Electromagnetic induction
Making electricity.
An electric current is produced when a magnet is moved into a coil of wire in a circuit. We say that the electric current has been induced, and the process is called induction. 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.
In creasing the induced current:
- Move the magnet faster
- Use a stronger magnet
- Increase the number of turns on the coil
- Increase the area of the coil
Generators
Generators induce a current by spinning a coil of wire inside a magnetic field, or by spinning a magnet inside a coil of wire. As this happens, a potential difference is produces, which causes a current to flow.
Ex. The bicycle dynamo (A type of electrical generator) has a wheel that touches the back tire. As it moves, the wheel turns a magnet inside a coil. This induces enough electricity to run the bicycle’s lights.
Making ac electricity
As coil is rotated in a magnetic field, the induced current reverses direction every have turn.
In an ac generator, each end of coil is attached to slip rings. They make electrical contact with the coil and rotate with it. Electrical contact with the external circuit is achieved using conducting brushes. These do not move, but they are in continuous contact with the rotating slip rings.
Transformers
A transformer is an electrical device that changes the voltage of an ac supply.
- A transformer that increases the voltage is called a step-up transformer
- A transformer that decreases the voltage is called a step-down transformer.
- Step down transformers are used in mains adapters and rechargers for mobile phones and CD players
- Transformers do not work with dc supplies.
Transformers need an alternating current that will create a changing magnetic field. A changing magnetic field also induces a changing current in coil.
- The primary coil is connected to an ac supply
- An alternating current passes through a primary coil wrapped around a soft iron core
- The changing current produces a changing magnetic field
- This induces an ac in the secondary coil.
There is no electrical connection between the primary and the secondary coils
Transformers only work if ac is supplied to the primary coil. If dc was applied, there would be no current in the secondary coil.
Voltage across primary/ voltage across secondary= number of turns on primary/ number of turns on secondary.
Vp/Vs=Np/NS