If a transformer increases voltage, does it create extra energy?

No — that would break conservation of energy. In an ideal transformer power in equals power out, so when the voltage goes up the current goes down by the same factor (and vice versa). Real transformers are very efficient but lose a little energy as heat in the core and windings.

Why is the core made of iron, and why is it laminated?

Iron is easily magnetised, so it channels the changing magnetic field from one coil to the other very effectively. It is built from thin insulated layers (laminations) to reduce wasteful swirling 'eddy' currents in the core, which would otherwise heat it up and waste energy.

Transformers and the National Grid | Physics

Changing voltage at will

Electricity leaves a power station, travels hundreds of kilometres, and arrives in your home at a safe, steady 230 volts. Along the way its voltage is raised to as much as 400,000 volts and then lowered again — many times. The device that performs this trick is the transformer, and without it the National Grid simply could not work. This lesson explains how transformers change voltage, the equation that governs them, and why the whole grid is built around them.

A transformer is a direct application of electromagnetic induction, so make sure you are comfortable with the generator effect first.

How a transformer works

A basic transformer has two coils of insulated wire wound on the same iron core:

the primary coil, connected to the input (the source), and
the secondary coil, connected to the output (the load).

The two coils are not electrically joined — current never flows directly from one to the other. Instead they are linked by magnetism:

An alternating current in the primary coil creates a constantly changing magnetic field.
The iron core carries this changing field round to the secondary coil.
The changing field through the secondary coil induces an alternating voltage in it (Faraday's law).

This is why transformers only work with AC: a steady direct current would create a constant field, and a constant field induces nothing. The core is iron because iron is easily magnetised and channels the field efficiently between the coils.

Step-up and step-down

The output voltage depends entirely on the ratio of turns on the two coils:

A step-up transformer has more turns on the secondary than the primary, so the output voltage is higher than the input.
A step-down transformer has fewer turns on the secondary, so the output voltage is lower than the input.

The relationship is the turns-ratio (transformer) equation:

Vp ÷ Vs = Np ÷ Ns

where Vp and Vs are the primary and secondary voltages, and Np and Ns are the numbers of turns on each coil.

Conservation of energy: nothing is free

A transformer can raise voltage, but it cannot create energy. In an ideal (100% efficient) transformer, the electrical power going in equals the power coming out:

Vp × Ip = Vs × Is   (power in = power out)

So if the voltage is stepped up, the current must step down by the same factor — and vice versa. A step-up transformer that doubles the voltage halves the current. This is the key to understanding the grid. (Recall that electrical power is P = V × I, covered in electrical power and energy.)

Worked examples

Example 1 — find the output voltage. A transformer has 200 primary turns and 1000 secondary turns. The primary voltage is 230 V. Vs = Vp × (Ns ÷ Np) = 230 × (1000 ÷ 200) = 230 × 5 = 1150 V (a step-up transformer).

Example 2 — find the number of turns. A step-down transformer must convert 230 V to 11.5 V. The primary has 4000 turns. How many secondary turns are needed? Ns = Np × (Vs ÷ Vp) = 4000 × (11.5 ÷ 230) = 4000 × 0.05 = 200 turns.

Example 3 — find the output current. An ideal transformer steps 12 V up to 240 V. The primary current is 5 A. What is the secondary current? Power in = Vp × Ip = 12 × 5 = 60 W. Since power out = power in, Is = 60 ÷ 240 = 0.25 A. The voltage rose ×20, so the current fell ×20.

The National Grid: why high voltage matters

The National Grid is the network of cables, pylons and transformers that carries electricity from power stations to homes and factories across the country. Its central problem is that long cables have resistance, and current flowing through resistance wastes energy as heat. That wasted power is:

P(lost) = I² × R

Notice the current is squared. Halving the current cuts the wasted power to a quarter; reducing it tenfold cuts the loss by a factor of one hundred. So the grid's strategy is to make the current as small as possible during transmission.

Because power = voltage × current, you can deliver the same power with a small current if you use a very high voltage. So the grid:

Uses a step-up transformer at the power station to raise the voltage to 275,000–400,000 V. This makes the transmission current tiny, so very little energy is wasted heating the cables.
Transmits the power efficiently across the country at this high voltage.
Uses step-down transformers at local substations to lower the voltage in stages, finally reaching the 230 V that is safe to use in homes.

This is the whole reason transformers exist on the scale they do: high voltage for efficient long-distance transmission, low voltage for safe use. For how that 230 V is used safely inside your house, see household electricity and safety.

Worked example: the grid pays off

Suppose a power station delivers 1,000,000 W (1 MW) along a cable with resistance 0.5 Ω.

At 1000 V: current I = P ÷ V = 1,000,000 ÷ 1000 = 1000 A. Power lost = I² × R = 1000² × 0.5 = 500,000 W — half the power wasted as heat!

At 100,000 V: current I = 1,000,000 ÷ 100,000 = 10 A. Power lost = I² × R = 10² × 0.5 = 50 W — almost nothing.

Raising the voltage by 100 times cut the loss by 10,000 times. That dramatic saving is exactly why the grid runs at hundreds of thousands of volts.

Try it yourself! 🧪

Model a transformer safely with low-voltage AC from a school signal generator or a battery-driven AC source — never mains.

You need: an iron C-core or nail bundle, two lengths of insulated wire, a low-voltage AC supply (1–6 V from a signal generator or school power pack set to AC), and a small AC voltmeter or low-voltage bulb.

Wind 20 turns of wire on one side of the core for the primary; connect it to the low-voltage AC supply.
Wind 40 turns on the other side for the secondary; connect it to the voltmeter or bulb.
Switch on. The secondary voltage should be about double the primary, because it has twice the turns.
Rewind the secondary with only 10 turns and try again — now the output is about half the input (a step-down).

You have built a working transformer and verified the turns-ratio rule.

⚠️ Safety: Use only a low-voltage AC source of a few volts. Never connect homemade coils, cores or transformers to mains sockets — mains and grid voltages are lethal. The core and wires can warm up, so switch off promptly between trials.

What we learned

A transformer uses two coils on an iron core to change the size of an alternating voltage, governed by Vp/Vs = Np/Ns. Step-up transformers raise voltage; step-down transformers lower it; and because power is conserved, raising voltage lowers current. The National Grid exploits this to transmit power at very high voltage and tiny current, slashing the I²R heat losses in the cables, before stepping the voltage back down to a safe 230 V for our homes.

Transformers and the National Grid

Key takeaways