Electromagnetic induction
Electromagnetic induction can be demonstrated using the apparatus shown below:
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The direction of the induced current is reversed when the magnet is moved out of the coil again.
It can also be reversed if the other pole of the magnet is moved into the coil.
A current can also be induced if:
- the magnet is held stationary and the coil is moved or,
- the magnet is rotated close to the coil.
Notice that no current is induced when the magnet is not moving, even if it is inside the coil.
If the magnet and the coil are held together and both are moved together no current is induced.
We say that a current is induced when the magnet moves relative to the coil.
Key points
- A current is induced in a wire when it moves relative to a magnet.
- The direction of the current can be reversed by reversing the direction the magnet moves or by turning the magnet around and reversing the direction of the magnetic field.
Summary of observations
| Direction of movement of magnet | Needle of ammeter | Current is induced? Yes or No |
| Magnet is stationary outside of coil | Remains at zero | No |
| South pole moves into coil | Deflects in one direction and then returns to zero | Yes, while the magnet is moving |
| Magnet is stationary inside of coil | Remains at zero | No |
| South pole moves out of coil | Deflects in the opposite direction and then returns to zero | Yes, while the magnet is moving |
| Direction of movement of magnet | Magnet is stationary outside of coil |
|---|---|
| Needle of ammeter | Remains at zero |
| Current is induced? Yes or No | No |
| Direction of movement of magnet | South pole moves into coil |
|---|---|
| Needle of ammeter | Deflects in one direction and then returns to zero |
| Current is induced? Yes or No | Yes, while the magnet is moving |
| Direction of movement of magnet | Magnet is stationary inside of coil |
|---|---|
| Needle of ammeter | Remains at zero |
| Current is induced? Yes or No | No |
| Direction of movement of magnet | South pole moves out of coil |
|---|---|
| Needle of ammeter | Deflects in the opposite direction and then returns to zero |
| Current is induced? Yes or No | Yes, while the magnet is moving |
Key points
The size of the induced current can be increased by:
- moving the magnet faster into and out of the coil;
- adding more turns to the coil;
- increasing the strength of the magnet.
The current induced when a magnet moves into and out of a coil is an alternating current.
The process is the basis of electricity generation in power stations and generators.