Induced potential and transformers
The generator effect (Higher tier only) (Separate Physics only)
If a wire moves through a magnetic field, or the magnetic field around a wire changes, a is induced across the ends of the wire. If the wire is part of a complete circuit, a flows. This is the .
You can increase the induced pd by:
- moving the wire or magnet faster
- using a stronger magnet
- using a coil with more turns
Reversing the movement, or the magnet, reverses the pd.
The induced current makes its own magnetic field, which opposes the change that caused it.
Key terms
Alternators, dynamos and microphones (Higher tier only) (Separate Physics only)
Generators rotate a coil in a magnetic field.
- An makes . It is connected to the circuit through . A graph of its pd against time is a wave, going positive then negative.
- A makes . It uses a , which swaps the connections every half turn. Its graph goes up and down in humps but stays positive.
Turning either one faster makes the peaks higher and the waves come closer together.
A uses the generator effect. Sound waves make a thin diaphragm vibrate. This moves a coil in a magnetic field, inducing a current that changes in the same pattern as the sound.
Key terms
Transformers (Higher tier only) (Separate Physics only)
A has a and a wound on the same . Iron is used because it is easily magnetised.
- An in the primary coil makes a in the iron core.
- This changing field induces an alternating pd across the secondary coil.
Transformers only work with ac: a steady direct current wouldn't make a changing field.
- A transformer has more turns on the secondary coil, so it increases the pd.
- A transformer has fewer turns on the secondary coil, so it decreases the pd.
Key terms
Transformer equations (Higher tier only) (Separate Physics only)
Both equations are on the equation sheet. Vₚ and nₚ are the pd and number of turns on the primary coil; Vₛ and nₛ are for the secondary.
Vₚ/Vₛ = nₚ/nₛ
Example 1: a transformer has 2000 turns on its primary and 100 on its secondary. The primary pd is 230 V.
- Vₛ = Vₚ × nₛ ÷ nₚ = 230 × 100 ÷ 2000
- Vₛ = 11.5 V (a step-down transformer)
If a transformer is 100% efficient, the power in equals the power out:
VₚIₚ = VₛIₛ
Example 2: at a power station, 25 000 V and 40 A are stepped up to 400 000 V.
- Iₛ = VₚIₚ ÷ Vₛ = 25 000 × 40 ÷ 400 000
- Iₛ = 2.5 A
This is why the uses step-up transformers: a high pd means a , so far less energy is wasted heating the cables.