loop of wire to the right through a magnetic field. A current flowed in the loop (Figure a). Experiment 2: He moved the magnet to the left, holding the loop still. Again, a current flowed in the loop (Figure b). Experiment 3: With both the loop and the magnet at rest, he changed the strength of the field (he used an electromagnet, and varied the current in the coil). Once again current flowed in the loop (Figure c). Thus, universal flux rule is that, whenever (and for whatever reason) the magnetic flux through a loop changes, an e.m.f. will appear in the loop.

d dt

(Where magnetic flux B.d a )

In experiment 2,

A changing magnetic field induces an electric field. d It is this “induced” electric field that accounts for the e.m.f. E dl dt B B Then E dl t .d a E t 4.1.1 Lenz’s Law In Faraday’s law negative sign represents the Lenz’s law. (The induced current will flow in such a direction that the flux it produces tends to cancel the change). For example if the magnetic flux is increasing then induced e.m.f will try to reduce and vice versa. Head office fiziks, H.No. 23, G.F, Jia Sarai, Near IIT, Hauz Khas, New Delhi-16 Phone: 011-26865455/+91-9871145498 Website: www.physicsbyfiziks.com Email: [email protected]

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 5. Maxwell's equations 5.1 Maxwell’s equation in free space 5.1.1 Electrodynamics before Maxwell’s (i)

0

(Gauss’ Law),

(ii) B 0

(No name),

B (iii) t (iv) B 0 J

(Farday’s Law), (Ampere’s law).

5.1.2 How Maxwell fixed Ampere’s Law From continuity equation and Gauss Law E E .J ( 0 .E ) . 0 . J 0 0 . t t t t E B 0 J 0 0 Thus t A changing electric field induces a magnetic field.

E Maxwell called this extra term the displacement current J d 0 . t E Integral form of Ampere's law B.dl 0 I enc 0 0 .d a t 5.1.4 Maxwell’s equation in free space (i)

0

(Gauss’ Law),

(ii) B 0

(No name),

B (iii) t

(Farday’s Law),

E (iv) B 0 J 0 0 t

(Ampere’s law with Maxwell's correction).

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 5.3 Boundary conditions on the fields at interfaces || || (a ) Dabove Dbelow f (c) E above E below || || (b) Babove Bbelow (d ) H above H below K f nˆ In particular, if there is no free charge or free current at the interface between medium1 and medium 2, then (a )1 E1 2 E2 0 (b) B1 B2

and

|| || (c) E1 E 2 . 1 || 1 || (d ) B1 B 2 0 1 2

6. Electromagnetic waves in free space 6.1 Poynting Theorem (“work energy theorem of electrodynamics”) The work necessary to assemble a static charge distribution is We

0 E 2 d , where E is the resulting electric field 2

The work required to get currents going (against the back emf) is

Wm

1 B 2 d , where B is the resulting magnetic field 2 0

This suggests that the total energy in the electromagnetic field is U em

1 B2 2 E d . 0 2 0

Suppose we have some charge and current configuration which at time t, produces fields E & B . In next instant, dt, the charges moves around a bit. The work is done by electromagnetic forces acting on these charges in the interval dt. According to Lorentz Force Law, the work done on a charge ‘q’ is F dl q( E v B) v dt qE v dt .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES Now q d and v J , so the rate at which work is done on all the charges in a

volume V is dW d 1 1 2 1 0E2 B d dt dt V 2 0 0

E B d a ,

S

where S is the surface bounding V. This is Poynting's theorem; it is the “work energy theorem” of electrodynamics. The first integral on the right is the total energy stored in the fields, U em . The second term evidently, represents the rate at which energy is carries out of V, across its boundary surface, by the electromagnetic fields. Poynting's theorem says, that, “the work done on the charges by the electromagnetic force is equal to the decrease in energy stored in the field, less the energy that flowed out through the surface”. The energy per unit time, per unit area, transported by the fields is called the Poynting vector

1 S EB . 0

S d a is the energy per unit time crossing the infinitesimal surface d a – the energy or energy flux density. Momentum density stored in the electromagnetic field is:dem 0 0 S

6.2 Waves in one dimension (Sinusoidal waves) 6.2.1 The wave equation A wave propagating with speed v in z-direction can be expressed as:

2 f 1 2 f z 2 v 2 t 2

The most general solution to the wave equation is the sum of a wave to the right (+z direction) and a wave to the left (-z direction): f z, t g z vt h z vt .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.2.2 Terminology Let us consider a function f z, t A cos k ( z vt ) A is the amplitude of the wave (it is positive, and represents the maximum displacement from equilibrium). The argument of the cosine is called the phase, and is the phase constant (normally, we use a value in the range 0 2 ). Figure given below shows this function at time t 0 . Notice that at z vt

k

, the phase

is zero; let's call this the “central maximum.” If 0 , central maximum passes the origin

at time t 0 ; more generally k is the distance by which the central maximum (and therefore the entire wave) is “delayed.” Central maximum

f z, 0

v

A

z

/k

Finally k is the wave number; it is related to the wavelength as advances by

2 , for when z k

2 , the cosine executes one complete cycle. k

As time passes, the entire wave train proceeds to the right, at speed v . Time period of one complete cycle is T

2 . kv

The frequency (number of oscillations per unit time) is

1 kv v . T 2

The angular frequency 2 kv In terms of angular frequency , the sinusoidal wave can be represented as f z, t A cos kz t .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES A sinusoidal oscillation of wave number k and angular frequency traveling to the left would be written f z, t A cos kz t .

Comparing this with the wave traveling to the right reveals that, in effect, we could simply switch the sign of k to produce a wave with the same amplitude, phase constant, frequency, and wavelength, traveling in the opposite direction. Central maximum

f z, 0

v

/k

z

6.2.3 Complex notation In view of Euler's formula,

ei cos i sin , the sinusoidal wave f z, t A cos kz t can be written as

f z, t Re Aei kz t , where Re denotes the real part of the complex number . This invites us to introduce the complex wave function i kz t f z, t Ae

with the complex amplitude A Aei absorbing the phase constant. The actual wave function is the real part of f :

f z, t Re f z , t . The advantage of the complex notation is that exponentials are much easier to manipulate than sines and cosines.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.2.4 Polarization In longitudinal wave, the displacement from the equilibrium is along the direction of propagation. Sound waves, which are nothing but compression waves in air, are longitudinal. Electromagnetic waves are transverse in nature. In a transverse wave displacement is perpendicular to the direction of propagation. There are two dimensions perpendicular to any given line of propagation. Accordingly, transverse waves occur in two independent state of polarization:

i kz t xˆ , “Vertical” polarization fv z , t Ae i kz t yˆ , “Horizontal” polarization fh z , t Ae i kz t nˆ or along any other direction in the xy plane f z, t Ae The polarization vector nˆ defines the plane of vibration. Because the waves are transverse, nˆ is perpendicular to the direction of propagation:

nˆ.zˆ 0 6.3 Electromagnetic waves in vacuum 6.3.1 The wave equation for E and B Write Maxwell’s equations in free space ( 0 and J 0 ) then, 2 E 2 B 2 2 E 0 0 2 B 0 0 2 and t t

Thus, E and B satisfy the wave equation 2 f

1 2 f . v t 2

So, EM waves travels with a speed v where 0 4 10 7 N

1

0 0 A

2

3 108 m / s c(velocity of light in free space)

2 , 0 8.86 1012 C

Nm 2

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.3.2 Monochromatic plane waves Suppose waves are traveling in the z-direction and have no x or y dependence; these are called plane waves because the fields are

uniform

perpendicular

over to

the

every

x

plane

direction

of

propagation. The plane waves can be represented as: ~ ~ ~ ~ E z , t E 0 e i kz t , B z , t B0 e i kz t

c

0

z

0 / c

~ ~ where E 0 and B0 are the (complex) y

~ ~ amplitudes (the physical fields, of course are the real parts of E and B ). That is, electromagnetic waves are transverse: the electric and magnetic fields are perpendicular to the direction of propagation. k Also B 0 ( zˆ E 0 ) There is nothing special about the z direction; we can generalize the monochromatic plane waves traveling in an arbitrary direction. The propagation vector or wave vector k points in the direction of propagation, whose magnitude is the wave number k. The scalar product k .r is the appropriate generalization of kz, so i k.r t 1 i k .r t ˆ 1 ˆ E r , t E 0 e n, B r , t E 0 e k nˆ kˆ E c c

where nˆ is polarization vector. The actual (real) electric and magnetic fields in a monochromatic plane wave with propagation vector k and polarization nˆ are

E r , t E0 cos k .r t nˆ ,

1 B r , t E0 cos k .r t c

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kˆ nˆ

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.3.3 Energy and Momentum in Electromagnetic Wave 1 1 2 The energy per unit volume stored in electromagnetic field is u 0 E 2 B 2 0

In case of monochromatic plane wave B 2

E2 0 0 E 2 2 c

1 1 2 So the electric and magnetic contributions are equal i.e. u E u B 0 E 2 B . 2 2 0 u uE u B 0 E 2 = 0 E0 2 cos 2 ( kz wt ) .

As the wave travels, it carries this energy along with it. The energy flux density (energy per unit area, per unit time) transported by the fields is given by the Pointing vector

1 S ( E B) 0 For monochromatic plane wave propagating in the z-direction, S c 0 E02 cos 2 ( kz wt ) zˆ cu zˆ . The energy per unit time, per unit area, transported by the wave is therefore uc. Electromagnetic fields not only carry energy, they also carry momentum. The momentum density stored in the field is For monochromatic plane wave,

1 2 S . c

1 1 0 E0 2 cos 2 ( kz wt ) zˆ u zˆ . c c

1 u 0 E02 , 2 ^ 1 Average of Poynting vector S c 0 E02 z , 2 ^ 1 Average momentum density 0 E02 z . 2c Average energy density

The average power per unit area transported by an electromagnetic wave is called the intensity

1 I S c 0 E02 . 2

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES Note: (a) When light falls on perfect absorber it delivers its momentum to the surface. In a p Act ,

time t the momentum transfer is

A

so the radiation pressure (average force per unit area) is P

c

1 p 1 I 0 E02 . A t 2 c

(b) When light falls on perfect reflector, the radiation pressure

P

ct

2I c

because the momentum changes direction, instead of being absorbed. 6.4 Electromagnetic waves in matter Inside matter, but in regions where there is no free charge or free current. If the medium

1 is linear and homogeneous, D E and H= B . 2E 2B 2 Now the wave equation inside matter is E 2 and B 2 . t t 2

Thus EM waves propagate through a linear homogenous medium at a speed

v

1 c where n 0 0 n

thus n r is the index of refraction (since r 1 for non-magnetic material). 1 1 1 The energy density u E 2 B 2 u E02 2 2 ^ 1 1 The Poynting vector S E B S v E02 z 2

1 Intensity I S vE02 2

Thus in a medium c→ v, 0 and 0

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.5 Electromagnetic waves in conductors Any initial free charge density f 0 given to conductor dissipate in a characteristic time / t / where σ is conductivity and f t e f 0 .

This reflects the familiar fact that if we put some free charge on conductor, it will flow out to the edges. The modified wave equation for E and B are, 2 E E 2 B B 2 2 E 2 and B t t t t ~ ~ ~ ~ ~ ~ The admissible plane wave solution is E z , t E 0 e i k z t , B z , t B 0 e i k z t

“wave number” k is complex. Let k k i where k and are real and imaginary part of k . 1/2

2 2 1 k 1 and 1 2 2 Thus, E z, t E 0 e z e i kz t , B z , t B 0 e z ei kz t

The distance it takes to reduce the amplitude by a factor of

d

1/ 2

1 is called the skin depth (d) e

1 ;

it is a measure of how far the wave penetrates into the conductor. The real part of k determines the wavelength, the propagation speed, and the index of refraction:

2 , v , k k

n

ck

Like any complex number, k can be expressed in terms of its modulus and phase:

k Kei 2

where K k k 2 2 1 and tan 1 k

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES The complex amplitudes E 0 E0 ei E and B 0 B0 ei B are related by

k Kei B 0 E 0 B0 ei B E0 ei E . Evidently the electric and magnetic fields are no longer in phase; in fact B E , the magnetic field lags behind the electric fields. 1/2

2 B0 K 1 E0

E z , t E0 e z cos kz t E xˆ Thus, B z, t B0 e z cos kz t E yˆ

Note: (a) In a poor conductor ( )

2

i.e independent of frequency.

(b) In a very good conductor ( )

, 2

d

1 2 1 f

(c) When an electromagnetic wave strikes a perfect conductor ( ) then all waves are reflected back i.e. E 0 R E 0 I

and

E 0T 0 .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 7. Applications of Electromagnetic waves 7.1 Reflection and Refraction 7.1.1 Normal incidence At z 0 , the combined field on the left E I E R and B I B R , must join the fields on the right ET & BT , in accordance with the boundary conditions. v v 0 R 2 1 0 I v2 v1

2v2 0T 0 I v1 v2

,

1

I

x

2

T v2

v1

Note:

T

I

Reflected wave is in phase if v2 v1 or n2 n1

z

y

and out of phase if v2 v1 or n2 n1 .

R

In terms of indices of refraction the real

v1

R

Interface

amplitudes are 0 R

n1 n2 0I n1 n2

,

0T

2n1 0 I . n1 n2

1 Since Intensity I vE02 , then the ratio of the reflected intensity to the incident intensity is 2 2

2

E n n I the Reflection coefficient R R 0 R 1 2 . I I E0 I n1 n2 The ratio of the transmitted intensity to the incident intensity is the Transmission coefficient 2

I v E 4n1n2 T T 2 2 0T . I I 1v1 E0 I n1 n2 2

R T 1

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 7.1.2 Oblique incidence In oblique incidence an incoming wave meets the boundary at an arbitrary angle I . Of course, normal incidence is really just a special case of oblique incidence with I 0 . First Law (law of refraction)

kR

The incident, reflected and transmitted wave vectors

kT

form a plane (called the incidence plane),

R

which also includes normal to the surface.

T

Second law (law of reflection)

z

I

The angle of incidence is equal to the angle of reflection i.e.

I R

Plane of Incidence

kI

Third Law: (law of refraction, or Snell’s law)

2

1

sin T n1 . sin I n2 7.1.3 Fresnel’s relation (Parallel and Perpendicular Polarization) Case I: (Polarization in the plane of incidence)

kR

Reflected and transmitted amplitudes E 0 R

E0 I and

cos T where cos I

and

2 E 0T

BR

E0 I

v n 11 1 2 2v2 2 n1

These are known as Fresnel’s equations.

ER

T

R

T

T

kT

z I

Notice that transmitted wave is always in phase with

I I

kI

1

2

the incident one; the reflected wave is either in phase , if , or 180 0 out phase if . At Brewster’s angle ( B ) reflected light is completely extinguished when , or

tan B

n2 n1

and I B 90o

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES When light enters from denser to rarer medium ( n1 n2 ) then after a critical angle ( C ) there is total internal reflection. sin 900 n1 sin c n2

sin c

n2 n1

at C , T 90o .

Reflection and Transmission coefficients are R

2

2 and T

2

R T 1

CaseII: (Polarization perpendicular to plane of incidence) 1 E0 R 1

E0 I

2 E 0T 1

and

E0 I

kR ER R

In this case B is not possible. 2

1 Thus R and 1

2 T 1

BR

T

T

2

z

I I

R T 1

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T

kT

kI I

1

2

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 8. Potential and Field formulation for Time Varying Fields 8.1 Scalar and vector potentials

B A

A E V t

Now from first Maxwell’s equation (i) 2V A ...............(1) t 0

From fourth Maxwell’s equation 2 2 A V A 0 0 2 A 0 0 J 0 …............(2) t t Equations (1) and (2) contain all the information of Maxwell’s equations. Thus we need to calculate only four components (one for V and three for A ) instead of six components (three for E and three for B ). 8.2 Gauge transformation Suppose we have two sets of potentials, V , A and V , A , which correspond to the

same electric and magnetic fields. Thus A A and V V . A A , It follows that V V . t

Conclusion: For any old scalar function , we can add to A , provided we

from V. None of these will affect the physical quantities E t and B . Such changes in V and A are called gauge transformations. simultaneously subtract

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 8.3 Coulomb and Lorentz gauge A 0 . Coulomb Gauge reads V Lorentz Gauge condition is A 0 0 . t 2 2 A V 2 Since A 0 0 2 A 0 0 A , 0 J and V t t t 0

Using Lorentz Gauge condition

2 2 2V 2 A V . A 0 0 2 0 J and 0 0 2 t 0 t The virtue of the Lorentz gauge is that it treats V and A on an equal footing: the same 2 2 differential operator 0 0 t 2

2

(called the d' Alembertian) occurs in both

equations: (i) 2 V

, 0

(ii)

2

A o J .

9.2 Waveguides Electromagnetic waves confined to the interior of a hollow pipe or wave guide. The wave guide is a perfect conductor, E 0 and B 0 inside the material itself, and hence the boundary conditions at the inner wall are: || E 0 and B 0

Free charges and currents will be induced on the surface in such a way as to enforce these constraints. Let us assume E.M. Waves that propagate inside the waveguide is represented by:

E x, y, z, t E 0 ( x, y)ei kz t ,

B x, y, z, t B 0 ( x, y)ei kz t .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES These electric and magnetic field must satisfies Maxwell's equations in the interior of the waveguide.Since confined waves are not (in general) transverse; in order to fit the boundary conditions we shall have to include longitudinal components Ez and Bz :

E 0 Ex xˆ E y yˆ Ez zˆ, B 0 Bx xˆ By yˆ Bz zˆ . Putting this into Maxwells equations(iii) and (iv) and compare R.H.S and L.H.S

E y Ex Ez E ikE y i Bx , ikEx z i By , i Bz y x x y By Bx Bz i B i i ikBy 2 Ex , ikBx z 2 E y , 2 Ez y c x y c x c Let us rewrite these six equations (i) (ii)

E y

Ex i Bz , y

(iv)

Ez ikE y i Bx , y

(v)

x

(iii) ikEx

Ez i By , x

By x

Bx i 2 Ez y c

Bz i ikBy 2 Ex y c

(vi) ikBx

Bz i 2 Ey x c

Equation (ii), (iii), (v), and (vi) can be solved for Ex , E y , Bx , and By :

(i) Ex

Ez B k z y k 2 x

i

c

(iii) Bx

2

i

c

2

Bz Ez 2 k 2 x c y k

(ii) E y

(iv) By

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i

c

2

i

c

2

Ez B k z x k 2 y

Bz Ez 2 k 2 y c x k

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES Finally, we will get coupled differential equation 2 2 2 (i) 2 2 k 2 Ez 0 x y c

2 2 2 (ii) 2 2 k 2 Bz 0 x y c

If Ez 0 we call these TE (“transverse electric”) waves; if Bz 0 they are called TM (“transverse magnetic”) waves; if both Ez 0 and Bz 0 , we call them TEM waves. Note: It turns out that TEM waves can not occur in a hollow waveguide. 9.2.1 TE Waves in Rectangular Waveguide Suppose we have a wave guide of rectangular shape with height a and with b a b , and we are interested in the propagation of TE waves. x

a

z b

The

problem

is

to

solve

y 2 2 2 2 2 2 k Bz 0 x y c

(since Ez 0 )

subject to boundary condition B 0 .

m x n y Thus Bz B0 cos cos where m 0,1,2..... and n 0,1,2...... a b

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES This solution is called the TE mn mode. (The first index is conventionally associated with the larger dimension.) 2

Wave number k is obtaine from equation k x2 kx2 k 2 0 by putting k x and k y . c

c m a n b where 2f . 2

So wave number k

If

2

2

2

2 2 c m 2 n 2 c m a n b mn or f mn b , 2 a

the wave number is imaginary, and instead of a traveling wave we have exponentially attenuated fields. For this reason mn or f mn is called cutoff frequency for the mode in question. The lowest cutoff frequency (fundamental mode) for the waveguide occurs for the mode TE10 :

10

c a

c . 2a

f10

or

Frequencies less than this will not propagate at all.

Wave number can be written more simply in terms of the cutoff frequency k

The wave velocity is v

Group velocity vg

k

c

1 mn

2

c f 1 mn f

2

1 dk

1 2 2 mn . c

d

2

c.

2

f c 1 mn c 1 mn c . f

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES

0

Wavelength inside the waveguide g

1 mn 377

Characteristic impedanceTE

1 mn

2

2

0

f 1 mn f

2

377

f 1 mn f

2

9.2.2 TM Waves in Rectangular Waveguide

The

problem

is

to

solve

2 2 2 2 2 k Ez 0 2 x y c

(since Bz 0 )

|| subject to boundary condition E 0 .

We will get similar expression as we have derived in TE waves. Thus

m x n y E z E0 sin sin where m 1,2,3..... and n 1,2,3...... a b This solution is called the TM mn mode. (The first index is conventionally associated with the larger dimension.) Formula for cutoff frequency mn , wave velocity v , group velocity vg and g are same as TE waves.

w Characteristic impedance TM 377 1 mn w The fundamental mode is TM 11 and

f11

c 2

2

2

a b 1

1

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2

.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 10. Radiation- from moving charges 10.1 Radiation Accelerating charges and changing current produce electromagnetic waves (radiation). If the source is localized near the origin, the total power passing out through the spherical shell of radius r is the integral of pointing vector:

1 P r S .da 0

r

E B .da

Source

The power radiated is the limit of this quantity as r goes to infinity:

Prad lim P r r

This is the energy (per unit time) that is transported out to infinity, and never comes back. 10.1.1 Power radiated by a point charge The electric field E and magnetic field B of an EM wave due to a point charge q, having acceleration a at any point P (position vector r ) is given by

qa sin qa sin E , B r r

z E

(where is the angle which r makes with z-direction) q 2 a 2 sin 2 0 q 2 a 2 S Thus r2 16 2 c

sin 2 2 r

a

rˆ

The total power radiated is evidently

P

r

y

q x

q 2 a 2 sin 2 q 2a2 P S .da 0 2 2 r 2 sin d d 0 6c 16 c r The direction of E can be determined from the following rule: (i) r and E is always perpendicular. (ii) a , r and E lies in one plane.

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Branch office Anand Institute of Mathematics, 28-B/6, Jia Sarai, Near IIT Hauz Khas, New Delhi-16 1

fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.12 Image problems.................................................................................... (15-17) 1.12.1 The Classic Image problem 1.12.2 Induced Surface Charge 1.12.3 Force and Energy 1.12.4 Other Image Problem 2. Magnetostatic.......................................................................................... (18-24) 2.1 Magnetic force on current element............................................................. (18) 2.1.1 Current in a wire 2.1.2 Surface current density 2.1.3 Volume current density 2.2 Continuity equation................................................................................... (19) 2.3 Biot-Savart law.......................................................................................... (19) 2.3.1 Magnetic field due to wire 2.3.2 Magnetic field due to Solenoid and Toroid 2.4 Ampere's Law........................................................................................... (20) 2.5 Magnetic Vector Potential........................................................................ (20) 2.6 Magnetostatic boundary condition........................................................... (21) 2.7 Multiple Expansion of Vector Potential................................................... (22) 2.8 Magnetisation........................................................................................... (23) 2.8.1 The Field of a magnetized Object (Bound Currents) 2.9 The Auxiliary field H.......................................................................... (23-24) 2.9.1 Ampere’s Law in in presence of magnetic materials 2.9.2 Magnetic Susceptibility and Permeability 2.9.3 Boundary Condition 3. Dynamics of charged particles in static and uniform electromagnetic fields 3.1 Charged particle in static electric field………………………..……. (25-26) 3.1.1 Charged particle enters in the direction of field (Linear motion) 3.1.2 Charged particle enters in the direction perpendicular to field (Parabolic motion) Head office fiziks, H.No. 23, G.F, Jia Sarai, Near IIT, Hauz Khas, New Delhi-16 Phone: 011-26865455/+91-9871145498 Website: www.physicsbyfiziks.com Email: [email protected]

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3.2 Charged particle in static magnetic field.......................................................... (27) 3.2.1 Charged particle enters in the direction perpendicular to field (Circular motion) 3.2.2 Charged particle enters in the direction making an angle with the field (Helical motion) 3.3 Charged particle in uniform electric and magnetic field (Cycloid motion).... (28)

4. Electromagnetic induction…………………………………………………... (29) 4.1 Faraday’s Law 4.1.1 Lenz’s Law 4.1.2 Inductance 4.1.3 Energy stored in the field

5. Maxwell's equations…………………………………………………..…... (30) 5.1 Maxwell’s equation in free space 5.1.1 Electrodynamics before Maxwell’s 5.1.2 How Maxwell fixed Ampere’s Law 5.1.3 Paradox of charging capacitor 5.1.4 Maxwell’s equation in free space 5.2 Maxwell’s equation in linear isotropic media …………………………... (30) 5.3 Boundary conditions on the fields at interfaces ……………………….... (31)

6. Electromagnetic waves in free space…………………………….….... (31-34) 6.1 Poynting Theorem…………………………………………………..….... (31) 6.2 Waves in one dimension (Sinusoidal waves) ……………….................... (32) 6.2.1 The wave equation 6.2.2 Terminology 6.2.3 Complex notation 6.2.4 Polarization Head office fiziks, H.No. 23, G.F, Jia Sarai, Near IIT, Hauz Khas, New Delhi-16 Phone: 011-26865455/+91-9871145498 Website: www.physicsbyfiziks.com Email: [email protected]

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.3 Electromagnetic waves in vacuum……………….................................... (35) 6.3.1 The wave equation for E and B 6.3.2 Monochromatic plane waves 6.3.3 Energy and momentum in Electromagnetic wave 6.4 Electromagnetic waves in matter………………..................................... (38) 6.5 Electromagnetic waves in conductors……………….............................. (39)

7. Applications of Electromagnetic waves………................................ (41-43) 7.1 Reflection and Refraction………………................................................ (41) 7.1.1 Normal incidence 7.1.2 Oblique incidence 7.1.3 Fresnel’s relation (Parallel and Perpendicular Polarization)

PART ‘B’ ADVANCED

8. Potential and Field formulation for Time Varying Fields……........ (44-45) 8.1 Scalar and vector potentials..................................................................... (44) 8.2 Gauge transformation.............................................................................. (44) 8.3 Coulomb and Lorentz gauge.................................................................... (45) 9. Waveguides……................................................................................... (45-49) 9.1 Rectangular Waveguides......................................................................... (45) 9.2.1 TE Waves in Rectangular Waveguide 9.2.2 TM Waves in Rectangular Waveguide 10. Radiation- from moving charges ....................................................... (50) 10.1 Radiation 10.1.1 Power radiated by a point charge

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1. Electrostatics The electric field at any point due to stationary source charges is called as electrostatic field. 1.1 Coulomb’s Law and Superposition Principle The electric force on a test charge Q due to a single point charge q, which is at rest and a distance R apart is given by Coulomb’s law

Q

1 Qq F . 4 0 R 2 R

R

The constant 0 is called the permittivity of free space. q

In mks units, 0 8.85 10

12

C2 N .m 2

1.1.1 Electric field If we have many point charges q1 , q2 ,...... at distances R1 , R2 ,...... from test charge Q, then according to the principle of superposition the total force on Q is q2Q 1 q1Q F F1 F2 ............ 2 R1 2 R2 ........ , 4 0 R1 R2

F QE

where E P

1 4 0

n

qi

R i 1

2 i

Ri

E is called the electric field of the source charges. Physically E P is the force per unit charge that would be exerted on a test charge placed at P. If charge is distributed continuously over some region, then

E r

1 4 0

1 ˆ Rdq . 2 R line

The electric field of a line charge is dq dl

E r

1 4 0

( r ) ˆ Rdl where is charge per unit length. R2 line

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES For surface charge dq da

E r

1 4 0

( r ) ˆ Rda where is charge per unit area. 2 R surface

For a volume charge dq d

E r

1 4 0

( r ) ˆ Rd where is charge per unit volume. 2 R volume

1.2 Gauss’s law 1.2.1 Field lines and Electric flux

Consider that a point charge q is situated at the origin: E

1 q 4 0 r 2

r

This field is represented by the field line as shown in figure below. The magnitude of the field is indicated by the density of the field lines: it's strong near the center

E

where the field line are close together, and weak farther out, where they are relatively far apart. The field strength (E) is proportional to the number of field lines per unit area (area perpendicular to the lines).

The flux of E through a surface S, E E .d a is a measure of the “number of field S

lines” passing through S. A charge outside the surface would contribute nothing to the total flux, since its field lines go in one side and out the other. It follows, then, that for any closed surface,

1

E .d a

Qenc

0

where Qenc is the total charge enclosed within the surface. This is Gauss’s law in integral form.

Gauss’s law in differential form:

.E

1 0

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.2.2 Applications of Gauss’s Law Gauss's law is always true, but it is not always useful. Gauss's law is useful for only three kinds of symmetry: 1. Spherical symmetry. Make your Gaussian surface a concentric sphere. 2. Cylindrical symmetry. Make your Gaussian surface a coaxial cylinder. 3. Plane symmetry. Make your Gaussian surface a “pillbox,” which extends equally above and below the surface. Gaussian surface r

Gaussian pillbox

R

Gaussian surface

1.3 Electric Potential 1.3.1 Curl of Electric field

Then integral around a closed path is zero i.e.

E .d l 0

This line integral is independent of path. It depends on two end points. Applying stokes theorem, we get E 0 . The electric field is not just any vector but only those vector whose curl is zero. P

So, we can define a function V r E .d l

where is some standard reference point. V then depends only on the point r. It is called the electric potential. Evidently, the potential difference between two points a and b is b

V b V a E .d l

E V

a

Potential obeys the superposition principle.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.3.2 Potential of localized charges Potential of a point charge q is V

1 q where R is the distance from the charge. 4 0 R n

1 4 0

R

For continuous volume charge distribution V ( r )

1 4 0

The

charges

The potential of a collection of point charge is V

potential

and V ( r )

of

line

and

surface

i 1

qi

.

i

(r ) d R are

V (r )

1 (r ) dl 4 0 R

1 ( r ) da . 4 0 R

1.4 Laplace’s and Poisson equations Since E V and .E 2V . 0 0 This is known as Poisson's equation. In regions where there is no charge, so that 0 , Poisson's equation reduces to Laplace's equation, 2V 0 . 1.5 Electrostatic boundary condition The boundary between two medium is a thin sheet of surface charge . above below

|| || and Eabove Ebelow E above E below nˆ 0 0

where nˆ is unit vector perpendicular to the surface, pointing upward. Vabove Vbelow

where

Vabove Vbelow n n 0

V V nˆ denotes the normal derivative of V (that is the rate of change in the n

direction perpendicular to the surface.)

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.6 Work and Energy in electrostatics The work done in moving a test charge Q in an external field E , from point a to b is b W F dl Q E dl Q V b V a b

a

a

q1

a

q2

If a and b r

W Q V r V QV r since V 0

Q

qi

b

In this sense potential is potential energy (the work it takes to create the system) per unit charge (just as the field is the force per unit charge).

1.6.1 The energy of point charge distribution q3

When the first charge q1 is placed, no work has been done. When q2 is placed work done W2 q2V1 where V1

is

the

potential

due

to q1

R13

so,

q 1 W2 q2 1 . 4 0 R12

Similarly W3

when

third

charge q3

is

placed

r1

q1

r3

R23 R12

r 2

q2

q q 1 q3 1 2 . 4 0 R13 R23

The work necessary to assemble the first three charges is W

In general, W

1 4 0

n

n

i 1 j 1 j i

qi q j Rij

1 8 0

n

n

i 1 j 1 j i

qi q j Rij

1 q1q2 q1q3 q2 q3 . 4 0 R12 R13 R23

1 n qiV ri , 2 i 1

where V(ri) is the potential at point ri (the position of qi) due to all other charges.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.6.2 Energy of continuous charge distribution For a volume charge density W

1 Vd and 2

W

0 2

E 2 d

all space

(i) Energy of a uniformly charged spherical shell of total charge q and radius R is W

q2 . 80 R

(ii) Energy stored in a uniformly charged solid sphere of radius R and charge q is W

1 3q 2 . 40 5R

1.7 Basic properties Conductors 1. E 0 inside a conductor. 2. 0 inside a conductor. 3. Any net charge resides on the surface. 4. A conductor is an equipotential. 5. E is perpendicular to the surface, just outside a conductor. Because the field inside a conductor is zero, boundary E above E below nˆ requires that the field immediately outside is E nˆ . 0 0 In terms of potential equation

condition

Vabove Vbelow V yields 0 n n n 0

These equations enable us to calculate the surface charge on a conductor, if we can determine E or V . 1 2 Force per unit area on the conductor is f nˆ . 2 0 This amounts to an outwards electrostatic pressure on the surface, tending to draw the conductor into the field, regardless the sign of . Expressing the pressure in terms of the field just outside the surface,

P

0 2 E . 2

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.8 Multiple Expansions 1.8.1 The Electric Potential and Field of a Dipole If we choose coordinates so that p (dipole moment) lies at the

z

origin and points in the z direction, then potential at (r , ) is: rˆ. p p cos Vdip r , . 2 4 o r 4 o r 2

E dip r ,

p

r

y

1 ˆ ˆ 3 p r r p 4 0 r 3

x

Note:

(a) When a dipole is placed in a uniform electric field ( E ), net force on the dipole is zero where p qd . and it experiences a torque p E (b) In non-uniform field, dipoles have net force F p E and torque p E .

(c) Energy of an ideal dipole p in an electric field E is U p.E .

(d) Interaction energy of two dipoles separated by a distance r is

U

1 p1 p 2 3 p1 rˆ p 2 rˆ . 4 0 r 3

1.8.2 Approximate potential at large distances Approximate potential at large distances due to arbitrary localized charge distribution

d ' r '

V r

R

P r

1 1 1 1 r ' d ' 2 r ' cos ' r ' d ' 2 4 0 r r r

' 2

r

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1 3 2 ' ' ' 2 cos 2 r d ...

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1 The first term (n = 0) is the monopole contribution (it goes like ). The second term r

(n = 1) is the dipole term (it goes like

1 ). The third term is quadrupole; the fourth r2

octopole and so on. The lowest nonzero term in the expansion provides the approximate potential at large r and the successive terms tell us how to improve the approximation if greater precision is required. The monopole and dipole terms Ordinarily, the multipole expansion is dominated (at large r) by the monopole term:

Vmon r

1 Q . 4 0 r

where Q d is the total charge of the configuration. If the total charge is zero, the dominant term in the potential will be the dipole (unless, of course, it also vanishes): 1 1 1 1 1 rˆ. p ' ' ' ' ' ' ' Vdip r r cos r d rˆ. r r d , 4 0 r 2 4 0 r 2 4 0 r 2 ' ' ' where dipole moment p r r d .

The dipole moment is determined by the geometry (size, shape and density) of the charge distribute. The dipole moment of a collection of point charge is n ' p qi ri . i 1

Note: Ordinarily, the dipole moment does change when we shift the origin, but there is an important exception: If the total charge is zero, then the dipole moment is independent of the choice of origin.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.9 Polarization

(Polarization) P dipole moment per unit volume

1.9.1 The Field of a polarized Object (Bound Charges) Suppose we have a piece of polarized material with polarization vector P containing a lot of microscopic

1 4 0

S

p

dipoles lined up. Then V r

R

1 1 1 P.d a ' '.P d ' . R 4 0 V R

The first term looks like the potential of a surface bound charge b P.nˆ (where nˆ is

the normal unit vector). The second term looks like the potential of a volume bound charge b .P .

1 Thus V r 4 0

S

b 1 b da ' d ' , this means the potential (and hence also the R 4 0 V R

field) of a polarized object is the same as that produced by a volume charge density b .P plus a surface charge density b P.nˆ . 1.10 The Electric Displacement 1.10.1 Gauss Law in the Presence of Dielectrics Within the dielectric, the total charge density can be written as b f where b is volume bound charge f free charge density. .D f where D 0 E P is known as the electric displacement. .D f Thus Gauss’ law reads, Or, in integral form D.d a Q fenc , where Q fenc denotes the total free charge enclosed in the volume.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.10.2 Linear Dielectrics (Susceptibility, Permittivity, Dielectric Constant) For any substances, the polarization is proportional to the field provided is not too strong: P E P 0 e E (Materials that obey this relation are called linear dielectrics) The constant of proportionality, e is called the electric susceptibility of the medium. The value of e depends on the microscopic structure of the substance and also on external conditions such as temperature. In linear media we have D 0 E P 0 E 0 e E 0 E (1 e ) E , where 0 (1 e ) This new constant is called the permittivity of the material. Also r

(1 e ) is called relative permittivity or dielectric constant, of the material. 0

1.10.3 Boundary Condition on D The boundary between two medium is a thin sheet of free surface charge f . ||

||

||

||

Dabove Dbelow f and D above D below P above P below

1.10.4 Energy in dielectric system W

1 D E d . 2 all space

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.12 Image problems 1.12.1 The Classic Image problem Suppose a point charge q is held a distance d above an infinite grounded conducting plane. We can find out what is the potential in the region above the plane. z z q q d d

y V 0

y d

q x x Forget about the actual problem; we are going to study a complete different situation. The new problem consists of two point charges +q at (0, 0, d) and –q at (0, 0,-d) and no conducting plane. For this configuration we can easily write down the potential: 1 q q V x, y, z 2 4 0 x 2 y 2 z d 2 x 2 y2 z d

(The denominators represent the distances from (x, y, z) to the charges +q and –q, respectively.) It follows that 1. V 0 when z 0 and 2. V 0 for x 2 y 2 z 2 d 2 , and the only charge in the region z 0 is the point charge +q at (0, 0, d). Thus the second configuration produces exactly the same potential as the first configuration, in the upper region z 0 .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 1.12.2 Induced Surface Charge The surface charge density induced on the conductor surface is 0

V z

z 0

qd

x, y

2 x 2 y 2 d 2

3

2

As expected, the induced charge is negative (assuming q is positive) and greatest at x=y=0. Q da q

The total induced charge 1.12.3 Force and Energy

The charge q is attracted towards the plane, because of the negative induced surface

1 q2 charge. The force: F zˆ . 4 0 2d 2 One can determine the energy by calculating the work required to bring q in from infinity. d

d 1 q2 1 q2 1 q2 W F .dl dz 4 0 4 z 2 4 0 4 z 4 0 4d d

1.12.4 Other Image Problem The method just described is not limited to a single point charge; any stationary charge distribution near a grounded conducting plane can be treated in the same way, by

introducing its mirror image.

R R'

r

R

a

q

V 0

b

q'

q a

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES Let us examine the completely different configuration, consisting of the point charge q together with another point charge q '

2 R q placed at a distance b R to the right of a a

the centre of sphere. No conductor, now-just two point charges. The potential of this configuration is V(r, )

V(r, )

1 q q' 40 R R

q 1 1 2 2 2 2 40 r a 2ra cos R (ra / R) 2ra cos

Clearly when r = R, V 0 Force The force on q, due to the sphere, is the same as the force of the image charge q, thus:

F

1 q 2 Ra 40 (a 2 R 2 )2

Energy To bring q in from infinity to a, we do work W

1 q 2R 40 2(a 2 R 2 )

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 2. MAGNETOSTATICS The magnetic field at any point due to steady current is called as magnetostatic field. 2.1 Magnetic force on current element

The magnetic force on a charge Q, moving with velocity v in a magnetic field B is,

F mag Q v B . This is known as Lorentz force law. In the presence of both electric and

magnetic fields, the net force on Q would be: F Q E v B 2.1.1 Current in a wire A line charge λ traveling down a wire at a speed v constitutes a current I v .

v t v

Magnetic force on a segment of current carrying wire is, F mag I dl B

P

2.1.2 Surface current density

When charge flows over a surface, we describe it by the surface current K . K

d dl

is the current per unit widthFlow

perpendicular to flow. dl Also K v where is surface charge density and v is its velocity. Magnetic force on surface current F mag K B da

K

2.1.3 Volume current density d J is the current per unit area-perpendicular to da da flow. Also J v where is volume charge density and v is its velocity. Magnetic force on volume current F mag J B d .Current crossing a surface S is J d a

J

Flow

S

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 2.2 Continuity equation General continuity equation J

t

This is the precise mathematical statements of local charge conservation. Thus for magnetostatic fields

0 and hence the continuity equation becomes: t J 0.

2.3 Biot-Savart law (magnetic field of steady line current) The magnetic field of a steady line current is given by 0 I Rˆ 0 dl ' Rˆ B r dl I 4 R 2 4 R 2

where 0 4 10 7

r

I

R

N ( permeability of free space) A2

dI '

For surface and volume current Biot - Savart law becomes: 0 K ( r ') Rˆ 0 J ( r ') Rˆ ( r ) da ' and (r ) d ' . 4 R2 4 R 2 2.3.1 Magnetic field due to wire

Let us find the magnetic field a distance d from a long straight wire carrying a steady current I. I B 0 (sin 2 sin 1 )ˆ 4d For Infinite wire: 1

1

I and 2 B 0 ˆ 2 2 2d

2

Note:

I

1. Force (per unit length) of attraction between two

Wire segment

long, parallel wires a distance d apart, carrying currents I1 and I2 in same direction are: f

0 I1 I 2 . 2 d

2. If currents are in opposite direction they will repel with same magnitude.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3. The magnetic field a distance d above the center of a circular loop of radius R, which

I R2 carries a steady current I is B 0 zˆ . 2 (R 2 d 2 )3/2 I At the center of the circle B(0) 0 zˆ 2R 2.3.2 Magnetic field due to Solenoid and Toroid The magnetic field of a very long solenoid, consisting of n closely wound turns per unit length of a cylinder of radius R and carrying a steady current I is: 0 nI zˆ inside the solenoid B outside the solenoid. 0 ^ 0 NI Magnetic field due to toroid is B 2 r 0

for points inside the coil for points outside the coil

where N is the total number of turns. 2.4 Ampere's Law In general we can write

B dl I where I 0 enc enc J .d a is the total current enclosed

by the amperian loop. B 0 J

Right hand Rule If the fingers of your right hand indicate the direction of integration around the boundary, then your thumb defines the direction of a positive current. 2.5 Magnetic Vector Potential A : Since B 0 B A For magnetostatic fields, A 0 and 2 A 0 J 0 J r ' If J goes to zero at infinity, A r d ' for volume current. 4 R 0 I 1 0 K For line and surface currents, A r dl '; A r da ' 4 R 4 R

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES

2.6 Magnetostatic boundary condition (Boundary is sheet of current, K ) Just as the electric field suffers a discontinuity at a surface charge, so the magnetic field is discontinuous at a surface current. Only this time it is the tangential component that changes.

B.d a 0

Since

Babove Bbelow

For tangential components || || r B.dl 0 I enc Babove B K B is parallel to surface but to K below 0 || || B is parallel to surface and along K B.dl 0 I enc Babove Bbelow Thus the component of B that is parallel to the surface but perpendicular to the current

is discontinuous in the amount 0 K . A similar amperian loop running parallel to the current reveals that the parallel component is continuous. The result can be summarized in a single formula:

B above B below 0 K nˆ , where nˆ is a unit vector perpendicular to the surface, pointing “upward”. Like the scalar potential in electrostatics, the vector potential is continuous across, an boundary: Aabove Abelow For . A 0 guarantees that the normal component is continuous, and A B , in the form A . dl B.d a line

S

Aabove Abelow But the derivative of A inherits the discontinuity of B : 0 K n n

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 2.7 Multiple Expansion of Vector Potential

1 We can always write the potential in the form of a power series in , where r is the r distance to the point in question. Thus we can always write

0 I 4

1 1 1 1 2 3 2 r d l ' r 2 r ' cos ' d l ' r 3 r ' 2 cos ' 2 d l ' ......... z First term, monopole dl 0 (no magnetic Ar

monopole)

0 m rˆ Second term, dipole Adip r 4 r 2 where

m

is

the

magnetic

dipole

moment:

m

m I d a I A where A is area vector Thus A dip

r

y

m sin ˆ r 0 4 r 2

x

Hence

B dip r A

1 0m ˆ 0 3 3 m rˆ rˆ m ˆ 2 cos r sin 4 r 4 r 3

Note:

(a) When a magnetic dipole is placed in a uniform magnetic field ( B ), net force on the dipole is zero and it experiences a torque m B . (b) In non-uniform field, dipoles have net force F m B and torque m B .

(c) Energy of an ideal dipole m in an magnetic field B is U m.B . (d) Interaction energy of two dipoles separated by a distance r is

U

1 m1 m2 3 m1 rˆ m2 rˆ . 4 0 r 3 .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES

2.8 Magnetisation M

Magnetization M is magnetic dipole moment per unit volume. 2.8.1 The Field of a magnetized Object (Bound Currents)

Consider a piece of magnetized material with magnetization M . Then the vector potential is given by 1 A r 0 M r d ' 0 4 R 4 v

1 d a . M r R

This means the potential(and hence also the field) of a magnetized object is the same as would be produced by a volume current J b M throughout the material, plus a surface current K b M nˆ , on the boundary. We first determine these bound currents, and then find the field they produce. 2.9 The Auxiliary field ( H ) 2.9.1 Ampere’s Law in in presence of magnetic materials In a magnetized material the total current can be written as J J b J f where J b is B bound current and J f is free current. Thus H J f where H M 0 In integral form H dl I fenc where I fenc is the total free current passing through the

amperian loop. H plays a role in magnetostatic analogous to D in electrostatic: Just as D allowed us to write Gauss's law in terms of the free charge alone, H permits us to express Ampere's law in terms of the free current alone- and free current is what we control directly. Note:

When we have to find B or H in a problem involving magnetic materials, first look for symmetry. If the problem exhibits cylindrical, plane, solenoid, or toroidal symmetry, then we can get H directly from the equation H dl I fenc .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 2.9.2 Magnetic Susceptibility and Permeability For most substances magnetization is proportional to the field H , M m H , where m is magnetic susceptibility of the material.

B 0 H M 0 1 m H B H

where

0 0 r 0 1 m ,

is

permeability of material.

2.9.3 Boundary Condition ( H ) The boundary between two medium is a thin sheet of free surface current K f . The Ampere’s law states that H above H below K f nˆ . And

H above H below M above M below

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3. Dynamics of charged particles in static and uniform electromagnetic fields (The Lorentz Force Law)

The magnetic force on a charge Q, moving with velocity v in a magnetic field B is,

F mag Q v B

This is known as Lorentz force law. In the presence of both electric and magnetic fields, the net force on Q would be: F Q E v B

3.1 Charged particle in static electric field 3.1.1 Charged particle enters in the direction of field (Linear motion) Q

2

1

E

The force on the charge Q in electric field E is F Q E . Acceleration of the charge particle in the direction of the electric field is a If r is the position vector at any time t then Let at t=0, r r0 C1 r0 r If initially u 0, r0 0 r

QE 2 t ut r0 2m

ut

F QE . m m QE 2 t 2m 2

r0

1

QE 2 QE t and v t. 2m m

The energy acquired by the charged particle in moving from point 1 to 2 is 2

2

2

W F .d l m a.d l m 1

1

1

2 1 dv .vdt m v.d v W mv 22 v12 2 dt 1

If the potential difference between points 1 to 2 is V then W QV

1 m v 22 v12 2

If the particle starts from rest i.e v1 0 and final velocity is v then

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES

W QV

1 2 mv v 2

2QV m

1 2 1 Q2E2 2 QE 2 t QE t QEr Kinetic energy of the particle K .E. mv m. 2 2 2 2m m 3.1.2 Charged particle enters in the direction perpendicular to field (Parabolic motion) Let us consider a charge particle enters in an electric field region with velocity v x at t=0. The electric field is in the y-direction and the field region has length l. After traversing a distance l it strikes a point P on a screen which is placed at a distance L from the field region.

L vx

P

y2 y1

Ey

y

x

l

QE y 1 Thus y a y t 2 2 2m QE y l y1 2m v x

x vx

2

and which represents parabolic path.

2

and y 2 L tan

QE y l Thus distance of point P from the center of the screen is, y1 y 2 2m v x

Angle of deviation in the field region, tan

2

L tan

dy QE y x dx mv x2

Angle of deviation in the field free region, tan

QE y mv x2

l

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3.2 Charged particle in static magnetic field

The magnetic force on a charge Q, moving with velocity v in a magnetic field B is,

F mag Q v B

This is known as Lorentz force law. 3.2.1 Charged particle enters in the direction perpendicular to field (Circular motion) If a charge particle enters in a magnetic field at angle of 90o, then motion will be circular with the magnetic force providing the centripetal acceleration.

y

2

For uniform circular motion: QvB m

v mv R R QB

v

R

where R is the radius of the circle and m is the mass of the charge

particle.

z

Momentum of the charged particle p QBR Kinetic energy (KE) Time period T

F

Q

x

B

p 2 Q2 B2 R2 2m 2m

2R 2m v QB

3.2.2 Charged particle enters in the direction making an angle with the field (Helical motion) If the charge particle enters in a magnetic field making an angle θ, then motion will be helical. v v sin

and

v|| v cos ,

mv and the radius of helix is R QB .

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B

||

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 3.3 Charged particle in uniform electric and magnetic field (Cycloid motion) F Q E v B Q E zˆ Bz yˆ By zˆ z ma m y yˆ z zˆ

E y z , z y B QB where (cylotron frequency ) m

E

Solving above differential equations, we get

o

E E y t t sin t , z t 1 cos t B B E 2 2 y Rt z R R 2 where R B This is the formula for a circle, of radius R, x

whose center is

0, Rt , R

travels in the

a

c

b

B

y-direction at constant speed, v R

E B

The curve generated in this way is called a cycloid.

d l Q v B vdt 0 .

Magnetic forces do not work because v B is perpendicular to v , so dWmag F mag

Magnetic forces may alter the direction in which a particle moves, but they can not speed up or slow down it.

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y

fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 4. Electromagnetic induction 4.1 Faraday’s Law v

v

I

I

B

I

B

B

b

a

changing magnetic field

c

Experiment 1: He pulled a loop of wire to the right through a magnetic field. A current flowed in the loop (Figure a). Experiment 2: He moved the magnet to the left, holding the loop still. Again, a current flowed in the loop (Figure b). Experiment 3: With both the loop and the magnet at rest, he changed the strength of the field (he used an electromagnet, and varied the current in the coil). Once again current flowed in the loop (Figure c). Thus, universal flux rule is that, whenever (and for whatever reason) the magnetic flux through a loop changes, an e.m.f. will appear in the loop.

d dt

(Where magnetic flux B.d a )

In experiment 2,

A changing magnetic field induces an electric field. d It is this “induced” electric field that accounts for the e.m.f. E dl dt B B Then E dl t .d a E t 4.1.1 Lenz’s Law In Faraday’s law negative sign represents the Lenz’s law. (The induced current will flow in such a direction that the flux it produces tends to cancel the change). For example if the magnetic flux is increasing then induced e.m.f will try to reduce and vice versa. Head office fiziks, H.No. 23, G.F, Jia Sarai, Near IIT, Hauz Khas, New Delhi-16 Phone: 011-26865455/+91-9871145498 Website: www.physicsbyfiziks.com Email: [email protected]

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 5. Maxwell's equations 5.1 Maxwell’s equation in free space 5.1.1 Electrodynamics before Maxwell’s (i)

0

(Gauss’ Law),

(ii) B 0

(No name),

B (iii) t (iv) B 0 J

(Farday’s Law), (Ampere’s law).

5.1.2 How Maxwell fixed Ampere’s Law From continuity equation and Gauss Law E E .J ( 0 .E ) . 0 . J 0 0 . t t t t E B 0 J 0 0 Thus t A changing electric field induces a magnetic field.

E Maxwell called this extra term the displacement current J d 0 . t E Integral form of Ampere's law B.dl 0 I enc 0 0 .d a t 5.1.4 Maxwell’s equation in free space (i)

0

(Gauss’ Law),

(ii) B 0

(No name),

B (iii) t

(Farday’s Law),

E (iv) B 0 J 0 0 t

(Ampere’s law with Maxwell's correction).

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 5.3 Boundary conditions on the fields at interfaces || || (a ) Dabove Dbelow f (c) E above E below || || (b) Babove Bbelow (d ) H above H below K f nˆ In particular, if there is no free charge or free current at the interface between medium1 and medium 2, then (a )1 E1 2 E2 0 (b) B1 B2

and

|| || (c) E1 E 2 . 1 || 1 || (d ) B1 B 2 0 1 2

6. Electromagnetic waves in free space 6.1 Poynting Theorem (“work energy theorem of electrodynamics”) The work necessary to assemble a static charge distribution is We

0 E 2 d , where E is the resulting electric field 2

The work required to get currents going (against the back emf) is

Wm

1 B 2 d , where B is the resulting magnetic field 2 0

This suggests that the total energy in the electromagnetic field is U em

1 B2 2 E d . 0 2 0

Suppose we have some charge and current configuration which at time t, produces fields E & B . In next instant, dt, the charges moves around a bit. The work is done by electromagnetic forces acting on these charges in the interval dt. According to Lorentz Force Law, the work done on a charge ‘q’ is F dl q( E v B) v dt qE v dt .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES Now q d and v J , so the rate at which work is done on all the charges in a

volume V is dW d 1 1 2 1 0E2 B d dt dt V 2 0 0

E B d a ,

S

where S is the surface bounding V. This is Poynting's theorem; it is the “work energy theorem” of electrodynamics. The first integral on the right is the total energy stored in the fields, U em . The second term evidently, represents the rate at which energy is carries out of V, across its boundary surface, by the electromagnetic fields. Poynting's theorem says, that, “the work done on the charges by the electromagnetic force is equal to the decrease in energy stored in the field, less the energy that flowed out through the surface”. The energy per unit time, per unit area, transported by the fields is called the Poynting vector

1 S EB . 0

S d a is the energy per unit time crossing the infinitesimal surface d a – the energy or energy flux density. Momentum density stored in the electromagnetic field is:dem 0 0 S

6.2 Waves in one dimension (Sinusoidal waves) 6.2.1 The wave equation A wave propagating with speed v in z-direction can be expressed as:

2 f 1 2 f z 2 v 2 t 2

The most general solution to the wave equation is the sum of a wave to the right (+z direction) and a wave to the left (-z direction): f z, t g z vt h z vt .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.2.2 Terminology Let us consider a function f z, t A cos k ( z vt ) A is the amplitude of the wave (it is positive, and represents the maximum displacement from equilibrium). The argument of the cosine is called the phase, and is the phase constant (normally, we use a value in the range 0 2 ). Figure given below shows this function at time t 0 . Notice that at z vt

k

, the phase

is zero; let's call this the “central maximum.” If 0 , central maximum passes the origin

at time t 0 ; more generally k is the distance by which the central maximum (and therefore the entire wave) is “delayed.” Central maximum

f z, 0

v

A

z

/k

Finally k is the wave number; it is related to the wavelength as advances by

2 , for when z k

2 , the cosine executes one complete cycle. k

As time passes, the entire wave train proceeds to the right, at speed v . Time period of one complete cycle is T

2 . kv

The frequency (number of oscillations per unit time) is

1 kv v . T 2

The angular frequency 2 kv In terms of angular frequency , the sinusoidal wave can be represented as f z, t A cos kz t .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES A sinusoidal oscillation of wave number k and angular frequency traveling to the left would be written f z, t A cos kz t .

Comparing this with the wave traveling to the right reveals that, in effect, we could simply switch the sign of k to produce a wave with the same amplitude, phase constant, frequency, and wavelength, traveling in the opposite direction. Central maximum

f z, 0

v

/k

z

6.2.3 Complex notation In view of Euler's formula,

ei cos i sin , the sinusoidal wave f z, t A cos kz t can be written as

f z, t Re Aei kz t , where Re denotes the real part of the complex number . This invites us to introduce the complex wave function i kz t f z, t Ae

with the complex amplitude A Aei absorbing the phase constant. The actual wave function is the real part of f :

f z, t Re f z , t . The advantage of the complex notation is that exponentials are much easier to manipulate than sines and cosines.

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.2.4 Polarization In longitudinal wave, the displacement from the equilibrium is along the direction of propagation. Sound waves, which are nothing but compression waves in air, are longitudinal. Electromagnetic waves are transverse in nature. In a transverse wave displacement is perpendicular to the direction of propagation. There are two dimensions perpendicular to any given line of propagation. Accordingly, transverse waves occur in two independent state of polarization:

i kz t xˆ , “Vertical” polarization fv z , t Ae i kz t yˆ , “Horizontal” polarization fh z , t Ae i kz t nˆ or along any other direction in the xy plane f z, t Ae The polarization vector nˆ defines the plane of vibration. Because the waves are transverse, nˆ is perpendicular to the direction of propagation:

nˆ.zˆ 0 6.3 Electromagnetic waves in vacuum 6.3.1 The wave equation for E and B Write Maxwell’s equations in free space ( 0 and J 0 ) then, 2 E 2 B 2 2 E 0 0 2 B 0 0 2 and t t

Thus, E and B satisfy the wave equation 2 f

1 2 f . v t 2

So, EM waves travels with a speed v where 0 4 10 7 N

1

0 0 A

2

3 108 m / s c(velocity of light in free space)

2 , 0 8.86 1012 C

Nm 2

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.3.2 Monochromatic plane waves Suppose waves are traveling in the z-direction and have no x or y dependence; these are called plane waves because the fields are

uniform

perpendicular

over to

the

every

x

plane

direction

of

propagation. The plane waves can be represented as: ~ ~ ~ ~ E z , t E 0 e i kz t , B z , t B0 e i kz t

c

0

z

0 / c

~ ~ where E 0 and B0 are the (complex) y

~ ~ amplitudes (the physical fields, of course are the real parts of E and B ). That is, electromagnetic waves are transverse: the electric and magnetic fields are perpendicular to the direction of propagation. k Also B 0 ( zˆ E 0 ) There is nothing special about the z direction; we can generalize the monochromatic plane waves traveling in an arbitrary direction. The propagation vector or wave vector k points in the direction of propagation, whose magnitude is the wave number k. The scalar product k .r is the appropriate generalization of kz, so i k.r t 1 i k .r t ˆ 1 ˆ E r , t E 0 e n, B r , t E 0 e k nˆ kˆ E c c

where nˆ is polarization vector. The actual (real) electric and magnetic fields in a monochromatic plane wave with propagation vector k and polarization nˆ are

E r , t E0 cos k .r t nˆ ,

1 B r , t E0 cos k .r t c

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kˆ nˆ

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.3.3 Energy and Momentum in Electromagnetic Wave 1 1 2 The energy per unit volume stored in electromagnetic field is u 0 E 2 B 2 0

In case of monochromatic plane wave B 2

E2 0 0 E 2 2 c

1 1 2 So the electric and magnetic contributions are equal i.e. u E u B 0 E 2 B . 2 2 0 u uE u B 0 E 2 = 0 E0 2 cos 2 ( kz wt ) .

As the wave travels, it carries this energy along with it. The energy flux density (energy per unit area, per unit time) transported by the fields is given by the Pointing vector

1 S ( E B) 0 For monochromatic plane wave propagating in the z-direction, S c 0 E02 cos 2 ( kz wt ) zˆ cu zˆ . The energy per unit time, per unit area, transported by the wave is therefore uc. Electromagnetic fields not only carry energy, they also carry momentum. The momentum density stored in the field is For monochromatic plane wave,

1 2 S . c

1 1 0 E0 2 cos 2 ( kz wt ) zˆ u zˆ . c c

1 u 0 E02 , 2 ^ 1 Average of Poynting vector S c 0 E02 z , 2 ^ 1 Average momentum density 0 E02 z . 2c Average energy density

The average power per unit area transported by an electromagnetic wave is called the intensity

1 I S c 0 E02 . 2

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES Note: (a) When light falls on perfect absorber it delivers its momentum to the surface. In a p Act ,

time t the momentum transfer is

A

so the radiation pressure (average force per unit area) is P

c

1 p 1 I 0 E02 . A t 2 c

(b) When light falls on perfect reflector, the radiation pressure

P

ct

2I c

because the momentum changes direction, instead of being absorbed. 6.4 Electromagnetic waves in matter Inside matter, but in regions where there is no free charge or free current. If the medium

1 is linear and homogeneous, D E and H= B . 2E 2B 2 Now the wave equation inside matter is E 2 and B 2 . t t 2

Thus EM waves propagate through a linear homogenous medium at a speed

v

1 c where n 0 0 n

thus n r is the index of refraction (since r 1 for non-magnetic material). 1 1 1 The energy density u E 2 B 2 u E02 2 2 ^ 1 1 The Poynting vector S E B S v E02 z 2

1 Intensity I S vE02 2

Thus in a medium c→ v, 0 and 0

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 6.5 Electromagnetic waves in conductors Any initial free charge density f 0 given to conductor dissipate in a characteristic time / t / where σ is conductivity and f t e f 0 .

This reflects the familiar fact that if we put some free charge on conductor, it will flow out to the edges. The modified wave equation for E and B are, 2 E E 2 B B 2 2 E 2 and B t t t t ~ ~ ~ ~ ~ ~ The admissible plane wave solution is E z , t E 0 e i k z t , B z , t B 0 e i k z t

“wave number” k is complex. Let k k i where k and are real and imaginary part of k . 1/2

2 2 1 k 1 and 1 2 2 Thus, E z, t E 0 e z e i kz t , B z , t B 0 e z ei kz t

The distance it takes to reduce the amplitude by a factor of

d

1/ 2

1 is called the skin depth (d) e

1 ;

it is a measure of how far the wave penetrates into the conductor. The real part of k determines the wavelength, the propagation speed, and the index of refraction:

2 , v , k k

n

ck

Like any complex number, k can be expressed in terms of its modulus and phase:

k Kei 2

where K k k 2 2 1 and tan 1 k

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES The complex amplitudes E 0 E0 ei E and B 0 B0 ei B are related by

k Kei B 0 E 0 B0 ei B E0 ei E . Evidently the electric and magnetic fields are no longer in phase; in fact B E , the magnetic field lags behind the electric fields. 1/2

2 B0 K 1 E0

E z , t E0 e z cos kz t E xˆ Thus, B z, t B0 e z cos kz t E yˆ

Note: (a) In a poor conductor ( )

2

i.e independent of frequency.

(b) In a very good conductor ( )

, 2

d

1 2 1 f

(c) When an electromagnetic wave strikes a perfect conductor ( ) then all waves are reflected back i.e. E 0 R E 0 I

and

E 0T 0 .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 7. Applications of Electromagnetic waves 7.1 Reflection and Refraction 7.1.1 Normal incidence At z 0 , the combined field on the left E I E R and B I B R , must join the fields on the right ET & BT , in accordance with the boundary conditions. v v 0 R 2 1 0 I v2 v1

2v2 0T 0 I v1 v2

,

1

I

x

2

T v2

v1

Note:

T

I

Reflected wave is in phase if v2 v1 or n2 n1

z

y

and out of phase if v2 v1 or n2 n1 .

R

In terms of indices of refraction the real

v1

R

Interface

amplitudes are 0 R

n1 n2 0I n1 n2

,

0T

2n1 0 I . n1 n2

1 Since Intensity I vE02 , then the ratio of the reflected intensity to the incident intensity is 2 2

2

E n n I the Reflection coefficient R R 0 R 1 2 . I I E0 I n1 n2 The ratio of the transmitted intensity to the incident intensity is the Transmission coefficient 2

I v E 4n1n2 T T 2 2 0T . I I 1v1 E0 I n1 n2 2

R T 1

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 7.1.2 Oblique incidence In oblique incidence an incoming wave meets the boundary at an arbitrary angle I . Of course, normal incidence is really just a special case of oblique incidence with I 0 . First Law (law of refraction)

kR

The incident, reflected and transmitted wave vectors

kT

form a plane (called the incidence plane),

R

which also includes normal to the surface.

T

Second law (law of reflection)

z

I

The angle of incidence is equal to the angle of reflection i.e.

I R

Plane of Incidence

kI

Third Law: (law of refraction, or Snell’s law)

2

1

sin T n1 . sin I n2 7.1.3 Fresnel’s relation (Parallel and Perpendicular Polarization) Case I: (Polarization in the plane of incidence)

kR

Reflected and transmitted amplitudes E 0 R

E0 I and

cos T where cos I

and

2 E 0T

BR

E0 I

v n 11 1 2 2v2 2 n1

These are known as Fresnel’s equations.

ER

T

R

T

T

kT

z I

Notice that transmitted wave is always in phase with

I I

kI

1

2

the incident one; the reflected wave is either in phase , if , or 180 0 out phase if . At Brewster’s angle ( B ) reflected light is completely extinguished when , or

tan B

n2 n1

and I B 90o

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES When light enters from denser to rarer medium ( n1 n2 ) then after a critical angle ( C ) there is total internal reflection. sin 900 n1 sin c n2

sin c

n2 n1

at C , T 90o .

Reflection and Transmission coefficients are R

2

2 and T

2

R T 1

CaseII: (Polarization perpendicular to plane of incidence) 1 E0 R 1

E0 I

2 E 0T 1

and

E0 I

kR ER R

In this case B is not possible. 2

1 Thus R and 1

2 T 1

BR

T

T

2

z

I I

R T 1

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T

kT

kI I

1

2

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 8. Potential and Field formulation for Time Varying Fields 8.1 Scalar and vector potentials

B A

A E V t

Now from first Maxwell’s equation (i) 2V A ...............(1) t 0

From fourth Maxwell’s equation 2 2 A V A 0 0 2 A 0 0 J 0 …............(2) t t Equations (1) and (2) contain all the information of Maxwell’s equations. Thus we need to calculate only four components (one for V and three for A ) instead of six components (three for E and three for B ). 8.2 Gauge transformation Suppose we have two sets of potentials, V , A and V , A , which correspond to the

same electric and magnetic fields. Thus A A and V V . A A , It follows that V V . t

Conclusion: For any old scalar function , we can add to A , provided we

from V. None of these will affect the physical quantities E t and B . Such changes in V and A are called gauge transformations. simultaneously subtract

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 8.3 Coulomb and Lorentz gauge A 0 . Coulomb Gauge reads V Lorentz Gauge condition is A 0 0 . t 2 2 A V 2 Since A 0 0 2 A 0 0 A , 0 J and V t t t 0

Using Lorentz Gauge condition

2 2 2V 2 A V . A 0 0 2 0 J and 0 0 2 t 0 t The virtue of the Lorentz gauge is that it treats V and A on an equal footing: the same 2 2 differential operator 0 0 t 2

2

(called the d' Alembertian) occurs in both

equations: (i) 2 V

, 0

(ii)

2

A o J .

9.2 Waveguides Electromagnetic waves confined to the interior of a hollow pipe or wave guide. The wave guide is a perfect conductor, E 0 and B 0 inside the material itself, and hence the boundary conditions at the inner wall are: || E 0 and B 0

Free charges and currents will be induced on the surface in such a way as to enforce these constraints. Let us assume E.M. Waves that propagate inside the waveguide is represented by:

E x, y, z, t E 0 ( x, y)ei kz t ,

B x, y, z, t B 0 ( x, y)ei kz t .

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES These electric and magnetic field must satisfies Maxwell's equations in the interior of the waveguide.Since confined waves are not (in general) transverse; in order to fit the boundary conditions we shall have to include longitudinal components Ez and Bz :

E 0 Ex xˆ E y yˆ Ez zˆ, B 0 Bx xˆ By yˆ Bz zˆ . Putting this into Maxwells equations(iii) and (iv) and compare R.H.S and L.H.S

E y Ex Ez E ikE y i Bx , ikEx z i By , i Bz y x x y By Bx Bz i B i i ikBy 2 Ex , ikBx z 2 E y , 2 Ez y c x y c x c Let us rewrite these six equations (i) (ii)

E y

Ex i Bz , y

(iv)

Ez ikE y i Bx , y

(v)

x

(iii) ikEx

Ez i By , x

By x

Bx i 2 Ez y c

Bz i ikBy 2 Ex y c

(vi) ikBx

Bz i 2 Ey x c

Equation (ii), (iii), (v), and (vi) can be solved for Ex , E y , Bx , and By :

(i) Ex

Ez B k z y k 2 x

i

c

(iii) Bx

2

i

c

2

Bz Ez 2 k 2 x c y k

(ii) E y

(iv) By

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i

c

2

i

c

2

Ez B k z x k 2 y

Bz Ez 2 k 2 y c x k

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES Finally, we will get coupled differential equation 2 2 2 (i) 2 2 k 2 Ez 0 x y c

2 2 2 (ii) 2 2 k 2 Bz 0 x y c

If Ez 0 we call these TE (“transverse electric”) waves; if Bz 0 they are called TM (“transverse magnetic”) waves; if both Ez 0 and Bz 0 , we call them TEM waves. Note: It turns out that TEM waves can not occur in a hollow waveguide. 9.2.1 TE Waves in Rectangular Waveguide Suppose we have a wave guide of rectangular shape with height a and with b a b , and we are interested in the propagation of TE waves. x

a

z b

The

problem

is

to

solve

y 2 2 2 2 2 2 k Bz 0 x y c

(since Ez 0 )

subject to boundary condition B 0 .

m x n y Thus Bz B0 cos cos where m 0,1,2..... and n 0,1,2...... a b

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES This solution is called the TE mn mode. (The first index is conventionally associated with the larger dimension.) 2

Wave number k is obtaine from equation k x2 kx2 k 2 0 by putting k x and k y . c

c m a n b where 2f . 2

So wave number k

If

2

2

2

2 2 c m 2 n 2 c m a n b mn or f mn b , 2 a

the wave number is imaginary, and instead of a traveling wave we have exponentially attenuated fields. For this reason mn or f mn is called cutoff frequency for the mode in question. The lowest cutoff frequency (fundamental mode) for the waveguide occurs for the mode TE10 :

10

c a

c . 2a

f10

or

Frequencies less than this will not propagate at all.

Wave number can be written more simply in terms of the cutoff frequency k

The wave velocity is v

Group velocity vg

k

c

1 mn

2

c f 1 mn f

2

1 dk

1 2 2 mn . c

d

2

c.

2

f c 1 mn c 1 mn c . f

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES

0

Wavelength inside the waveguide g

1 mn 377

Characteristic impedanceTE

1 mn

2

2

0

f 1 mn f

2

377

f 1 mn f

2

9.2.2 TM Waves in Rectangular Waveguide

The

problem

is

to

solve

2 2 2 2 2 k Ez 0 2 x y c

(since Bz 0 )

|| subject to boundary condition E 0 .

We will get similar expression as we have derived in TE waves. Thus

m x n y E z E0 sin sin where m 1,2,3..... and n 1,2,3...... a b This solution is called the TM mn mode. (The first index is conventionally associated with the larger dimension.) Formula for cutoff frequency mn , wave velocity v , group velocity vg and g are same as TE waves.

w Characteristic impedance TM 377 1 mn w The fundamental mode is TM 11 and

f11

c 2

2

2

a b 1

1

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2

.

Branch office Anand Institute of Mathematics, 28-B/6, Jia Sarai, Near IIT Hauz Khas, New Delhi-16 49

fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES 10. Radiation- from moving charges 10.1 Radiation Accelerating charges and changing current produce electromagnetic waves (radiation). If the source is localized near the origin, the total power passing out through the spherical shell of radius r is the integral of pointing vector:

1 P r S .da 0

r

E B .da

Source

The power radiated is the limit of this quantity as r goes to infinity:

Prad lim P r r

This is the energy (per unit time) that is transported out to infinity, and never comes back. 10.1.1 Power radiated by a point charge The electric field E and magnetic field B of an EM wave due to a point charge q, having acceleration a at any point P (position vector r ) is given by

qa sin qa sin E , B r r

z E

(where is the angle which r makes with z-direction) q 2 a 2 sin 2 0 q 2 a 2 S Thus r2 16 2 c

sin 2 2 r

a

rˆ

The total power radiated is evidently

P

r

y

q x

q 2 a 2 sin 2 q 2a2 P S .da 0 2 2 r 2 sin d d 0 6c 16 c r The direction of E can be determined from the following rule: (i) r and E is always perpendicular. (ii) a , r and E lies in one plane.

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