Transformers Revision Notes for OCR GCSE Physics A (Gateway Science Suite)

7.3.5 Transformers

infoNote

A transformer is an electrical device that changes the potential difference (voltage) of an alternating current supply.

infoNote

Key Points

infoNote

A basic transformer consists of two coils of wire, which are not connected. These coils are wrapped around an iron core.

infoNote

Process: 1. An alternating current is supplied to the primary coil.

2. The current flows through the primary coil and produces a magnetic field.

Since the current keeps changing direction, the magnetic field does as well 3. The iron core is easily magnetised, so the changing magnetic field passes through it.

4. As a result, the changing magnetic field induces a potential difference in the secondary coil.

The magnetic field is constantly changing, so the potential difference induced will be alternating. 5. If the secondary coil is part of a complete circuit, then alternating current will flow.

infoNote

Transformers do not work with direct current (DC) as the magnetic field produced by DC does not change.

Transformers are used:

To increase the potential difference (voltage) of an alternating current supply, before it is transmitted across the National Grid.
To lower the high voltage of electricity used in power lines to levels suitable for use in homes.

AC in the first coil creates a changing magnetic field
This changing magnetic field cuts through the secondary coil
Induces a current in the secondary coil
- Which is also AC
- If primary current was DC, the magnetic field it produces would be constant, not inducing anything in the secondary coil
More coils on secondary: Step up transformer
- Voltage will be increased, as the changing field will cut through more of the secondary wire inducing a larger pd
Fewer coils on secondary: Step down transformer
- Smaller pd forms on secondary

\dfrac{\text{number of coils on primary}}{\text{number of coils on secondary}} = \dfrac{\text{pd of primary}}{\text{pd of secondary}}

This only works with current too if the transformer is 100% efficient. Unless it states this, assume not and just use this to find voltage