Transformers and the National GridIB MYP Physics: Revision notes

Section 1

How a transformer is built

A transformer changes the size of an alternating voltage. It has three parts:

a primary coil, connected to the input supply
a secondary coil, which gives the output
a soft iron core that links the two coils

The coils are not connected by wire. Instead, an alternating current in the primary coil produces a changing magnetic field in the iron core. The core carries this field through the secondary coil, where it induces an alternating voltage.

Key termstransformerprimary coilsecondary coilsoft iron core

Common mistake

Transformers only work with alternating current. A steady direct current gives a constant magnetic field, so no voltage is induced in the secondary coil.

Section 2

The turns ratio equation

The voltages across the coils depend on how many turns each coil has:

$\frac{V_p}{V_s}=\frac{N_p}{N_s}$

where $V$ is voltage and $N$ is the number of turns (p = primary, s = secondary).

Worked example: a transformer has 200 turns on the primary and 50 turns on the secondary. The input is 240 V. Then $V_s = 240\times\frac{50}{200}=60$ V.

Key termsturns ratio

Exam tip

Write the equation first, then put in the numbers, then rearrange. The ratio always has primary on one side and secondary on the other.

Section 3

Step-up and step-down transformers

A step-up transformer raises the voltage. The secondary coil has more turns than the primary.
A step-down transformer lowers the voltage. The secondary coil has fewer turns than the primary.

A phone charger uses a step-down transformer to turn 230 V mains into a few volts. Power stations use step-up transformers.

Key termsstep-up transformerstep-down transformer

Section 4

Power in an ideal transformer

Transformers are very efficient. For an ideal (100% efficient) transformer, the power put in equals the power given out:

$V_p I_p = V_s I_s$

So if the voltage goes up, the current must go down (and the other way round). You do not get extra energy for free.

Worked example: a transformer takes 12 V at 2.0 A. The power is $12\times 2.0=24$ W. If the output voltage is 6.0 V, the current is $24\div 6.0=4.0$ A.

Key termsideal transformer

Common mistake

A step-up transformer does not increase power. The voltage rises but the current falls by the same factor.

Section 5

Why transmit electricity at high voltage?

Cables have resistance, so they get hot when a current flows. The power wasted as heat is:

$P=I^2R$

The power sent is $P=VI$ , so for the same power, a higher voltage means a smaller current. Because the loss depends on the square of the current, a small current wastes far less energy.

Worked example: 100 kW is sent through cables of resistance 2 $\Omega$ . At 1000 V the current is 100 A and the loss is $100^2\times2=20\,000$ W (20%). At 10 000 V the current is 10 A and the loss is $10^2\times2=200$ W (0.2%).

Key termspower loss

Exam tip

Say two things in an explanation: higher voltage gives a smaller current, and a smaller current gives a much smaller $I^2R$ loss.

Section 6

The National Grid: generation, transmission, distribution

The National Grid is the network that delivers electricity to homes and businesses.

Generation: power stations produce electricity at about 25 kV.
Step-up: a step-up transformer raises it to 400 kV or so.
Transmission: overhead cables on pylons (or underground cables) carry it long distances.
Step-down: substations reduce the voltage in stages.
Distribution: local cables deliver 230 V to homes, which is safe to use.

Key termsNational Griddistribution

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Transformers and the National Grid

A company in Shenzhen designs a phone charger that contains a transformer. The charger is plugged into a 230 V alternating mains supply and must deliver 5 V to the phone. The primary coil of the transformer has 1150 turns.
Calculate the number of turns on the secondary coil of the charger. Assume the transformer is ideal.2 marks
A student in Lagos builds a model transformer from two coils of insulated wire wound on a soft iron core. She connects the primary coil to a low-voltage alternating supply and a lamp to the secondary coil, and the lamp lights. She then replaces the alternating supply with a battery: the lamp flashes briefly when she connects it, then stays off.
Explain why the lamp lights continuously when the primary coil is connected to an alternating supply.2 marks
A power station in Poland generates electricity at 25 kV. Transformers raise this to 400 kV for transmission through overhead cables to a city, where further transformers reduce it in stages to 230 V for homes. The transmission cables have a total resistance of 20 Ω.
Outline how transformers are used to carry electricity from the power station to homes.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).