GCSE Physics revision

Transformers (HT only)

Induced potential, transformers and the National Grid (physics only) (HT only)

AQA 4.7.3.4

Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Explain transformer operation and use coil-turn, potential-difference, current and power relationships to calculate transfers and explain efficient high-voltage transmission.

MOST 🎯🎯

Apply the specified scientific knowledge of transformers to a relevant example.

SOME 🎯🎯🎯

Analyse an unfamiliar example of transformers using the specified scientific ideas.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

Why Transformers Are Needed

Electrical energy from power stations is transmitted to homes via the National Grid through cables.
A high current flowing through cables causes friction, wasting energy as heat.
Transformers solve this problem by changing voltage and current levels to minimise energy loss.

Step-Up Transformers

A step-up transformer is placed at the power station to increase voltage, for example from 10,000 V to 400,000 V.
Increasing the voltage reduces the current, which significantly reduces energy lost as heat in the cables.
You can identify a step-up transformer because the secondary coil has more turns than the primary coil.

2. Step-Up TransformersAQA

Notes

A step-up transformer is placed at the power station to increase voltage, for example from 10,000 V to 400,000 V.
Increasing the voltage reduces the current, which significantly reduces energy lost as heat in the cables.
You can identify a step-up transformer because the secondary coil has more turns than the primary coil.

Step-Down Transformers

A step-down transformer is placed before electricity enters homes to reduce the voltage to a safe level, such as 230 V.
You can identify a step-down transformer because the secondary coil has fewer turns than the primary coil.
In the UK, mains electricity is supplied at 230 V, 50 Hz as an alternating current (AC).

Structure of a Transformer

A transformer consists of an iron core with two coils of wire wrapped around it.
The primary coil receives the incoming electrical energy, and the secondary coil delivers the changed voltage output.
An iron core is used because it is easily magnetised, allowing a magnetic field to pass efficiently between the two coils.

4. Structure of a TransformerAQA

Notes

A transformer consists of an iron core with two coils of wire wrapped around it.
The primary coil receives the incoming electrical energy, and the secondary coil delivers the changed voltage output.
An iron core is used because it is easily magnetised, allowing a magnetic field to pass efficiently between the two coils.

How a Transformer Works

An alternating potential difference (AC) in the primary coil creates an alternating magnetic field in the iron core.
This alternating magnetic field passes through the secondary coil and induces an alternating potential difference in it.
AC must be used in the primary coil because only a changing magnetic field can induce a current in the secondary coil; DC would not work.
Because the magnetic field alternates direction, the current produced in the secondary coil is also an alternating current (AC).
Exam Answer: How Does a Transformer Work?
Step 1: An alternating potential difference is applied to the primary coil.
Step 2: This creates an alternating magnetic field in the iron core.
Step 3: The alternating magnetic field induces an alternating potential difference in the secondary coil.

5. How a Transformer WorksAQA

Notes

An alternating potential difference (AC) in the primary coil creates an alternating magnetic field in the iron core.
This alternating magnetic field passes through the secondary coil and induces an alternating potential difference in it.
AC must be used in the primary coil because only a changing magnetic field can induce a current in the secondary coil; DC would not work.
Because the magnetic field alternates direction, the current produced in the secondary coil is also an alternating current (AC).
Exam Answer: How Does a Transformer Work?
Step 1: An alternating potential difference is applied to the primary coil.
Step 2: This creates an alternating magnetic field in the iron core.
Step 3: The alternating magnetic field induces an alternating potential difference in the secondary coil.

The Transformer Turns Equation

The ratio of voltages across the coils equals the ratio of the number of turns in each coil.
The equation is: V_p / V_s = N_p / N_s where V_p and V_s are the primary and secondary voltages, and N_p and N_s are the number of turns.
If the number of turns doubles, the voltage also doubles by the same factor.

6. The Transformer Turns EquationAQA

Notes

The ratio of voltages across the coils equals the ratio of the number of turns in each coil.
The equation is: V_p / V_s = N_p / N_s where V_p and V_s are the primary and secondary voltages, and N_p and N_s are the number of turns.
If the number of turns doubles, the voltage also doubles by the same factor.

The Transformer Power Equation

For an ideal transformer, power is conserved, meaning the power input equals the power output.
This gives the equation: V_p × I_p = V_s × I_s, where I_p and I_s are the primary and secondary currents.
When voltage increases in a step-up transformer, the current decreases proportionally to keep power constant.

7. The Transformer Power EquationAQA

Notes

For an ideal transformer, power is conserved, meaning the power input equals the power output.
This gives the equation: V_p × I_p = V_s × I_s, where I_p and I_s are the primary and secondary currents.
When voltage increases in a step-up transformer, the current decreases proportionally to keep power constant.

Worked Example Using Both Equations

If a primary coil has a voltage of 10 V and a current of 5 A, the power input is P = V × I = 10 × 5 = 50 W.
If the number of turns doubles, the secondary voltage doubles to 20 V, and the secondary current halves to 2.5 A.
The output power is 20 × 2.5 = 50 W, confirming that power is the same on both sides of an ideal transformer.

8. Worked Example Using Both EquationsAQA

Notes

If a primary coil has a voltage of 10 V and a current of 5 A, the power input is P = V × I = 10 × 5 = 50 W.
If the number of turns doubles, the secondary voltage doubles to 20 V, and the secondary current halves to 2.5 A.
The output power is 20 × 2.5 = 50 W, confirming that power is the same on both sides of an ideal transformer.

Key Vocabulary Summary

Alternating current (AC) is a current that continuously changes direction, essential for transformer operation.
The National Grid is the network of cables and transformers that distributes electrical energy across the UK.
An ideal transformer is assumed to be 100% efficient, meaning no energy is lost within the transformer itself.

9. Key Vocabulary SummaryAQA

Notes

Alternating current (AC) is a current that continuously changes direction, essential for transformer operation.
The National Grid is the network of cables and transformers that distributes electrical energy across the UK.
An ideal transformer is assumed to be 100% efficient, meaning no energy is lost within the transformer itself.