Electricity generated at a power station would waste most of its energy as heat before reaching your home if it were transmitted at low voltage. Transformers solve this by stepping voltage up for long-distance transmission — where high voltage means low current and therefore low energy losses — then stepping it back down to safe levels for domestic use.
How does a transformer work?
A transformer transfers electrical energy between two circuits using electromagnetic induction. It works only with alternating current (AC) because the changing current produces a continuously changing magnetic field.
Structure
A basic transformer has:
- A primary coil (input) of wire wound around an iron core.
- A secondary coil (output) of wire wound on the same iron core.
- A soft iron core that concentrates and transfers the changing magnetic flux between the coils.
Mechanism
- Alternating current in the primary coil creates a constantly changing magnetic field in the iron core.
- This changing magnetic field passes through the secondary coil.
- The changing flux induces an alternating voltage in the secondary coil (Faraday's law of electromagnetic induction).
- No electrical connection exists between the coils — energy is transferred via the magnetic field only.
What are step-up and step-down transformers?
The ratio of primary to secondary voltage depends on the number of turns of wire on each coil:
- Step-up transformer: more turns on the secondary coil than the primary (Ns > Np) → secondary voltage is higher than primary voltage.
- Step-down transformer: fewer turns on the secondary coil than the primary (Ns < Np) → secondary voltage is lower than primary voltage.
What is the turns ratio equation?
The relationship between the voltages and the number of turns is:
Vp / Vs = Np / Ns
Where:
- Vp = primary (input) voltage (V)
- Vs = secondary (output) voltage (V)
- Np = number of turns on the primary coil
- Ns = number of turns on the secondary coil
Worked example 1 — step-up transformer
A primary coil has 200 turns and a primary voltage of 25 V. The secondary coil has 1000 turns. Find the secondary voltage.
Vp / Vs = Np / Ns 25 / Vs = 200 / 1000 25 / Vs = 0.2 Vs = 25 / 0.2 = 125 V
The transformer steps the voltage up by a factor of 5 (the turns ratio: 1000/200 = 5).
Worked example 2 — step-down transformer
A step-down transformer has a primary voltage of 230 V and a secondary voltage of 11.5 V. How many turns are on the secondary coil if there are 2000 primary turns?
230 / 11.5 = 2000 / Ns 20 = 2000 / Ns Ns = 2000 / 20 = 100 turns
What is the power equation for an ideal transformer?
For an ideal (100 % efficient) transformer, power in = power out:
Vp × Ip = Vs × Is
Where Ip and Is are the primary and secondary currents. Stepping voltage up necessarily steps current down by the same factor, and vice versa.
Worked example — finding secondary current
A transformer steps voltage from 25 V to 125 V (step-up). The primary current is 10 A. Find the secondary current.
Vp × Ip = Vs × Is 25 × 10 = 125 × Is 250 = 125 × Is Is = 250 / 125 = 2 A
The current decreases by the same factor (5) as the voltage increased. Energy (power) is conserved — only voltage and current change.
Why does the National Grid use high voltage?
Transmitting electricity at high voltage keeps the current low, which dramatically reduces the power wasted as heat in the cables.
The power wasted (P_loss) in a transmission cable depends on the current and resistance:
P_loss = I² × R
Because power loss depends on the square of the current, halving the current reduces power loss by a factor of four (not two). If the current is reduced to one tenth, power loss falls to one hundredth.
| Voltage | Current (for same power) | Relative power loss (P = I²R) |
|---|---|---|
| 25 kV | 400 A | 400² × R = 160,000R |
| 400 kV | 25 A | 25² × R = 625R |
Transmitting at 400 kV instead of 25 kV reduces cable power loss by a factor of 256 for the same power delivered.
In the UK National Grid, electricity is stepped up to as high as 400,000 V (400 kV) for long-distance transmission, then stepped down by a series of step-down transformers to 33 kV (large factories), 11 kV (local substations), and finally 230 V (230 V AC, 50 Hz) for homes.
Frequently asked questions
Why do transformers only work with alternating current?
Electromagnetic induction requires a changing magnetic field to induce a voltage. Alternating current (AC) constantly changes direction and magnitude, so it produces a continuously changing magnetic flux in the transformer's iron core, which induces a voltage in the secondary coil. Direct current (DC) produces a constant (unchanging) magnetic field, so there is no change in flux and no induced voltage — a DC transformer simply does not work after the initial switch-on transient.
What is the iron core in a transformer made of and why?
The core is made of soft iron (iron with low carbon content), which is magnetically soft — it magnetises and demagnetises easily as the alternating magnetic field reverses many times per second (50 Hz in the UK). Hard magnetic materials would retain their magnetisation and oppose the rapid changes. The core is also laminated (made of thin sheets insulated from each other) to reduce eddy currents — induced currents within the core that would waste energy as heat.
Are real transformers 100 % efficient?
No real transformer is perfectly efficient, though well-designed large grid transformers can reach efficiencies of 98–99 %. Energy is lost in several ways: as heat due to the resistance of the copper coils (I²R losses in the wire); as heat in the iron core from eddy currents and magnetic hysteresis; and as sound (a faint hum at twice the supply frequency). For GCSE calculations, transformers are assumed to be ideal (100 % efficient) unless stated otherwise.
What happens to the current when a transformer steps up the voltage?
For an ideal transformer, power (P = V × I) is conserved, so if the voltage increases, the current must decrease by the same factor. For example, if a step-up transformer doubles the voltage, the current is halved. This is exactly the reason that high voltage is used for transmission — high voltage means low current, and low current means much less power lost as heat (P_loss = I² × R). The relationship Vp × Ip = Vs × Is allows you to calculate the output current for any input voltage and current.
For Socratic GCSE physics with Professor Newton — predicting the behaviour of transformers before checking the equations — visit aitutors.me.