Physics - 15 Electromagnetism - 15.7 Transformers & 15.8 Transformers in Action

Transformer

device that increases or decreases voltage

What is a transformer made from?

soft iron laminated core wrapped in insulated wire, connected to two different circuits

Why is iron used in a transformer?

it is easily magnetised and demagnetised

Step-up transformer

increases voltage and decreases current

Step-down transformer

decreases voltage and increases current

Primary coil

coil supplied with current in a transformer

Secondary coil

coil that outputs current in a transformer

Transformer circuit symbol

Why is an alternating current used in a transformer?

a direct current would not provide the necessary changing magnetic field, so no current would be induced

How does a transformer work? [3]

- an alternating current is supplied to the primary coil

- this produces an alternating magnetic field

- the magnetic field induces an alternating current in the secondary coil

In a step-up transformer, the primary coil has (more/less) turns than the secondary coil

less

In a step-down transformer, the primary coil has (more/less) turns than the secondary coil

more

The potential difference produced is ... to the number of turns on the coil

proportional

What is conserved when a transformer functions (assuming 100% efficiency)?

power

Transformer word equation

(potential difference across primary coil)/(potential difference across secondary coil) = (number of turns in primary coil)/(number of turns in secondary coil)

Transformer symbol equation

Vₚ/Vₛ = Nₚ/Nₛ

The heating effect of a resistor in a wire is proportional to ...

the square of the current

The national grid

system of cables and transformers linking power stations to consumers

When are step-up transformers used in the national grid? [4]

- to increase p.d. and decrease current in transmission wires

- to reduce the heating effect of the current

- less energy is lost by heating

- which makes the transfer more efficient

When are step-down transformers used in the national grid? [2]

- to decrease p.d. and increase current in consumer use

- to reduce danger of p.d. by making it lower

Mains electricity voltage

230V