Transformers and Their Operation — AQA GCSE Physics (Separate)
Transformers change the size of an alternating potential difference and are essential to the National Grid.
How a transformer works
A transformer has two coils of insulated wire — the primary and secondary — wound on an iron core.
- An alternating current in the primary coil creates a changing magnetic field in the iron core.
- This changing magnetic field passes through the secondary coil and induces an alternating potential difference in it (electromagnetic induction).
Transformers only work with alternating current, because a changing magnetic field is needed.
Step-up and step-down
- A step-up transformer has more turns on the secondary coil and increases the voltage.
- A step-down transformer has fewer turns on the secondary coil and decreases the voltage.
The transformer equation
$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$
- V = potential difference, N = number of turns, p = primary, s = secondary.
For an ideal (100% efficient) transformer, power in = power out: $$V_p \times I_p = V_s \times I_s$$
Use in the National Grid
Step-up transformers raise the voltage for efficient transmission (low current → less energy wasted as heat); step-down transformers lower it for safe use.
Exam tips
- A transformer needs a.c. to create a changing magnetic field.
- Step-up = more secondary turns (↑ voltage); step-down = fewer (↓ voltage).
- Learn the turns ratio equation and the ideal-transformer power equation.
- High voltage in the grid = lower current = less wasted energy.