Electromagnetism

Unit 21

Electric Electric motors motors are are machines machines that that use use magnetism magnetism and and electricity electricity to to make make things things move. move. We use d.c. motors to drive electric trains. We use d.c. motors to drive electric trains. The The electric current is supplied to the train from electric current is supplied to the train from overhead overhead wires wires or or from from the the rails rails below. below.

Electromagnetism

contents

ƒ Force on a Current-carrying Conductor ƒ D.C. Motors

ƒ http://www.youtube.com/watch?v=HQdLFEiVeCA

Unit 21.1: Magnetic Effect of a Current

Demonstrating the magnetic effect of a current—Oersted’s Experiment • When the circuit is closed, a compass A placed above the wire XY would point to the East. Another compass B is placed below the wire would point to the West. • A current-carrying conductor produces a magnetic field around it.

Fig. 21.4 Oersted’s experiment

A Straight Wire

1. A straight wire carrying a current produces circular lines of force.

What happens when the direction of current is reversed? The direction of the magnetic field will also be reversed!

A Straight Wire

The strength of the magnetic field in a straight wire is stronger when • (the circular lines of force are closer) i.e. nearer the wire

• a larger current flows through it.

Direction of arrow Direction of current or magnetic field

current is directed out of paper (point of arrow)

current is directed into paper (tail of arrow) Current-carrying wire is perpendicular to the plane of board.

A Flat Circular Coil

A flat coil carrying a current produces circular lines of force around the wires and almost parallel lines of force in the middle.

Unit 21.1: Magnetic Effect of a Current Test Yourself 21.1 1.

A current flows in a long straight wire in the direction shown in Figure 21.17. Draw, in the diagram, the pattern and direction of the magnetic field produced.

Answer:

Unit 21.1: Magnetic Effect of a Current

Test Yourself 21.1 2.

(a) Draw the magnetic field lines around a current-carrying solenoid. (b) Name three ways to increase the magnetic field strength of a solenoid.

Answer: (a)

Fig 21.9 pp 418

(b) 3 ways to increase magnetic field of solenoid: • Increase the no. of turns per unit length of the solenoid, • Increase the magnitude of the current • Place a soft iron core in the solenoid.

force on a current-carrying conductor 2. Current-carrying conductor The setup investigates the interaction between a current and a magnetic field. soft iron c-core

thick bare wire

2V power pack or lead-acid accumulator powerful magnadur magnet

force on a current-carrying conductor 3. Fleming’s left-hand rule The direction of the force can be deduced by using this rule. Motion Motion (thumb) (thumb)

Field Field (first (first finger) finger)

Current Current (second (second finger) finger)

the fingers are at right angles to one another

force on a current-carrying conductor Fleming’s left-hand rule To explain the force exerted on the wire, consider the combined magnetic fields due to the current flowing through the straight wire and the magnets.

N S

magnetic field between two magnadur magnets

magnetic field due to the current in the wire

force on a current-carrying conductor Fleming’s left-hand rule The two fields acting in the same direction combine to give a stronger field, but the two fields opposing each other combine to give a weaker field. The unbalanced fields on both sides exert produce a force that exerts on the wire.

combined magnetic field

Further explanation Unit 21.2: Force on Current-carrying Conductors Worked Example 21.1 Figure 21.20(a) shows a wire placed between two magnetic poles. (a) If the current in the wire flows from A to B, in which direction does a force act on the wire? (b) What will happen if the current flows from B to A instead?

Fig. 21.20(a)

Unit 21.2: Force on Current-carrying Conductors Worked Example 21.1 – Solution (a) By using Fleming’s Left-Hand Rule, we find that the force acts vertically downward on the wire AB (Figure 21.20(b)). (b) If the current flows from B to A, the force reverses in direction and acts vertically upward.

Fig. 21.20(b)

Unit 21.2: Force on Current-carrying Conductors Why does a current-carrying conductor experience a force when placed in a magnetic field?

Fig. 21.21(a) & (b) Separate magnetic fields of a current flowing through a wire and of two magnetic poles

Unit 21.2: Force on Current-carrying Conductors Why does a current-carrying conductor experience a force when placed in a magnetic field?

Fig. 21.21(c) Superimposed field patterns of (a) and (b)

Unit 21.2: Force on Current-carrying Conductors Why does a current-carrying conductor experience a force when placed in a magnetic field?

Fig. 21.21(d) Combined magnetic field when the wire is placed between the poles of the magnet

Unit 21.2: Force on Current-carrying Conductors Why does a current-carrying conductor experience a force when placed in a magnetic field? From Fig. 21.21(d), we can see that there is a stronger field on one side of the wire at A, since all the magnetic field lines are in the same direction. At B, the combined field is weaker due to opposing magnetic field lines. A force then acts on the wire from the stronger field to the weaker field.

Fig 21.21(d) Combined magnetic field when the wire is placed between the poles of the magnet.

force on a current-carrying conductor 4. Force on a beam of charged particles

Fleming’s left hand rule can be applied to all moving charges. The conventional current travels in an opposite direction to that of the electron flow.

motion

field (magnetic)

Conventional current flow

Electron current flow

force on a current-carrying conductor force on a beam of charged particles

motion (force) direction of positively charged particle before entering the magnetic field

positively charged particle

magnetic field into paper

x x x x

x x x x

x x x x

path of positively charged particle (part of a circle)

x x x x x

x x x x

current

force on a current-carrying conductor force on a beam of charged particles direction of electron or negatively charged particle before entering the magnetic field

electron or negatively charged particle

current magnetic field into paper

x x x x

x x x x

x x x x

x

motion (force)

x x x x

path of electron or negatively charged particle (part of a circle)

x x x x

Unit 21.2: Force on Current-carrying Conductors Force on a moving charge in a magnetic field

Fig. 21.22(a) A positively charged particle in a magnetic field is deflected upwards in a circular path.

• When a beam of positive charges enter the magnetic field region, it is deflected upwards in a circular path as the moving charges experience a force perpendicular to its direction of motion. • The direction of the force can be predicted by Fleming’s Left-hand rule.

Unit 21.2: Force on Current-carrying Conductors Forces between two parallel current-carrying wires • Currents in opposing directions cause repulsion.

Fig. 21.24 Combined magnetic field due to currents in the opposite direction

Unit 21.2: Force on Current-carrying Conductors Forces between two parallel current-carrying wires • Currents in similar directions cause attraction.

Fig. 21.25 Combined magnetic field due to currents in the same direction.

d.c. motors 5. Turning effect on a current carrying coil A currentcarrying coil placed in a magnetic field of a horseshoe magnet experiences a turning effect.

d.c. motors turning effect on a current carrying coil A catapult field is produced when the field produced by the coil superimposes on the field of the horseshoe magnet.

d.c. motors turning effect on a current carrying coil The turning effect can be increased by ƒ increasing the number of turns on the coil ƒ increasing the magnitude of the current ƒ inserting a soft iron core within the coil to concentrate the magnetic lines of force

d.c. motors principles of a d.c. motor ƒ make use of the turning effect of a current-carrying coil in magnetic field field to to convert convert electrical energy to mechanical (kinetic) energy ƒ works on direct current ƒ are the basic components in electric fans, hair dryers and many other electrical appliances

an opened up d.c. motor

d.c. motors principles of a d.c. motor a.

when the circuit is closed, current flows from the battery through P and X, through the coil and back to the battery through Y and Q

ƒ using Fleming’s left-hand rule, the left side of the coil experiences a downward force and the righthand side experiences an equal upward force

d.c. motors principles of a d.c. motor b.

this pair of forces causes the coil to rotate anticlockwise until it reaches a vertical position

ƒ at this point, current is cut off because neither X nor Y is in contact with P or Q

d.c. motors principles of a d.c. motor (c)

momentum of the coil carries it slightly beyond this vertical position

ƒ half-ring Y will then touch P while X comes into contact with Q ƒ turning forces act again and coil continues to rotate in the the same direction

d.c. motors principles of a d.c. motor If a soft iron cylinder is placed between the curved poles of the magnet in a motor: ƒ a radial field will be created ƒ radial field keeps the pair of forces acting on the coil almost constant as it turns ƒ this arrangement increases the magnetic field strength and thus increases the turning effect for a given current in the coil

Unit 21.3: Force on a Current-carrying Rectangular Coil in a Magnetic Field How does a d.c motor work? – When current flows through the coil ABCD, using Fleming’s left-hand rule, a downward force will act on side AB, and an upward force on side CD. – The coil thus rotates anticlockwise about axis PQ until it reaches a vertical position. – Here, the current is cut off because X and Y are both not in contact with the carbon brushes – The turning effect of the coil, however, carries it past the vertical position. – This reverses the current direction in the wire arm CD and now a downward force acts on it. – Similarly, an upward force acts on wire arm AB. – Hence, the coil continues to rotate in the anticlockwise direction.

Unit 21.3: Force on a Current-carrying Rectangular Coil in a Magnetic Field How does a d.c motor work? • The purpose of the split-ring commutator is to reverse the direction of the current in the coil every half a revolution to ensure that the coil will always turn in one direction. • To increase the turning effect of the coil, we can: 1. Insert a soft iron core or cylinder into the coil to concentrate the magnetic field lines. 2. Increase the number of turns in the coil 3. Increase the current

Unit 21.3: Force on a Current-carrying Rectangular Coil in a Magnetic Field Key Ideas 1. The d.c. motor works on the principle that a current-carrying coil in a magnetic field experiences a turning effect. 2. The function of a split-ring commutator is to reverse the direction of current in the coil when the coil passes the vertical position so that it continues to turn in the same direction. 3. The turning effect on the coil can be increased by (a) increasing the current in the coil (b) having more turns on the coil, or (c) inserting a soft iron core or cylinder into the coil.

Unit 21.3: Force on a Current-carrying Rectangular Coil in a Magnetic Field Test Yourself 21.3 1. In the d.c. motor, what change(s) must be made so that the coil rotates clockwise instead of anti-clockwise? Answer: To change the direction of rotation to turn clockwise, we can do one of the following: • reverse the poles of the magnets, or • reverse the direction of the current, by switching the terminals of the battery

Unit 21.3: Force on a Current-carrying Rectangular Coil in a Magnetic Field Test Yourself 21.3 2. Explain the purpose of the rheostat in the d.c. motor. Answer: The resistance of the rheostat is varied so that the current flowing in the coil can be controlled. By lowering the resistance, the current will increase and the turning force on the coil will increase. This results in an increased speed of rotation.

Unit 21.3: Force on a Current-carrying Rectangular Coil in a Magnetic Field Test Yourself 21.3 3. State the energy conversion that takes place in the d.c. motor. Answer: Electrical energy to mechanical energy.

Purpose of the split ring commutator • To reverse the direction of the current in the coil every half a revolution whenever the commutator changes contact from one brush to another. This is to ensure that the current continue to flow in the same direction in the coil. Q: How will the p.d. against time graph look like?

Electromagnetism is shown by

Fleming’s Left hand rule

helps to determine the direction of Force on a current-

carrying conductor in a magnetic field results in

Turning effect is increased by increasing (a) number of turns (b) current

Turning effect on a current-carrying coil is the basis of

Electric Electric motor motor

Force on a beam of charged particles in a magnetic field