IGCSE Physics Force on a Current-Carrying Conductor Questions and Worked Solutions (4.5.4) | 0625

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IGCSE Physics 0625 · Topic 4.5.4

IGCSE Physics Force on a Current-Carrying Conductor Questions and Worked Solutions

50 topic-wise practice questions on Force on a Current-Carrying Conductor (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 questions16 Core24 Extended10 Challenge131 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: Force on a Current-Carrying Conductor

A current-carrying conductor in a magnetic field experiences a force. The force reverses if the current or the field is reversed. Moving charged particles experience the same kind of force.

Key formulas

Fleming's left-hand rulethumb = force (motion), first finger = field (N→S), second finger = current (+ → −)Extended

Prerequisite knowledge

  • Magnetic effect of a current (4.5.3)
  • Fields (4.1)

Exam tips & common mistakes

  • Use the LEFT hand for motors (force on a current)
  • For electron beams, point the second finger opposite to the electron motion
  • The force is always perpendicular to both field and current

Key facts from the syllabus (4.5.4)

  • Experiment: a loose wire or rolling rod between magnet poles moves when current flows
  • Reversing the current or the field reverses the force; reversing both leaves it unchanged
  • Larger current or stronger field gives a larger force
  • No force when the current is parallel to the field
  • Extended use Fleming's left-hand rule for directions
  • Extended beams of charged particles are deflected; electrons move opposite to conventional current

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

1
Core[3 marks]

Describe an experiment to show that a force acts on a current-carrying conductor in a magnetic field.

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Method: Catapult experiment.

Place a wire (or loose rod on rails) between the poles of a magnet; pass a current; the wire moves (is catapulted) at right angles to the field
2
Core[2 marks]

State the effect of reversing the current on the force on a wire in a magnetic field.

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Method: Force reverses.

The force reverses direction
3
Core[2 marks]

State the effect of reversing the magnetic field on the force on a current-carrying wire.

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Method: Force reverses.

The force reverses direction
4
Core[2 marks]

Both the current and the field are reversed. State the direction of the force.

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Method: Two reversals cancel.

The force is in the original direction
5
Core[2 marks]

State two ways to increase the force on a wire in a magnetic field.

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Method: Current, field.

Increase the current; use a stronger magnetic field
6
Core[1 mark]

State the direction of the force when the current is parallel to the field.

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Method: No force.

There is no force
7
Core[2 marks]

Explain why a loose wire between magnet poles jumps when the current is switched on.

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Method: Force on current in field.

The magnetic field of the current interacts with the magnet's field, producing a force
8
Core[2 marks]

Give one device that uses the force on a current-carrying conductor.

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Method: Motor, loudspeaker.

Electric motor (or loudspeaker)
9
Core[2 marks]

State what happens to a loose wire carrying current between magnetic poles when the current is switched off.

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Method: No force.

It stops moving (no force)
10
Core[2 marks]

State the effect of increasing the current on the force on a wire in a magnetic field.

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Method: Larger force.

The force increases
11
Core[2 marks]

A wire experiences an upward force. The magnet is turned round. State the direction of the new force.

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Method: Reversed.

Downwards
12
Core[3 marks]

Describe an experiment using two rails and a copper rod to show the force on a conductor.

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Method: Rolling rod.

Place a copper rod across two horizontal rails between magnet poles; connect the rails to a supply; when current flows the rod rolls along the rails
13
Core[2 marks]

State what happens to the rod in the previous question if the current is reversed.

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Method: Rolls opposite.

It rolls the other way
14
Core[2 marks]

State one factor apart from current that affects the size of the force on a wire.

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Method: Field strength or length.

The strength of the magnetic field (or length of wire in the field)
15
Core[2 marks]

Explain why a loudspeaker coil moves when current flows.

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Method: Force on conductor.

The current-carrying coil is in a magnetic field, so it experiences a force
16
Core[2 marks]

State the energy transfer when a current-carrying wire is pushed by a magnetic field.

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Method: Electrical → kinetic.

Electrical → kinetic
17
Extended[1 mark]

Name the hand rule used to find the direction of the force on a current-carrying conductor.

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Method: Fleming's left-hand rule.

Fleming's left-hand rule
18
Extended[3 marks]

State what each finger represents in Fleming's left-hand rule.

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Method: FBI.

Thumb: force (motion); first finger: field (N to S); second finger: current
19
Extended[3 marks]

A wire carries current to the right in a field pointing into the page. State the direction of the force.

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Method: Left-hand rule.

Upwards
20
Extended[3 marks]

A wire carries current into the page in a field pointing from left to right. State the direction of the force.

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Method: Left-hand rule.

Downwards
21
Extended[3 marks]

A wire carries current upwards in a field pointing to the right. State the direction of the force.

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Method: Left-hand rule.

Into the page
22
Extended[3 marks]

A beam of electrons moves to the right through a magnetic field pointing into the page. State the direction of the force on the beam.

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Method: Conventional current is opposite to electron flow.

Downwards (conventional current is to the left)
23
Extended[3 marks]

A beam of positive particles moves upwards through a magnetic field pointing out of the page. State the direction of the force.

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Method: Left-hand rule with current upwards.

To the right
24
Extended[2 marks]

Explain why a beam of charged particles moves in a curved path in a magnetic field.

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Method: Force perpendicular to velocity.

The force is always at right angles to the motion, so the beam changes direction continuously
25
Extended[2 marks]

Explain why an alpha particle and a beta particle are deflected in opposite directions in a magnetic field.

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Method: Opposite charges.

They have opposite charges, so the forces on them are in opposite directions
26
Extended[2 marks]

Explain why gamma rays are not deflected in a magnetic field.

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Method: Uncharged.

Gamma rays have no charge
27
Extended[2 marks]

A wire experiences a force upwards. The current is doubled and reversed. Describe the new force.

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Method: Twice as big, downwards.

Twice as large, acting downwards
28
Extended[2 marks]

Explain why the force on a current-carrying wire is zero when it is parallel to the field lines.

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Method: Current must cut across field.

The force only occurs when the current has a component across the field
29
Extended[3 marks]

Current flows out of the page in a field pointing to the left. State the direction of the force.

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Method: Left-hand rule.

Downwards
30
Extended[3 marks]

Current flows to the left in a field pointing into the page. State the direction of the force.

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Method: Left-hand rule.

Downwards
31
Extended[3 marks]

Current flows downwards in a field pointing out of the page. State the direction of the force.

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Method: Left-hand rule.

To the left
32
Extended[3 marks]

A wire experiences a force into the page when the field points upwards. State the current direction.

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Method: Rearrange rule.

To the left
33
Extended[3 marks]

Electrons move upwards through a field pointing into the page. State the direction of the force on them.

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Method: Conventional current downwards.

To the right
34
Extended[3 marks]

Protons move to the left through a field pointing out of the page. State the direction of the force.

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Method: Current to left.

Upwards
35
Extended[2 marks]

Explain why a beam of electrons in a cathode ray tube can be bent by a magnet.

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Method: Moving charges.

Moving electrons are a current, which experiences a force in a magnetic field
36
Extended[2 marks]

Explain why the speed of charged particles in a magnetic field does not change, even though they are deflected.

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Method: Force perpendicular.

The force is always perpendicular to the motion, so it changes direction only
37
Extended[3 marks]

Alpha and beta particles enter the same magnetic field. Compare the size and direction of deflection.

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Method: Opposite, beta more.

Opposite directions; beta deflected much more because of its much smaller mass
38
Extended[2 marks]

State the direction of the force on a wire carrying current parallel to the field.

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Method: No force.

There is no force
39
Extended[2 marks]

State the rule used to find the direction of force on a current in a magnetic field.

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Method: Left-hand.

Fleming's left-hand rule
40
Extended[2 marks]

In Fleming's left-hand rule, state which finger represents current.

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Method: Second finger.

The second (middle) finger

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 show the force on a current-carrying conductor in a magnetic field and how to show the effect of reversing the current and the field.

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Method: Catapult/rails.

Place a loose wire or rolling rod between magnet poles; switch on — it moves; reverse the current — it moves the other way; turn the magnet round — it moves the other way; reverse both — original direction
42
ChallengeCore[3 marks]

Explain why reversing both the current and the magnetic field leaves the force in the same direction.

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Method: Two reversals.

Each reversal reverses the force, so two reversals cancel
43
ChallengeCore[4 marks]

A rod rests on two horizontal rails in a magnetic field. When current flows it rolls. Suggest two changes that would make it roll faster and one that would make it roll the opposite way.

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Method: Force factors.

Faster: larger current or stronger magnet; opposite: reverse the current (or the field)
44
ChallengeExtended[4 marks]

A horizontal wire carries current to the east in a magnetic field directed north. Use Fleming's left-hand rule to find the direction of the force.

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Method: First finger north (field), second finger east (current); the thumb points up.

Vertically upwards
45
ChallengeExtended[4 marks]

A beam of electrons travels to the right in a magnetic field directed out of the page. Find the direction of the force on the electrons, explaining the use of conventional current.

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Method: Electrons moving right are a conventional current to the left. First finger out of the page, second finger to the left: thumb points up.

Upwards
46
ChallengeExtended[4 marks]

Explain why a beam of charged particles moves in a circle in a uniform magnetic field and why its speed does not change.

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Method: Force ⊥ velocity.

The force is always at right angles to the motion, providing a centripetal force; no work is done, so the speed is constant
47
ChallengeExtended[4 marks]

Protons and electrons enter the same magnetic field at the same speed. Compare their deflections.

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Method: Opposite charge, mass.

They curve in opposite directions; electrons curve much more tightly because their mass is far smaller
48
ChallengeExtended[4 marks]

Explain how the deflection of radiation in a magnetic field can show which of alpha, beta and gamma is present.

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Method: Deflection direction and size.

Gamma is undeflected; alpha and beta curve in opposite directions; beta curves much more because it is far lighter
49
ChallengeExtended[4 marks]

A wire in a magnetic field experiences an upward force of 0.050 N. State the force if (a) the current doubles, (b) the current and field both reverse, (c) the wire is turned parallel to the field.

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Method: Proportions.

(a) 0.10 N upward (b) 0.050 N upward (c) zero
50
ChallengeExtended[3 marks]

State Fleming's left-hand rule and explain how it differs in purpose from the right-hand rule.

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Method: Motor vs generator.

Left hand: thumb force, first finger field, second finger current — for motors (force on current); the right-hand rule gives induced current for generators
All IGCSE Physics topics← 4.5.3 Magnetic Effect of a Current4.5.5 The d.c. Motor →

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