O Level Physics Chapter :22: Electric Motor and Electromagnetic Induction

Prepared By: Shakil Raiman

22.1: Movement from Electricity: If a current passed through

a piece of wire held at right angles to the magnetic field of a magnet the wire will move. This motion is the result of a force created by overlapping magnetic fields around the wire and the magnet.

22.2: Motor Effect: When a current flows along a wire a cylindrical magnetic field is

created around the wire. If the wire is placed between the poles of a magnet, the two fields overlap.

In certain places, the fields are in the same direction and so reinforces each other, producing strong magnetic field. In other places, the fields are in opposite directions, producing a weaker field.

The wire experiences a force, pushing it from the stronger part of the field to the weaker part. This is called motor effect.

22.2: Motor Effect: If the direction of the current or the

direction of the magnetic field is reversed, the wire experiences a force in the opposite direction. We can predict the direction of the motion of the wire by using Fleming’s left-hand rule.

How to Increase the force: by increasing the current by using stronger magnet

22.3: Use of Motor Effect:Moving Coil Loudspeaker: Signals from a source, such as an

amplifier, are fed into the coil of the speaker as currents that are continually changing in size and direction. This produces varying magnetic field around the coil which interacts with the magnetic field of the permanent magnet. This causes rapidly varying forces on the wires of the coil, which causes the speaker cone to vibrate. These vibrations create the sound waves we hear.

22.3: Use of Motor Effect:Electric Motor: Motor converts electrical energy to kinetic

energy. As current passes around the loop of wire,

one side of it will experience a force pushing it upwards. The other side will feel a force pushing it downward, so the loop will rotate.

Current following through the loop changes direction at every half turn which ensures the continuous rotation.

How to increase the rotation or force: Increase the number of turns or loops of wire Increase the strength of the magnetic field Increase the current flowing through the loop of wire

22.3: Use of Motor Effect:Difference of Practical Electric Motor from the previous

one: The permanent magnets are replaced with curved

electromagnets capable of producing strong magnetic field for longer time which increases the rotational effect.

The single loop is replaced with several coils of wire wrapped on the same axis. This makes the motor more powerful and allows it to turn more smoothly.

The coils are wrapped on a laminated soft iron core which causes more field passes through the coil. This makes the motor more efficient and more powerful.

22.4: Electromagnetic Induction:

Electric Motor: If we move a wire across a magnetic field

at right angles, a voltage is induced or generated in the wire. If the wire is part of a complete circuit, a current flows. This Phenomenon is called electromagnetic induction.

The size of the induced voltage and current can be increased by: moving the wire more quickly using a stronger magnet wrapping the wire into a coil so that more pieces of

wire move through the magnetic field.

22.5: Faraday’s Law:Faraday’s Law of Electromagnetic

Induction: The size of the induced voltage across the

ends of a wire (coil) is directly proportional to the rate at which the magnetic lines of flux are being cut.

22.6: Use of Electromagnetic Induction:Electric Generator: Generator converts kinetic energy to

electrical energy. As the coil rotates, its wires cut through the

magnetic field lines. So, there is rate of change of flux linkage which produces induced current in the coil.

In the figure the wire of the coil moves up through the field and moves down through the field for each turn. As a result, the current induced in the coil flows first in one direction and then in the opposite direction.

This kind of current is called alternating current. A generator produces alternating current is called an alternator.

22.6: Use of Electromagnetic Induction:Electric Generator: A generator which produces direct

current is called a dynamo.

22.6: Use of Electromagnetic Induction:Electric Transformer: When alternating current is passed

through primary coil, the magnetic field around it is continuously changes. This changing magnetic filled is passed towards the secondary coil by iron core.

The secondary coil cuts the field lines. So, there is rate of change of flux cut

by secondary coil. So, there is induced voltage in the secondary coil.

The relationship between voltage and no of turns is shown on right side:

22.6: Use of Electromagnetic Induction:Electric Transformer: There are two kinds of transformer. Step-up transformer to

make voltage high and step down transformer to make voltage low.

The relationship between voltage and no of turns is :

If a transformer is 100% efficient:P

in=P

out

VP×I

P=V

s×I

s

22.7: National Grid or Transmission Line: After the generator or before transmission line step-up transformer is used to make the

voltage high. In a step up transformer the no of turns in the secondary coil is more. After the transmission line or before household and industry step-down transformer is used

to make the voltage low. In a step-down transformer the no of turns in the secondary coil is less than primary coil.

22.8: Why High Voltage: At transmission line power is lost as heat energy as

P=I2R If voltage is increased, current will decrease. So, power loss will be decreased. More efficient. Less current needs thin wire, which cost less. So, building transmission line becomes cheaper.

Thank You All

Wish you all very good luck and excellent result.