A transformer has 100 primary turns and 400 secondary turns, with 50\ V across the primary. Find the secondary voltage. [2 marks]

SEAB Original-style practice: A transformer has 200 turns on the primary coil and 50 turns on the secondary coil. The primary is connected to a 240\ V a.c. supply. (a) Calculate the secondary voltage. (b) State whether this is a step-up or step-down transformer, and explain.

(a) Transformer relationship: V_sV_p = N_sN_p, so V_s = V_p × N_sN_p = 240 × 50200 = 240 × 0.25 = 60\ V. (b) It is a step-down transformer, because the secondary has fewer turns than the primary, so the secondary voltage (60\ V) is lower than the primary voltage (240\ V). Markers reward the turns relationship with correct substitution, the secondary voltage, and identifying it as step-down because fewer secondary turns give a lower output voltage.

SingaporePhysicsSyllabus dot point

How can a changing magnetic field generate a voltage, and how do generators and transformers use this?

Describe electromagnetic induction and explain the a.c. generator and the transformer

A focused answer to the O-Level Physics outcome on electromagnetic induction. Inducing a voltage by a changing magnetic field, factors affecting its size, the a.c. generator, and the transformer with its turns relationship.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

SEAB wants you to describe electromagnetic induction, the production of a voltage by a changing magnetic field, to know what makes the induced voltage larger, and to explain how the a.c. generator and the transformer work. The big idea is the reverse of the motor effect: instead of a current producing motion, motion (a changing field) produces a voltage.

The answer

Electromagnetic induction

When the magnetic field through a coil changes, a voltage is induced across the coil. If the coil is part of a complete circuit, this voltage drives an induced current. The field can be changed by moving a magnet near the coil, moving the coil, or changing a current in a nearby coil.

If the movement stops, the field stops changing and the induced voltage falls to zero. This is why a magnet held still inside a coil induces nothing.

What makes the induced voltage larger

The induced voltage is bigger when:

the magnet (or coil) moves faster,
the magnet is stronger,
the coil has more turns.

Reversing the direction of the movement reverses the direction of the induced voltage and current.

The a.c. generator

An a.c. generator (alternator) turns motion into electricity. A coil is rotated in a magnetic field. As it spins, the magnetic field through it changes continuously, inducing a voltage. Because the field through the coil increases and decreases and reverses as the coil turns, the induced voltage alternates in direction, producing alternating current (a.c.). Slip rings connect the spinning coil to the external circuit.

The transformer

A transformer changes the size of an alternating voltage. It has two coils, the primary and the secondary, wound on a soft iron core. An alternating current in the primary produces a changing magnetic field in the core, which induces an alternating voltage in the secondary. The voltages relate to the numbers of turns:

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

A step-up transformer has more secondary turns, giving a higher output voltage.
A step-down transformer has fewer secondary turns, giving a lower output voltage.

Transformers work only on a.c., because they need a continuously changing field.

Worked example

A step-up transformer is needed to change $120\ \text{V}$ a.c. into $600\ \text{V}$ a.c. The primary coil has $300$ turns. (a) Find the number of secondary turns. (b) Explain why the transformer would not work on a steady d.c. supply.

Step 1: Apply the turns relationship

From $\dfrac{V_s}{V_p} = \dfrac{N_s}{N_p}$ , rearrange for $N_s$ :

N_s = N_p \times \frac{V_s}{V_p} = 300 \times \frac{600}{120} = 300 \times 5 = 1500\ \text{turns}

Step 2: Check it is step-up

The secondary has $1500$ turns, more than the primary's $300$ , so the voltage is stepped up from $120\ \text{V}$ to $600\ \text{V}$ , as required.

Step 3: Explain the d.c. problem

A steady d.c. supply gives a constant current and so a constant magnetic field in the core. With no changing field, no voltage is induced in the secondary, so the transformer produces no output. It works only on a.c., where the field is always changing.

The transformer needs $1500$ secondary turns and must be fed a.c. to function.

Examples in context

Example 1. Power stations and the grid. Generators at power stations induce huge alternating voltages by spinning coils in strong magnetic fields. Step-up transformers then raise the voltage for transmission across the grid (which reduces energy lost as heat in the cables), and step-down transformers lower it again to a safe level before it reaches homes.

Example 2. Phone chargers. A phone charger contains a small transformer that steps the high mains voltage down to a few volts safe for the phone. Because transformers need alternating current, this is one reason mains electricity is supplied as a.c. rather than d.c.

Try this

Q1. State what is needed to induce a voltage in a coil. [1 mark]

Cue. A changing magnetic field through the coil (for example, a moving magnet).

Q2. A transformer has $100$ primary turns and $400$ secondary turns, with $50\ \text{V}$ across the primary. Find the secondary voltage. [2 marks]

Cue. $V_s = V_p \times \dfrac{N_s}{N_p} = 50 \times \dfrac{400}{100} = 200\ \text{V}$ (a step-up transformer).

Q3. Explain why a transformer does not work with a steady d.c. supply. [2 marks]

Cue. A steady d.c. gives a constant magnetic field; with no changing field, no voltage is induced in the secondary, so there is no output.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA magnet is pushed into a coil of wire connected to a sensitive meter. (a) State what the meter shows, and explain why. (b) State two ways to make the induced voltage larger.

Show worked answer →

(a) The meter shows a reading (a deflection), meaning a voltage is induced. As the magnet moves, the magnetic field through the coil changes, and a changing magnetic field through a coil induces a voltage across it.

(b) Move the magnet faster, or use a stronger magnet. (Using more turns on the coil also increases the induced voltage.)

Markers reward a voltage induced by the changing field through the coil, and two valid ways to increase it, typically faster movement and a stronger magnet (or more turns).

Original5 marksA transformer has

200

turns on the primary coil and

50

turns on the secondary coil. The primary is connected to a

240\ \text{V}

a.c. supply. (a) Calculate the secondary voltage. (b) State whether this is a step-up or step-down transformer, and explain.

Show worked answer →

(a) Transformer relationship: $\dfrac{V_s}{V_p} = \dfrac{N_s}{N_p}$ , so $V_s = V_p \times \dfrac{N_s}{N_p} = 240 \times \dfrac{50}{200} = 240 \times 0.25 = 60\ \text{V}$ .

(b) It is a step-down transformer, because the secondary has fewer turns than the primary, so the secondary voltage ( $60\ \text{V}$ ) is lower than the primary voltage ( $240\ \text{V}$ ).

Markers reward the turns relationship with correct substitution, the secondary voltage, and identifying it as step-down because fewer secondary turns give a lower output voltage.

What this dot point is asking

The answer

Electromagnetic induction

What makes the induced voltage larger

The a.c. generator

The transformer

Examples in context

Try this

Exam-style practice questions

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