What is a transformer?

A device that changes the size of an alternating voltage using electromagnetic induction between two coils (primary and secondary) wound on a soft-iron core.

State the transformer equation relating voltages and turns.

Ns/Np = Vs/Vp — the ratio of secondary to primary turns equals the ratio of secondary to primary voltage.

What is a step-up transformer?

One with more turns on the secondary than the primary (Ns > Np), so the output voltage is greater than the input voltage.

What is a step-down transformer?

One with fewer turns on the secondary than the primary (Ns < Np), so the output voltage is less than the input voltage.

For an ideal transformer, how are input and output power related?

They are equal, so Ip·Vp = Is·Vs — power is conserved (100% efficiency).

How do you calculate the efficiency of a transformer?

Efficiency = (Is·Vs)/(Ip·Vp) × 100% — output power divided by input power.

AQA A-Level Physics 7408

Measurements and Their Errors

Topic overview Use of SI units and their prefixes Limitation of physical measurements Estimation of physical quantities

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Fields and Their Consequences

Transformers

3 things to know
Papers 2 and 3

Overview

The transformer equation, efficiency, and why the National Grid transmits at high voltage.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What is the transformer equation, and why must a transformer use alternating current?

Numbers

Why is power sent at high voltage across the National Grid?

Numbers

Why is the voltage stepped down before electricity reaches homes?

Questions from this subtopic

Each one with the model answer underneath.

1
What is a transformer?
Model answer
A device that changes the size of an alternating voltage using electromagnetic induction between two coils (primary and secondary) wound on a soft-iron core.
2
State the transformer equation relating voltages and turns.
Model answer
Ns/Np = Vs/Vp — the ratio of secondary to primary turns equals the ratio of secondary to primary voltage.
3
What is a step-up transformer?
Model answer
One with more turns on the secondary than the primary (Ns > Np), so the output voltage is greater than the input voltage.
4
What is a step-down transformer?
Model answer
One with fewer turns on the secondary than the primary (Ns < Np), so the output voltage is less than the input voltage.
5
For an ideal transformer, how are input and output power related?
Model answer
They are equal, so Ip·Vp = Is·Vs — power is conserved (100% efficiency).
6
How do you calculate the efficiency of a transformer?
Model answer
Efficiency = (Is·Vs)/(Ip·Vp) × 100% — output power divided by input power.

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