The TransformerCambridge IGCSE Physics: Revision notes
Section 1
What Is a Transformer Made Of?
A simple transformer consists of two separate coils of insulated wire, called the primary coil and the secondary coil, wound around a common soft iron core. The two coils are not directly connected to each other electrically — energy passes between them only through the changing magnetic field in the core.
Section 2
How Does a Transformer Work?
An alternating current in the primary coil creates a continuously changing magnetic field in the soft iron core. Because this magnetic field is changing, it links with the secondary coil and, by electromagnetic induction, induces an alternating e.m.f. in the secondary coil. A transformer will only work with alternating current — a steady direct current produces a constant (non-changing) magnetic field, which cannot induce an e.m.f.
A transformer does not work with direct current, because a constant current gives a constant magnetic field that cannot induce an e.m.f. in the secondary coil.
Section 3
Step-Up or Step-Down?
A transformer is called a step-up transformer if the secondary voltage is greater than the primary voltage, and a step-down transformer if the secondary voltage is smaller than the primary voltage. This depends on the ratio of the number of turns on each coil.
The voltages and turns are related by:
Vp / Vs = Np / Ns
where Vp and Np are the voltage and number of turns on the primary coil, and Vs and Ns are the voltage and number of turns on the secondary coil.
If the primary coil has 100 turns and the secondary coil has 1000 turns, the secondary voltage will be ten times the primary voltage — a step-up transformer.
Section 4
Transformers and Power: Efficiency
In an ideal transformer with 100% efficiency, no energy is wasted, so the electrical power delivered to the primary coil equals the electrical power delivered by the secondary coil:
Ip × Vp = Is × Vs
where Ip and Is are the primary and secondary currents. This means that when a transformer steps voltage up, it steps current down by the same factor, and vice versa.
A step-up transformer increases voltage but decreases current — total power stays the same for an ideal transformer.
Section 5
Why Transmit Electricity at High Voltage?
Step-up transformers are used at power stations to transmit electricity through the National Grid at very high voltages. The power lost as heat in the transmission cables is given by:
P = I² × R
where I is the current in the cables and R is their resistance. Because power loss depends on the square of the current, transmitting electricity at a high voltage (and therefore low current, for the same power delivered) greatly reduces the power wasted as heat in the cables. Step-down transformers are then used near where the electricity is needed to reduce the voltage to a safer level for consumers.
Doubling the voltage (and halving the current for the same power) cuts cable power losses to a quarter, since power loss depends on current squared.
Must Know
- Transformers only work with alternating current, since they rely on a changing magnetic field
- A soft iron core links the primary and secondary coils magnetically without any direct electrical connection
- Vp / Vs = Np / Ns links voltage to the turns ratio
- Step-up transformers have more secondary turns than primary; step-down have fewer
- For 100% efficiency, Ip × Vp = Is × Vs, so a step-up in voltage means a step-down in current
- High-voltage transmission reduces current, which cuts power losses (P = I²R) in cables
That's the notes covered.
Carry on to the next subtopic.