Electromagnetic Induction

Q: When is no voltage induced in a coil in a magnetic field?

No voltage is induced if there is no relative motion between the coil and magnetic field, or if the coil moves parallel to the magnetic field lines (no lines are being cut by the conductor).

Q: Why do transformers only work with alternating current?

Transformers require a constantly changing magnetic flux in the iron core to induce a voltage in the secondary coil. Direct current produces a constant magnetic field, so no voltage is induced in the secondary coil.

Q: Are transformer formulas provided on the exam formula sheet?

No, you must recall both the turns ratio () and 100% efficiency power formula () from memory for Edexcel IGCSE Physics exams.

Physics· 6(d) 6.15–6.20· 12 min read

1. 1. Core: Electromagnetic Induction Fundamentals★★☆☆☆⏱ 3 min

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📘 Definition

Electromagnetic Induction

The production of a voltage across a conductor when it cuts through magnetic field lines, or when the magnetic field passing through a coil changes over time.

Induced voltage only occurs when there is relative change between the conductor and the magnetic field. If the conductor is stationary relative to the field, or moves parallel to field lines, no voltage is produced. The size of the induced voltage depends on three key qualitative factors:

Stronger magnetic field strength
More turns of wire on the coil
Faster relative motion between the conductor and magnetic field

📐 Worked Example

A student moves a single straight wire between the poles of a permanent bar magnet. State three changes they could make to increase the size of the induced voltage.

1
1. Replace the permanent bar magnet with a stronger magnet to increase magnetic field strength.
2
1. Wrap the wire into a coil with multiple turns instead of using a single straight wire.
3
1. Move the wire through the magnetic field at a faster speed to cut more field lines per second.

Exam tip:

When describing induction practicals, always link changes you suggest to cutting more magnetic field lines per second to earn full AO3 marks.

2. 2. Core: Generating Alternating Electricity★★☆☆☆⏱ 3 min

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Grid electricity is generated using simple AC generators, which work on the principle of electromagnetic induction. Generators use either a rotating coil of wire inside a fixed magnetic field, or a rotating magnet inside a fixed coil of wire. As rotation occurs, the magnetic field through the coil constantly changes direction, producing an alternating voltage (and alternating current if the circuit is complete).

The same factors that increase induced voltage for simple induction apply to generators, plus one additional factor:

Stronger magnet
More turns on the coil
Faster rotation speed of the coil/magnet
Wrapping the coil around a soft iron core to concentrate the magnetic field

📐 Worked Example

A simple school generator has a coil rotating between two fixed bar magnets. Explain why increasing the rotation speed of the coil increases the size of the induced voltage.

1
1. Faster rotation means the coil cuts through magnetic field lines much more frequently per second.
2
1. This increases the rate of change of the magnetic field passing through the coil.
3
1. A higher rate of magnetic field change produces a larger induced voltage.

Exam tip:

Do not confuse generators and motors: generators convert kinetic energy to electrical energy, while motors convert electrical energy to kinetic energy.

3. 3. Higher Tier Only: Transformer Structure & Operation★★★☆☆HL only⏱ 3 min

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📘 Definition

Transformer

A device that changes the size of an alternating voltage, consisting of two separate insulated wire coils wrapped around a shared soft iron core.

The coil connected to the input a.c. supply is called the primary coil, and the coil connected to the output circuit is called the secondary coil. Alternating current in the primary coil produces a constantly changing magnetic field in the iron core, which induces an alternating voltage in the secondary coil. There are two types of transformers:

Step-up transformer: Has more turns on the secondary coil than the primary coil, so output voltage is higher than input voltage.
Step-down transformer: Has fewer turns on the secondary coil than the primary coil, so output voltage is lower than input voltage.

📐 Worked Example

A transformer has 200 turns on the primary coil and 800 turns on the secondary coil. Is this a step-up or step-down transformer? Explain your answer.

1
1. Compare the number of turns on the secondary coil ( $N_{s} = 800$ ) to the primary coil ( $N_{p} = 200$ ).
2
1. $N_{s} > N_{p}$ , so this is a step-up transformer.
3
1. The output voltage will be 4 times higher than the input voltage.

Exam tip:

If asked why transformers do not work with d.c., always state that d.c. produces a constant magnetic field, so no voltage is induced in the secondary coil.

4. 4. Higher Tier Only: Transformer Calculations & National Grid★★★★☆HL only⏱ 3 min

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You must recall two key transformer equations for your exam, as they are not provided on the formula sheet. First, the turns ratio equation links voltage and number of turns on each coil:

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

Second, for 100% efficient transformers (assumed in all exam questions unless told otherwise), input power equals output power:

V_p \times I_p = V_s \times I_s

Transformers are critical for efficient national grid electricity transmission. When transmitting electricity over long distances, energy is wasted as heat in cables due to wire resistance. Heat loss is proportional to the square of the current ( $P_{l oss} = I^{2} R$ ), so reducing current drastically reduces wasted energy. Step-up transformers increase voltage at power stations, which reduces current for the same fixed power ( $P = V I$ ). Step-down transformers then reduce voltage to a safe 230V level for domestic and commercial use.

📐 Worked Example

A 100% efficient step-up transformer has an input voltage of 25,000 V and 1000 turns on the primary coil. The secondary coil has 20,000 turns. Calculate the output voltage, and the output current if the input current is 40 A.

1
Step 1: Use the turns ratio equation to find $V_{s}$
2
$\frac{25000}{V_s} = \frac{1000}{20000}$
3
Rearrange to solve for $V_{s}$ :
4
$V_s = \frac{25000 \times 20000}{1000} = 500000 V$
5
Step 2: Use the power efficiency equation to find $I_{s}$
6
$25000 \times 40 = 500000 \times I_s$
7
Rearrange to solve for $I_{s}$ :
8
$I_s = \frac{25000 \times 40}{500000} = 2 A$

Exam tip:

Always show full working for calculation questions, and include correct units for your final answer to earn all available marks. Round answers to 2-3 significant figures unless instructed otherwise.

5. Common Pitfalls

Wrong move:

Stating voltage is induced when a conductor is stationary in a constant magnetic field

Why:

Induced voltage requires relative motion or a changing magnetic field to cut field lines; no change means no induction

Correct move:

Always specify that relative motion or a changing magnetic field is required for electromagnetic induction to occur

Wrong move:

Confusing step-up and step-down transformer turn ratios, e.g. saying step-up transformers have fewer secondary turns

Why:

Step-up transformers increase voltage, which requires more secondary turns per the turns ratio equation

Correct move:

Remember: More secondary turns = higher secondary voltage = step-up transformer; fewer secondary turns = lower secondary voltage = step-down transformer

Wrong move:

Applying transformer equations to direct current (d.c.) circuits

Why:

Transformers require alternating current to produce a changing magnetic field in the core; d.c. produces a constant field, so no voltage is induced in the secondary coil

Correct move:

Only use transformer equations for a.c. circuits, and explicitly note d.c. inputs will not work if asked

Wrong move:

Claiming high transmission voltage increases current and heat loss

Why:

For fixed power ( $P = V I$ ), voltage and current are inversely proportional: higher voltage means lower current, which reduces $I^{2} R$ heat loss

Correct move:

Link high grid transmission voltage directly to low current, which reduces wasted heat energy in cables

Wrong move:

Omitting units or using incorrect units for transformer calculation answers

Why:

Exam marks are awarded for correct units as well as correct numerical values

Correct move:

Always add units (V for voltage, A for current) to your final answer, and check unit consistency in your working

6. Quick Reference Cheatsheet

Concept	All Tiers	Higher Tier Only
Induction requirement	Relative motion / changing magnetic field	Same as core
Induced voltage factors	Stronger field, more coil turns, faster relative motion/rotation	Same as core
Transformer definitions	Not assessed	Step-up: $N_{s} > N_{p}$ → $V_{s} > V_{p}$ ; Step-down: $N_{s} < N_{p}$ → $V_{s} < V_{p}$
Must-recall formulas	None for this topic	$\frac{V _{p}}{V _{s}} = \frac{N _{p}}{N _{s}}$ ; $V_{p} I_{p} = V_{s} I_{s}$ (100% efficiency)
National grid use	Not assessed	Step-up at power stations to reduce heat loss; step-down for safe domestic use

7. Frequently Asked

When is no voltage induced in a coil in a magnetic field?

No voltage is induced if there is no relative motion between the coil and magnetic field, or if the coil moves parallel to the magnetic field lines (no lines are being cut by the conductor).

Why do transformers only work with alternating current?

Transformers require a constantly changing magnetic flux in the iron core to induce a voltage in the secondary coil. Direct current produces a constant magnetic field, so no voltage is induced in the secondary coil.

Are transformer formulas provided on the exam formula sheet?

No, you must recall both the turns ratio ( $V_{p} / V_{s} = N_{p} / N_{s}$ ) and 100% efficiency power formula ( $V_{p} I_{p} = V_{s} I_{s}$ ) from memory for Edexcel IGCSE Physics exams.

Going deeper

What's Next

Now that you have mastered electromagnetic induction for Edexcel IGCSE Physics, you can move on to practice exam-style questions for this topic, and revise related magnetism and electricity concepts. This topic is frequently tested in both Paper 1 (core induction and generator content) and Paper 2 (Higher tier transformer content), so make sure you can recall all required formulas and qualitative explanations. You should also practice multi-step transformer calculation questions to ensure you can rearrange equations correctly under timed exam conditions.