Modern Physics

 BANSALCLASSES ES § BANSALCLASS TARGET  IIT   IIT JEE   2007

XII (ALL)

QUESMOa MMEJM  MODERN PHYSICS

QUESTION  FOR SHORT  ATOMIC

ANSWER

PHYSICS

Q. l

In the photoelectric the photoelectric effect, why  the existence  of a cutoff  in favour of of the photon the photon effect, why does  does the  existence of  cu toff frequency speak  in favour  theory and theory and against  against the wave the wave theory?

Q. 2 

Explain the Explain the statement  statement that one's on e's eyes could not could not detect  detect faint starlight starlight if  light  light were we re not  particle-like.  not particle-like.

Q. 3

energy be given  given by How can a photon energy be  by E = h/when = h/when the very the very presence of the frequency/in the frequency/in the formula the formula implies that light is light is a wave? a wave?

Q. 4

p of a photon is  photon is given  given by c, the speed  speed oflight, does not does  not appear   appear  in The momentum The momentum p of  by p = h = hIX. Why i  Why  iss it that  it that c, the  in this expression?

Q. 5

Given that E that E = h/'for   Doppler shift in shift in frequency  frequency of  radiation from fro m a receding light source sou rce = h/'for  a photon,  a photon, the  the Doppler  of radiation  a receding light would seem to indicate a indicate a reduced  reduced energy for energy for the emitted photons. Is this  this in  fact true? If so, what the emitted photons. Is  in fact so, what happened energy principle? to the conservation the conservation of   of energy

Q. 6

 atomic hydrogen h ydrogen yet  what region of the spectrum? Any series Any series of   of  atomic  yet to be observed be observed will probably be found be found in  in what the spectrum?

Q.7

absorb a photon  photon whose energy exceeds its exceeds its binding  binding energy( 13.6 eV)? Can a hydrogen atom absorb a e V)?

Q. 8 

Only a Only a relatively  relatively small number  of  lines can be observed tubes, es, whereas  of  Balmer lines can be observed from laboratory discharge tub  stellar spectra. Explain this in this in terms  terms of th e small small densi density, ty, high temperature, a large number  are observed  are observed in  in stellar volume of  gases in ases in stellar  stellar atmospheres. and large and large volume of 

Q. 9 

Wnat is Wnat is the origin wavelength X  of figure figure shown? Why shown? Why is it an important clue to the photon the origin ofthe cutoff wavelength X an important clue to the photon nature of nature of x rays? mm

e u a < >u

30 40 50 60 70 80 90 Wavelength  Wavelength  (pm)

Q. 10

hydrogen be caused  caused to  emit x  rays? If so, describe  Ifnot, why  not? Can atomic Can atomic hydrogen be  to emit  x rays? so, describe how.  how. Ifnot,  why not?

Q. l l

 theory, which does not does not work  work very well even for  even for  helium  helium (Z = (Z = 2),  2), gives such a such a good  good Why is it that it that B ohr  theory, account of the characteristic that portion po rtion that originates originates deep the characteristic x-ray spectra ofthe elements, the elements, or  or at least at least of   of that within the atom? the atom? potential of  hydrogen is ydrogen is 13.6 V  tubefilled filledwith with The ionization The ionization potential of  13.6 V.. Yet to obtain to obtain discharge in a cathode a cathode ray  ray tube hydrogen, a very high high voltage ( ~10 ~ 10 V) has to has to be applied across the tube.  tube. Explain this clearly. Also be applied across the explain why explain why the gas must  obtain discharge. gas must be  be at low pressure low pressure to  to obtain

Q.12

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QUESTION  FOR SHORT  ATOMIC

ANSWER

PHYSICS

Q. l

In the photoelectric the photoelectric effect, why  the existence  of a cutoff  in favour of of the photon the photon effect, why does  does the  existence of  cu toff frequency speak  in favour  theory and theory and against  against the wave the wave theory?

Q. 2 

Explain the Explain the statement  statement that one's on e's eyes could not could not detect  detect faint starlight starlight if  light  light were we re not  particle-like.  not particle-like.

Q. 3

energy be given  given by How can a photon energy be  by E = h/when = h/when the very the very presence of the frequency/in the frequency/in the formula the formula implies that light is light is a wave? a wave?

Q. 4

p of a photon is  photon is given  given by c, the speed  speed oflight, does not does  not appear   appear  in The momentum The momentum p of  by p = h = hIX. Why i  Why  iss it that  it that c, the  in this expression?

Q. 5

Given that E that E = h/'for   Doppler shift in shift in frequency  frequency of  radiation from fro m a receding light source sou rce = h/'for  a photon,  a photon, the  the Doppler  of radiation  a receding light would seem to indicate a indicate a reduced  reduced energy for energy for the emitted photons. Is this  this in  fact true? If so, what the emitted photons. Is  in fact so, what happened energy principle? to the conservation the conservation of   of energy

Q. 6

 atomic hydrogen h ydrogen yet  what region of the spectrum? Any series Any series of   of  atomic  yet to be observed be observed will probably be found be found in  in what the spectrum?

Q.7

absorb a photon  photon whose energy exceeds its exceeds its binding  binding energy( 13.6 eV)? Can a hydrogen atom absorb a e V)?

Q. 8 

Only a Only a relatively  relatively small number  of  lines can be observed tubes, es, whereas  of  Balmer lines can be observed from laboratory discharge tub  stellar spectra. Explain this in this in terms  terms of th e small small densi density, ty, high temperature, a large number  are observed  are observed in  in stellar volume of  gases in ases in stellar  stellar atmospheres. and large and large volume of 

Q. 9 

Wnat is Wnat is the origin wavelength X  of figure figure shown? Why shown? Why is it an important clue to the photon the origin ofthe cutoff wavelength X an important clue to the photon nature of nature of x rays? mm

e u a < >u

30 40 50 60 70 80 90 Wavelength  Wavelength  (pm)

Q. 10

hydrogen be caused  caused to  emit x  rays? If so, describe  Ifnot, why  not? Can atomic Can atomic hydrogen be  to emit  x rays? so, describe how.  how. Ifnot,  why not?

Q. l l

 theory, which does not does not work  work very well even for  even for  helium  helium (Z = (Z = 2),  2), gives such a such a good  good Why is it that it that B ohr  theory, account of the characteristic that portion po rtion that originates originates deep the characteristic x-ray spectra ofthe elements, the elements, or  or at least at least of   of that within the atom? the atom? potential of  hydrogen is ydrogen is 13.6 V  tubefilled filledwith with The ionization The ionization potential of  13.6 V.. Yet to obtain to obtain discharge in a cathode a cathode ray  ray tube hydrogen, a very high high voltage ( ~10 ~ 10 V) has to has to be applied across the tube.  tube. Explain this clearly. Also be applied across the explain why explain why the gas must  obtain discharge. gas must be  be at low pressure low pressure to  to obtain

Q.12

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Q.13  Q.13  X-rays are X-rays are produced  produced when a when a fast  fast electron hits a hits  a proper  proper target. What happens to happens  to the electron? the electron? Q.14

Why does Why does the  the tail  tail of  of a comet always point away from the from  the sun?  sun?

Q.15

A neutron A neutron pion at rest at rest decays into two into two gamma  gamma photons. 7t° —-> y + y Why cannot Why cannot a  a singl  singlee photon phot on be  be born?  born? What conservation law conservation  law is in contradiction in contradiction with it? with  it?

Q.16  Q.16  What is What is so special so special about e/m about  e/m rather  rather than e end e  end m separately? m separately? Q.17 Q.1 7

Why is it advisable it advisable to  to view  view a  a TV screen TV screen from a from a distance  distance of   of  about ten  about ten feet?  feet?

Q. 18

The electrical The electrical conductivity of conductivity  of a gas increases gas increases when X-rays or  y-rays   y-rays pass through it. Explain this  phenomenon  phenomenon..

Q.19

In photoelectric In photoelectric emission exchange of  exchange of energy energy takes place among... (photon (phot on a  and nd electro  electron/' n/' photon, electron and lattice). and lattice).

Q.20

The thresh The threshold  for  photoemission for  are respectively v  v v v old frequencies frequenci es for   photoemission for  three  three metals numbered 1,2,3 numbered 1,2,3 are respectively and V and Vjj > v  > v  >  v . An incident  An incident radiation of  radiation  of  frequency v requency v  > v . . . cause photoemission from3 but... cause  photoemissionfrom1 (fill in (fill in the gaps the gaps with wit h may, may, may not / wil / willl certainly).  p

2

0

3

 NUCLEAR

3

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PHYSICS

Q. 1

Why does Why does the  the relative  relative importance of the Coulomb the Coulomb force compared to compared to the str the strong ong nuclear force for ce increase at large mass numbers?

Q.2

In your In your body, are body, are there  there more mor e neutrons than protons? More protons pro tons than electrons? Discuss Discuss

Q. 3

Why is the binding the binding energy per  figure)low low at low mass low mass numbers? At high At high mass numbers? energy  per  nucleon (see  nucleon (seefigure) Region of   greatest

r-^stability

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80 100 120 MO 161 161)) 180 200 220 24 0

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Mass number, A

Q.4

Aradioactive nucleus nucleus ca  can n emit a  emit a positron,  positron, e . This corresponds to a proton a proton in  in the nucleus the nucleus being converted to converted  to a neutron The neutron The mass  mass ofa neutron, neutron , however, i however,  iss greater than that o that  off a proton. How  proton. How thai  thai can positron can positron emission occur?

Q.5

In beta In beta decay the decay the emitted  emitted electrons form a form  a continuous  continuous spectrum, but spectrum,  but in alpha in alpha decay the alpha the  alpha particles  did d this cause in the explanation of   of beta how were form a form a discrete  discrete spectrum. What difficulties difficulties di cause in the explanation beta decay, decay, and how were these difficultiesfinally difficultiesfinallyovercome? overcome?

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Q.6

How do neutrinos differ fro m photons? Each has zero charge and (presumably) zero rest mass and travels at the speed oflight.

Q.7

In radioactive dating with U, how do you get around the fact that you do not know how much U was present in the rocks to begin with? (Hint: What is the ultimate decay product of  U?) 238

 2 3 8

 2 38

Q.8

If it is so much harder  to get a nucleon out of a nucleus than to get an electron out of an atom, why try?

Q.9

In the generalized equation for thefissiono f  U by thermal neutrons, U + n -> X + Y + bn, do you expect the Q of the reaction to depend on the identity of X and Y?

Q.10

The half-life of  U is 7.0 x 10 y. Discuss the assertion that ifit had turned out to be shorter  by a factor of 10 or  so, there would not be any atomic bombs today.

Q.ll

The binding energy curve of figuretells us that any nucleus more massive than A « 5 6 can release energy  by the fission process. Only very massive nuclides seem to do so,  however. Why cannot lead, for example, release energy by the fission process?

235

235

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Region of   greatest ^"stability Fission

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Bp

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2 3 9 f l c

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Pu

7 He 4

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80 1 00 120 140 160 180 200 220 240

Mass number, A

Q.12  Elements up to mass number  w 5 6 are created by thermonuclear fusion in the cores of  stars. Why are heavier elements not also created by this process? Q.13

Which would generate more radioactive waste products: - afissionreactor  or a fusion reactor?

Q. 14

How can Becquerel rays, i.e., the combination of a- , P- and y-rays, be separated?

Q.15  When a nucleus undergoes a-decay, is the product atom electrically neutral? In (3-decay? Q.16

Experimental results in radioactivity show small variations from the results predicted by theory. Explain this.

Q.17

If a nucleus emits only a y-rays photon, does its mass number change? Does its mass change?

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ONLY  ONE   OPTION  IS   CORRECT. Take approx. 2 minutes for   answering each question.

Q. 1 £ Q.2

£ Q.3

Let n   and n be respectively the number  of photons emitted by a red bulb and a blue bulb of  equal  power  in a given time. (B)n n (D) data insufficient (A)n = n   r 

 b

r 

 b

r 

 b

r 

 b

10~  W of   5000 A light is directed on a photoelectric cell. If the current in the cell is 0.16 pA, the  percentage of incident photons which produce photoelectrons, is (A) 0.4% (B) .04% (C) 20% (D) 10% 3

A proton and an electron are accelerated by same potential difference have de-Broglie wavelength X   p and A,. (A) X e = X  (B) < (C) X e > X (D) none of  these.  p e

 p

Q ,4

Two electrons are moving with the same speed v. One electron enters a region of uniform electric field while the other enters a region of  uniform magneticfield,then after sometime ifthe de-Broglie wavelengths of the two are X {  and X   2 , then: (A)

Q.5

= X  2 

(B)Aj > X  2

(C) X l  < X  2

(D) X  1 > X  2 or  X l  < X  2

In a photo-emissive cell, with exciting wavelength X, the maximum kinetic energy of  electron is K. If the  3X

exciting wavelength is changed to — the kinetic energy of the fastest emitted electron will be: (A) 3K/4

(B) 4K/3

(C) less than 4K/3

(D) greater than 4K/3

Q.6

If the frequency of  light in a photoelectric experiment is doubled, the stopping potential will (A) be doubled (B) halved (C) become more than doubled (D) become less than double

Q.7

An electron with initial kinetic energy of  100 eV is acceleration through a potential difference of 5 0 V  Now the de-Broglie wavelength of  electron becomes (B)VL5A

^

( A ) l A 

(C) V3 A

Q.8

If h is Planck's constant is SI system, the momentum of a photon of  wavelength 0.01 A is: (C)10 h (A) 10" h (B)h  ^(D) 10 h 2

£ Q. 9

(D) 12.27 A

12

2

The stopping potential for the photo electrons emitted from a metal surface of work function 1.7 eV is 10.4 V. Identify the energy levels corresponding to the transitions in hydrogen atom which will result in emission of wavelength equal to that of  incident radiation for the above photoelectric effect (C)n=2tol (A)n = 3 to 1 (B)n = 3 to 2  (D) n =  4 t o l

Q.10  When a photon of light collides with a metal surface, number  of  electrons, (if any) coming out is (A) only one (B) only two (C) infinite (D) depends upon factors

£ Q. 11

Two radioactive material Aj and ^ have decay constants of  10 X 0 and X 0. If initially they have same number  of  nuclei, the ratio of  number  of  their undecayed nuclei will be (1/e) after  a time  L

( ) r A

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Q.12

£

The frequency and the intensity of a beam oflig ht falling on the surface of photoelectric material are increased by a factor of two. This will: (A) increase the maximum energy of the photoelectrons, as well as photoelectric current by a factor  of two. (B) increase the maximum kinetic energy of the photo electrons and would increase the photoelectric current by a factor of two. (C) increase the maximum kinetic energy ofthe photoelectrons by a factor  of  greater than two and will have no effect on the magnitude of photoelectric current produced. (D) not produce any effect on the kinetic energy ofthe emitted electrons but will increase the photoelectric current by a factor of two.

QJo

Light comingfroma discharge tubefilledwith hydrogen falls on the cathode of the photoelectric cell. The work function of the surface of  cathode is 4eV Which one of the following values of the anode voltage (in Volts) with respect to the cathode will likely to make the photo current zero. (D )- 10 (A) - 4 ( B) -6 (C) - 8

Q. 14

A point source of  ligth is used in a photoelectric effect. If the source is removed farther fromthe emitting metal, the stopping potential: (A) will increase (B) will decrease (C) will remain constant (D) will either increase or   decrease.

QJ/5

A point source causes photoelectric effect from a small metal plate. Which of the following curves may represent the saturation photocurrent as a function of the distance between the source and the metal ?

(A) Q.16

(B)

(D)

Let Kj be the maximum kinetic energy of photoelectrons emitted by a light of wavelength A,  and K  corresponding to X . If  = 2"k , then:

2

2

2

(B) K, - 2K  

(A) 2Kj = K 

2

Q. 17

(C)

2

(C)K, 2K  2

In a photoelectric experiment, the potential difference V that must be maintained  betweenthe illuminated surface and the collector so as just to prevent any electron from reaching the collector is determined for  differentfrequenciesf of the incident illumination. The graph obtained is shown. The maximum kinetic energy ofthe electrons emitted atfrequency f, is Vi (^ ) ( f7fT3i i-f o)

(A) iff. M

v

( C ) h ( f   - f  ) 1

B

0

( D ) e V ( f  - f  ) 1

1

0

Q.18

Radiation oftwo photon energies twice andfivetimes the work function of  metal are incident sucessively on the metal surface. The ratio of the maximum velocity of photoelectrons emitted is the two cases will be (A) 1 :2 (B )2 . 1 (C) 1 4 (D )4 : 1

Q.19

Cut off  potentials for a metal in photoelectric effect for  light of wavelength X ,X  and X  is found to be Vj, V  and V  volts if  Vj, V  and V are inArithmetic Progression and A,,, X  and A  will be: (B) Geometric Progression (A) Arithmetic Progression (C) Harmonic Progression (D) None x

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Q. 20 C

Q.21

 p

Photons with energy 5 eV are incident on a cathode C, on a photoelectric cell. The maximum energy of the emitted photoelectrons is 2 eV. When photons of  energy 6 eV are incident on C, no photoelectrons will reach the anode A if the stopping potential ofA relative to C is (B)-3V (C)-1 V (A)3 V (D) 4 V In a photoelectric experiment, the collector plate is at 2.0V with respect to the emitter plate made of copper  cp -  4.5eV). The emitter is illuminated by a source of monochromatic light ofwavelength 200nm. (A) the minimum kinetic energy ofthe photoelectrons reaching the collector  is 0. (B) the maximum kinetic energy of the photoelectrons reaching the collector  is 3,7eV. (C) if the polarity of the battery is reversed then answer  to part A will be 0. (D) if the polarity of the battery is reversed then answer  to part B will be 1,7eV.

Q.22

By increasing the intensity of incident light keepingfrequency (v > v )fixedon the surface of  metal (A) kinetic energy of the photoelectrons increases (B) number  of emitted electrons increases (C) kinetic energy and number  of electrons increases (D) no effect

Q.23

In a photoelectric experiment, electrons are ejected from metals X and Y by light of  intensity I and frequency f. The potential difference V required to stop the electrons is measured for various frequencies. IfY has a greater work function than X; which one of the following graphs best illustrates the expected results? V V Vi V

0

X

X / /

<

f

(B)

Y/

(C) o

4 0

(D) •f

o

Q. 2,4 Monochromatic light with a frequency well above the cutoff frequency is incident on the emitter  in a  photoelectric effect apparatus. The frequency of the light is then doubled while the intensity is kept constant. How does this affect the photoelectric current? (A) The photoelectric current will increase. (B) The photoelectric current will decrease. (C),The photoelectric current will remain the same. (D) None of  these Q. 2 5

In a hypothetical system a particle of  mass m and charge - 3 q is moving around a very heavy particle having cahrge q. Assuming Bohr's model to be true to this system, the orbital velocity of  mass m when it is nearest to heavy particle is 3q

Q. 26 "

3q  

2

3q

2

3q

de-Broglie wavelength of an electron in the nth B ohr  orbit is \ and the angular momentum is J , then: n

(B) l n oc 7~

(A) J x n

(C) X n cc j

** rt

q s *  

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Question Bank on Modern Physics

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Q.27

The angular momentum  of an electron in the hydrogen atom is — .  Here h is Planck's constant. The 2tc

kinetic energy of this electron  is: (A)4.53 eV (B)1.51eV

(C)3. 4eV

(D)6.8eV

Q.28  Consider  the following electronic energy level diagram  of  H-atom: Photons associated with shortest  and longest wavelengths would be emitted from the  atom by the transitions labelled: (A) D  and C respectively (B) C  and A respectively (C) C  and D respectively (D) Aand C respectively Q.29

-  n = 4

D

-n = 3

B -n = 2 n

th

=j

n

n

n

n

Ifthe electron in a hydrogen atom were in the energy level with n= 3, how much energy in joule would  be required to ionise the atom? (Ionisation energy  of  H -atomis 2.18   10"" J): (A) 6.54 x 10 " (B) 1.43 x 10 " (C) 2.42 x 10~ (D) 3.14 10 " x

19

Q.31

C

In a hydrogen atom,  the binding energy of the electron in the n  state is E , then thefrquencyof r  evolutionof the electron in the nth orbits is: (A)2E /nh . (C)E /nh (D)E n/h (B) 2E n/h n

Q.30

- n = oo

A

19

18

19

x

20

In hydrogen and hydrogen like atoms,  the ratio of  difference of  energies E - E and E - E varies with its atomic number  z and n as: ( A ) z /n (C )z /n (D)z°n° (B) zVn 4 n

2

2

2 n

2 n

n

4

Q.32

In a hydrogen atom, the electron is in nth excited state. It may come down to second excited state  by .  emitting ten different wavelengths. What  is the value of n: (A) 6 (B) 7 (C) 8 (D) 5

Q.33

Difference between  nth and (n+1 )th Bohr's radius of'H' atom is equal to it's (n-1 )th Bohr's radius, the value ofnis: (A) 1 (B) 2 (C) 3 (D) 4

Q.34

An electron in hydrogen atom after absorbing energy photons  can jump between energy states n   and n (n, > nj). Then it may return to ground state after emitting six different wavelengths in emission spectrum. the energy of emitted photons is either equal to, less than  or  greater than the absorbed photons. Then  nj and n   are: (A) n  = 4, n = 3  (B)n =  5 , n j = 3  (C)n = 4, n, = 2 (D) n   = 4 , ^ = 1 t

2

|

2

2

}

2

2

2

Q.35

The electron in a hydrogen atom makes transitionfromM shell to L. The ratio of  magnitudes of  initial to final centripetal acceleration  of the electron is (B)81:16 (C )4 :9 (D)16:81 (A) 9: 4

Q.36

The electron in a hydrogen atom makes  a transition n, —> n  whose nj and n are the principal quantum numbers of the two  states. Assume the Bohr model to be valid. The frequency of  orbital motion of the electron in the initial state  is 1/27 of  that in thefinalstate. The  possible values  of n  and n  are (6)^=3, ^=1 (A) n = 4 , n = 2 (0)^ = 8,^=1 (0)^ = 6,^ = 3 2

2

t

2

t

Q.37

2

The radiu s of B ohr' sfirstorbit is a . The  electron in n  orbit has a radiu s: (B)a /n (C)n a   (D)a /n (A) na   th

0

0

(fe Bansal Classes

0

2

0

Question Bank on Modern Physics

0

2

8]

Q.38 ^

The ionisation potential of hydrogen atom is 13.6 volt. The energy required to remove an electron from the second orbit of  hydrogen is: (B )6.8e V (A) 3.4 eV (C)13 .6eV (D)27. 2eV

Q.39  Electron in a hydrogen atom is replaced by an identically charged particle muon with mass 207 times that of  electron. Now the radius of K  shell will be (A) 2.56 x 10~ A  (B) 109.7 A  (C) 1.21 x 10~  A   (D)22174.4A 3

Q.40

3

Monochromatic radiation of  wavelength X   is incident on ahydrogen sample containing in ground state. Hydrogen atoms absorb the light and subsequently emit radiations of ten different wavelengths. The value of X is (A) 95 nm (B)103nm (C) 73nm (D)8 8nm

Q.41  When a hydrogen atom, initially at rest emits, a photon resulting in transition n = 5 -> n = 1, its recoil speed is about (A) 10^ m/s (B) 2 x 10" m/s (C) 4.2 m/s (D) 3.8 x l( T   m/s 2

2

Q. 42

An electron collides with afixedhydrogen atom in its ground state. Hydrogen atom gets excited and the colliding electron loses all its kinetic energy. Consequently the hydrogen atom may emit a photon corresponding to the largest wavelength of the Balmer series. The min. K.E. of colliding electron will be (A) 10.2 eV (B) 1.9 eV (C)12. 1eV (D)13. 6eV

Q.43

Thefrequency of  revolution of  electron in n Bohr orbit is v . The graph between log n and log (v  / v,) may be th

n

n

Q. 44  Consider  the spectral line resulting from the transition n = 2 —» n = 1 in the atoms and ions given below. The shortest wavelength is produced by: (A) hydrogen atom (B) deuterium atom (C) singly ionized helium (D) doubly ionized lithium Q.45

In an atom, two electrons move around the nucleus in circular orbits of  radii R and 4R. The ratio of the time taken by them to complete one revolution is: (neglect electric interaction) (A) 1 : 4 (B) 4 : 1 (C) 1 : 8 (D) 8 : 1

Q.46

The electron in hydrogen atom in a sample is in n  excited state, then the number of  different spectrum lines obtained in its emission spectrum will be: (A) 1 + 2 + 3 + +(n - 1) (B) 1 + 2 + 3 + + (n) x (  _  l) (C) 1 + 2 + 3 + +(n +1) (D) 1 2 x 3 x lh

x

Q.47 2Mj 2

(C) M < 2Mj

(D) M, < 10(m + m )

2

n

 p

The decay constant of a radio active substance is 0.173 (years)" . Therefore : (A) Nearly 63% of the radioactive substance will decay in (1/0.173) year. (B) halflife of the radio active substance is (1/0.173) year. (C) one -forth of the radioactive substance will be left after nearly 8 years. (D) all the above statements are true.

 Bansal Classes

1

Question Bank  on Modern Physics

 [15]

 ANSWER KEY ONLY ONE OPTION IS CORRECT.

Q.6

C

Q.7

A

Q.13

D

Q.14

C

C

Q.20

B

Q.21

B

Q.26

A

Q.27

B

Q.28

C

A

Q.33

D

Q.34

C

Q.35

D

Q.39

A

Q.40

A

Q.41

C

Q.42

C

C

Q.46

B

Q.47

C

Q.48

A

Q.49

A

Q.52

C

Q.53

B

Q.54

A

Q.55

B

Q.56

B

B

Q.59

D

Q.60

B

Q.61

B

Q.62

C

Q.63

C

Q.65

C

Q.66

D

Q.67

A

Q.68

B

Q.69

B

Q.70

D

Q.72

C

Q.73

C

Q.74  A

Q.75

C

Q.76

B

Q.77

A

Qi

C

Q.2

B

Q.3

C

Q4

D

Q.5

Q.8

D

Q.9

A

Q.10

A

Q. ll

B

Q.12

Q.15

D

Q.16

C

Q.17

C

Q.18

A

Q.19

Q.22

B

Q.23

A

Q.24

B

Q.25

A

Q.29

A

Q.30

C

Q.31

D

Q.32

Q.36

B

Q.37

C

Q.38

A

Q.43

C

Q.44

D

Q.45

Q.50

B

Q.51

C

Q.57

A

Q.58

Q.64

B

Q.71

D

Q.78

C

D

ONE OR MORE THAN ONE OPTION MAYBE

CORRECT

Q.l

A,C

Q2

B

Q.3

B

Q4

AC,D

Q.5

A,C,D

Q.6

A

Q.7

B

Q.8

A

Q.9

AB

Q.10

A,C

Q.ll

AB,D

Q.12

B

Q.13

A,D

Q.14

A,C

Q.15

C,D

Q.16

A,C

TARGET IIT JEE   2007

XII (ALL)

 MODERN PHYSICS

CONTENTS

 KEYCONCEPTS  EXERCISE-I  EXERCISE-II  EXERCISE-III  ANSWER  KEY

KEY

CONCEPTS

(b)

CATHODE RAYS : Generated in a discharge tube in which a high vaccum is maintained . They are electrons accelerated by high p.d. (l Ot o 15 K.V.)

(c)

1 K.E. of C.R. particle accelerated by a p.d. V is — mv'

l. (a)

= eV.

2m

Can be deflected by Electric & magnetic fields . red(7.6xl0~ m) *—

vioIet(3.6*l(r m)

7

ELECTROMAGNETIC SPECTRUM Ordered arrangement of the big family of electro magnetic waves (EMW) either in ascending order of frequencies or ofwave lengths Speed ofE.M.W. in vacuum C = 3 x 10 m/s = v X

2.

3*10 m

3m

4

7

3xlO" m

3xl0^m

l2

infrared

Ultra viol et Ga mma rays

Radio waves

8

II

PLANK  S  QUANTUM THEORY : A beam ofEMW is a stream of discrete  packets of energy called  PHOTONS , each photon having afrequencyv and energy = E = h v .

3.

Micro waves (e.g. radar)

10 10 10 4

6

10 10 10

s

10

12

X-rays

\

Visible light

14

10

16

I0 1 0 ° i8

2

Frequency (Hz)

h = plank's constant = 6.63 x 10"   Js . 34

4.

(0 (ii)

PHOTO ELECTRIC EFFECT : The phenomenon of the emission of electrons , when metals are  exposed to light (of a certain minimum frequency) is called photo electric effect. Results : Can be explained only on the basis of the quantum theory (concept of photon). Electrons are emitted  ifthe incident light hasfrequency v > v   (thresholdfrequency)emission of electrons is independent ofintensity. The wave length corresponding to v is called threshold wave length 0 . v is different for different metals .  Number of electrons emitted per second depends on the intensity of the incident light . Q

0

(iii) (iv) (v)

 X 

0

EINSTEINS

PHOTO

ELECTRIC

Photon energy = K.

EQUATION

:

E. of electron + work function

.

2 , h v = —  mv +  = h v 0

(vi)

STOPPING POTENTIAL O R C UT O F F PO TENTIAL

:

The minimum value of the retarding potential to prevent electron emission is : cutofr = ( K E)  Note: The number of photons incident on a surface per unit time is called photon flux. eV

ma x

5.

WAVE NATURE OF MATTER : Beams of electrons and other forms ofmatter exhibit wave properties including interference and diffraction

with a de Broglie wave length given by

X = — P

(wave length of a praticle) .

 _  Y Z

2

> P+

+ a   + Energy

A _ 4

Z + 1

2

4

Y + v (antinuetrino) A

: The disintegration is a random phenomenon. Whcih atom disintegrates first is  purely a matter of chance .  Number of nuclei disintegrating per second is given ; (disintegration /s /gm is called specific activity) . STASTISTICAL LAW

0

(i)

dN dN , — a N —>—=-A,N = activity . dt dt XT

X T

Where N = No. of nuclei present at time t (ii)

N = N e~ o

X  _ ! + a + b (B) X +in

-*

(C) x

-> X

 A

A

Q.37

 A

Z

z

—> x +/ A

z

A

Z

(

D)

A

z

0

Xv 2 +

e_!

z

The volume and mass of a nucleus are related as (A) v qc m (B) v cc 1/m (C) v cc m

2

[JEE 2002 (Scr)] A A

3

- XZ7 -2 „ + C

i+8

[JEE 2003 (Scr)] (D) v oc 1/m 2

Q.38

The nucleus of  element X (A= 220)  undergoes a-decay. If  Q-value of the reaction is 5.5 MeV, then the kinetic energy of  a -particle is :   [JEE 2003 (Scr)] (B)10.8 MeV (C)2 .7Me V (D)None (A) 5.4MeV

Q.39

A radioactive sample emits n P-particles in 2 sec. In next 2 sec it emits 0.75 n  P-particles, what is the mean life of th e sample?  [JEE 2003 ]

 and