IGCSE Physics Magnetic Effect of a Current Questions and Worked Solutions (4.5.3) | 0625

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IGCSE Physics 0625 · Topic 4.5.3

IGCSE Physics Magnetic Effect of a Current Questions and Worked Solutions

50 topic-wise practice questions on Magnetic Effect of a Current (Unit 4: Electricity and Magnetism), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions24 Core16 Extended10 Challenge124 marksTextbook §4.5 · pp. 229–250
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: Magnetic Effect of a Current

A current produces a magnetic field. The field around a straight wire is a set of circles; the field of a solenoid is like a bar magnet's. This effect is used in electromagnets, relays and loudspeakers.

Key formulas

No equations to learn in this subtopic — use the right-hand grip rule for direction.

Prerequisite knowledge

  • Magnetic fields (4.1)
  • Current (4.2.2)

Exam tips & common mistakes

  • Right-hand grip: thumb = current, fingers = field direction
  • Soft-iron core makes an electromagnet much stronger
  • Viewed from an end, clockwise current = S pole

Key facts from the syllabus (4.5.3)

  • Straight wire: concentric circles around the wire
  • Solenoid: field like a bar magnet outside; strong and uniform inside
  • Experiment: iron filings or plotting compasses on a card around the wire or solenoid
  • Relays: a small current magnetises a coil that closes a switch in another circuit
  • Loudspeakers: a varying current in a coil in a magnetic field makes the cone vibrate
  • Extended the field around a wire weakens with distance; a solenoid's field is strongest inside
  • Extended increasing current strengthens the field; reversing current reverses the field

Textbook: §4.5 Electromagnetic effects, pp. 229–250

1
Core[2 marks]

Describe the magnetic field pattern around a straight current-carrying wire.

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Method: Concentric circles.

Concentric circles centred on the wire
2
Core[2 marks]

Describe the magnetic field pattern of a solenoid.

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Method: Like a bar magnet.

Similar to a bar magnet outside; strong, uniform, parallel lines inside
3
Core[3 marks]

Describe an experiment to show the field pattern around a straight wire.

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Method: Card, iron filings, compasses.

Pass the wire vertically through a horizontal card; sprinkle iron filings and tap; use plotting compasses to show direction; switch on a large current
4
Core[2 marks]

Name the rule used to find the direction of the field around a straight wire.

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Method: Right-hand grip rule.

The right-hand grip rule
5
Core[2 marks]

Current flows upwards in a vertical wire. State the direction of the field when viewed from above.

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Method: Right-hand grip.

Anticlockwise
6
Core[3 marks]

Describe how a relay uses the magnetic effect of a current.

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Method: Coil attracts armature.

Current in a coil magnetises a soft-iron core, which attracts an iron armature, closing (or opening) switch contacts in a second circuit
7
Core[1 mark]

Give one application of a relay.

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Method: Car starter motor.

e.g. car starter motor circuit (or automatic switching of high-current devices)
8
Core[3 marks]

Describe how a loudspeaker uses the magnetic effect of a current.

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Method: Varying current in coil, magnet, cone.

A varying current in a coil in a permanent magnet's field makes the coil move in and out, vibrating the cone to produce sound
9
Core[2 marks]

State two ways of increasing the strength of an electromagnet.

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Method: Current, turns, core.

Increase the current; increase the number of turns (or add a soft-iron core)
10
Core[2 marks]

Explain why an electric bell keeps ringing while the switch is held down.

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Method: Make-and-break.

The electromagnet attracts the armature (striking the gong), breaking the circuit; the armature springs back, remaking the circuit, and the cycle repeats
11
Core[2 marks]

Explain why the core of a relay is soft iron.

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Method: Temporary magnet.

Soft iron magnetises when current flows and loses its magnetism when it stops
12
Core[2 marks]

State one advantage of an electromagnet over a permanent magnet.

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Method: Switch on/off, adjust strength.

It can be switched on and off (and its strength varied)
13
Core[2 marks]

Current flows downwards in a vertical wire. State the direction of the field when viewed from above.

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Method: Right-hand grip.

Clockwise
14
Core[2 marks]

Describe the field inside a long solenoid.

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Method: Uniform.

Strong, uniform field with straight parallel lines
15
Core[3 marks]

Describe how to show the field pattern around a solenoid using iron filings.

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Method: Card through solenoid.

Thread the solenoid through a card; pass a current; sprinkle iron filings on the card and tap; the filings show a bar-magnet-like pattern
16
Core[2 marks]

State why a plotting compass is useful when investigating the field of a wire.

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Method: Shows direction.

It shows the direction of the field at each point
17
Core[2 marks]

Name the component in a relay that is attracted by the electromagnet.

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Method: Armature.

The (iron) armature
18
Core[2 marks]

Explain why a relay is used to switch on a car's starter motor.

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Method: Small current controls large.

A small current from the ignition switch can safely switch on the very large starter motor current
19
Core[3 marks]

Explain how a loudspeaker converts an electrical signal into sound.

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Method: Force on coil.

The varying current in the coil creates a varying magnetic field that interacts with the permanent magnet; the coil and cone move back and forth, producing sound waves
20
Core[2 marks]

State one advantage of an electromagnet in a scrapyard crane.

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Method: Can release.

It can be switched off to drop the load
21
Core[2 marks]

State how the strength of an electromagnet changes if its current is doubled.

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Method: Stronger.

It becomes stronger
22
Core[2 marks]

Explain why a magnetic door lock uses an electromagnet rather than a permanent magnet.

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Method: Controllable.

It can be released by switching off the current
23
Core[2 marks]

State the shape of the magnetic field outside a solenoid.

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Method: Bar magnet.

Like that of a bar magnet
24
Core[2 marks]

State the purpose of the soft-iron core in an electromagnet.

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Method: Increase strength.

It greatly increases the strength of the magnetic field
25
Extended[2 marks]

State how the strength of the field around a straight wire changes with distance from the wire.

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Method: Weaker further away.

It gets weaker further from the wire (circles further apart)
26
Extended[2 marks]

State where the magnetic field of a solenoid is strongest.

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Method: Inside.

Inside the solenoid
27
Extended[2 marks]

The current in a straight wire is increased. State the effect on the field.

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Method: Stronger.

The field becomes stronger (lines closer together)
28
Extended[2 marks]

The direction of the current in a solenoid is reversed. State the effect on its field.

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Method: Reverses.

The field reverses direction (N and S poles swap)
29
Extended[2 marks]

Describe how to find which end of a solenoid is the N pole.

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Method: Right-hand grip / plotting compass.

Grip with the right hand, fingers in the direction of current; the thumb points to the N pole (or use a compass)
30
Extended[3 marks]

Two parallel wires carry currents. Describe how the field around each changes if the current in one wire is doubled.

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Method: Field ∝ current.

The field around that wire becomes twice as strong; the other is unchanged
31
Extended[2 marks]

State the effect on a solenoid's field of adding more turns per unit length.

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Method: Stronger.

The field becomes stronger
32
Extended[2 marks]

State what happens to the field around a straight wire if the current is switched off.

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Method: Field disappears.

The magnetic field disappears
33
Extended[2 marks]

Describe the shape of the field lines around a straight wire as you move further away.

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Method: Larger circles, wider spacing.

Larger concentric circles, spaced further apart
34
Extended[2 marks]

Compare the field at the ends of a solenoid with the field inside.

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Method: Weaker at ends.

The field is weaker at the ends and spreads out
35
Extended[2 marks]

The current in a solenoid is halved. State the effect on the field strength.

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Method: Halved strength.

The field becomes weaker (about half as strong)
36
Extended[3 marks]

Two solenoids carry the same current. Solenoid A has twice the turns per metre. Compare their fields.

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Method: More turns, stronger.

A's field is stronger (about twice as strong)
37
Extended[2 marks]

Explain how to reverse the poles of an electromagnet.

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Method: Reverse current.

Reverse the direction of the current
38
Extended[2 marks]

A compass needle is placed under a straight wire running north–south. Current is switched on. Explain why the needle moves.

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Method: Field of current.

The current produces a magnetic field around the wire, which exerts a force on the needle
39
Extended[2 marks]

State what happens to the compass deflection in the previous question when the current is reversed.

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Method: Opposite.

The needle deflects the opposite way
40
Extended[2 marks]

State the effect on the field around a wire of increasing the distance from it to double.

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Method: Weaker.

The field becomes weaker (about half as strong)

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

Describe an experiment to find the pattern and direction of the magnetic field around a straight current-carrying wire.

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Method: Card, compasses.

Pass a vertical wire through a horizontal card; switch on a large current; place plotting compasses around the wire and mark their directions; sprinkle iron filings and tap; concentric circles appear
42
ChallengeCore[4 marks]

Explain how a relay allows a small current to switch on a large current, describing each part.

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Method: Coil, armature, contacts.

Current in the coil magnetises the soft-iron core, which attracts the iron armature; the armature pivots and closes the contacts of the second (high-current) circuit
43
ChallengeCore[4 marks]

Explain how a loudspeaker produces sound from an electrical signal.

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Method: Force on coil.

An alternating current in the coil, which sits in a permanent magnet's field, produces a varying force; the coil and cone vibrate at the signal's frequency, producing sound
44
ChallengeCore[3 marks]

Explain why electromagnets are used in electric door locks and bells rather than permanent magnets.

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Method: Switchable.

They can be switched on and off by controlling the current
45
ChallengeCore[4 marks]

Explain why a relay is used in a car's starter circuit, describing the currents in each circuit.

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Method: Small vs large current.

The ignition switch carries only a small current to the relay coil; the relay closes a separate circuit carrying the very large starter motor current, so the switch and thin wires are not damaged
46
ChallengeExtended[4 marks]

Two long straight wires carry currents. Describe how the field around one wire changes if (a) the current doubles, (b) the current reverses, (c) you move twice as far away.

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Method: Strength and direction.

(a) Stronger (about twice) (b) Field direction reverses (c) Weaker (about half)
47
ChallengeExtended[4 marks]

Use the right-hand grip rule to find which end of a solenoid is the N pole if, viewed from the left end, the current flows clockwise.

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Method: Grip rule.

The left end is a S pole (clockwise viewed from an end means S); the right end is N
48
ChallengeExtended[4 marks]

Explain why the field inside a solenoid is uniform and strong while outside it is weaker and spreads out.

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Method: Fields of turns add.

Inside, the fields from all turns add in the same direction; outside, they partly cancel and the lines spread out
49
ChallengeExtended[3 marks]

State three ways to increase the strength of the field of a solenoid.

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Method: Current, turns, core.

Increase the current, increase the number of turns per unit length, add a soft-iron core
50
ChallengeExtended[4 marks]

A compass is placed inside a solenoid. Describe what happens when the current is switched on, then reversed.

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Method: Aligns with field.

The needle aligns with the field along the axis; when the current reverses, the needle swings round 180°
All IGCSE Physics topics← 4.5.2 The a.c. Generator4.5.4 Force on a Current-Carrying Conductor →

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