Do transformers work with alternating or direct electrical supplies?

Transformers only work with alternating electrical supplies (i.e. alternating voltage/current). 

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What do transformers do?

Transformers change the voltage and current of an alternating current. 

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What does a step-down transformer do?

A step-down transformer decreases the voltage and increases the current. 

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What does a step-up transformer do?

A step-up transformer increases the voltage and decreases the current.

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Why is it important to use step-up transformers before electricity is transmitted across the national grid?

It is important to have a very high voltage and low current to minimise energy loss during transmission.


A high current would heat the wire causing energy loss via heat.

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How do transformers work?

  1. When supplied with an alternating current, the primary coil will generate an alternating magnetic field.
  2. This then induces an alternating magnetic field in the iron core.
  3. This, in turn, induces an alternating voltage in the secondary coil, which - if the circuit is complete - will lead to an alternating current.

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Will a step-up transformer have more turns on the primary coil, or the secondary coil?

On a step-up transformer, the secondary coil will have more turns. 

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Will a step-down transformer have more turns on the primary coil, or the secondary coil?

A step-down transformer will have more turns on the primary coil.

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What is the formula that links voltage in the primary coil (Vp), voltage in the secondary coil (Vs), turns in the primary coil (Np) and turns in the secondary coil (Ns)?

Vs​Vp​​ = Ns​Np​​


Vp = voltage at the primary (V)

Vs = voltage at the secondary (V) 

Np = number of turns on the primary coil

Ns = number of turns on the secondary coil 

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What is the formula that links voltage across the primary coil (Vp), voltage across the secondary coil (Vs), current in the primary coil (Ip) and current in the secondary coil (Is)?

Vp​Ip​ = Vs​Is​


Vp = voltage across the primary coil (V)

Ip = current in the primary coil (A)

Vs = voltage across the secondary coil (V)

Is = current in the secondary coil (A)

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