O Level Physics: Electricity and magnetism — Practice Questions (Cambridge 5054)

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Each question practises a skill tested in the June 2025 Paper 22. After each answer there is a tip and the real question to try next.

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Questions

1. A student wants to find the resistance of a length of metal wire using a battery, a switch, an ammeter, a voltmeter and connecting wires. Describe how the ammeter and the voltmeter must be connected, and state how the resistance is then calculated from their readings. [3]

2. A 40 cm length of metal wire has a potential difference of 2.0 V across it and a current of 0.50 A in it. (a) Calculate the resistance of the 40 cm length. (b) Another piece cut from this wire (same material and thickness) has a current of 0.50 A when the potential difference across it is 3.0 V. State whether this length is longer or shorter than 40 cm, and calculate its length. [4]

3. A room heater has a power of 1.5 kW and runs on a 230 V mains supply. The fuses available are 3 A, 5 A, 10 A and 13 A. Calculate the current in the heater, and choose the most suitable fuse, explaining your choice. [3]

4. A lighting circuit in a house is protected by a 5 A fuse. Each lamp on the circuit is connected in parallel and takes a current of 0.35 A. (a) Calculate the current in the fuse when 6 lamps are switched on. (b) Calculate the greatest number of these lamps that can be switched on at once without the fuse melting. [3]

5. (a) Explain how a fuse cuts off a circuit when too much current flows. (b) Give two reasons why a circuit breaker (trip switch) may be preferred to a fuse. [3]

6. A simple d.c. motor has a rectangular coil between the poles of two magnets. (a) Why does the motor need a split-ring commutator? (b) State what happens to the direction of rotation when only the current is reversed, and when both the current and the magnetic field are reversed together. (c) A straight wire in the motor carries a current towards the east, in a magnetic field pointing towards the north. State the direction of the force on the wire. [4]

7. In a d.c. motor, the coil is horizontal. The two sides of the coil are each 0.040 m from the axis of rotation. One side has an upward force of 1.6 N and the other side has a downward force of 1.6 N. (a) Find the combined moment of the two forces about the axis, with its unit. (b) As the coil rotates from horizontal towards vertical, its turning effect decreases. Explain why. [4]

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Answers

1. The ammeter in series with the wire, so the same current passes through both [1]. The voltmeter in parallel across the wire only, to measure the potential difference across it [1]. Resistance R = V ÷ I, the voltmeter reading divided by the ammeter reading [1].

Tip: also check that cells in the battery all point the same way; cells connected in opposite directions cancel out and give zero current.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 5.

2. (a) R = V ÷ I = 2.0 ÷ 0.50 = 4.0 Ω [1]. (b) New resistance = 3.0 ÷ 0.50 = 6.0 Ω, which is larger, so the wire is longer [1]. Resistance is proportional to length, so length = 40 × (6.0 ÷ 4.0) [1] = 60 cm [1].

Tip: for wire of the same material and thickness, resistance is directly proportional to length; set up a ratio rather than guessing.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 5.

3. I = P ÷ V = 1500 ÷ 230 [1] = 6.5 A [1]. Choose the 10 A fuse: it is the smallest rating above the normal current, so the heater works normally but the fuse melts quickly if a fault makes the current rise [1].

Tip: convert kW to W first; a 3 A or 5 A fuse would blow in normal use, and a 13 A fuse gives less protection than necessary.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 6.

4. (a) In parallel, the currents add: 6 × 0.35 = 2.1 A [1]. (b) 5 ÷ 0.35 = 14.3 [1], so the greatest whole number is 14 lamps [1].

Tip: round down, not up, when finding a maximum number: 15 lamps would take 5.25 A and melt the fuse.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 6.

5. (a) The fuse wire heats up and melts, which breaks the circuit and stops the current [1]. (b) Any two [2]: it can be reset instead of being replaced; it acts faster than a fuse; it is easy to see which circuit has switched off; there is no risk of fitting the wrong fuse.

Tip: “the fuse breaks” is too vague; say that the wire gets hot and melts, so the circuit is broken.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 6.

6. (a) It reverses the current in the coil every half turn so that the coil keeps turning in the same direction [1]. (b) Reversing only the current reverses the direction of rotation [1]; reversing both current and field together leaves the rotation in the same direction as before [1]. (c) Vertically upwards [1].

Tip: for Fleming’s left-hand rule, point the first finger along the field, the second finger along the current, and read the force from the thumb.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 7.

7. (a) Each force gives a moment of 1.6 × 0.040 = 0.064 N m, and both turn the coil the same way [1], so total moment = 2 × 0.064 = 0.128 N m (0.13 N m) [1]. (b) The forces stay vertical, so as the coil turns, the perpendicular distance from the line of each force to the axis gets smaller [1]; the moment (force × perpendicular distance) therefore decreases, and it becomes zero when the coil is vertical [1].

Tip: the upward and downward forces do not cancel as moments, because they are on opposite sides of the axis and both turn the coil in the same direction.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 7.

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Where marks are usually lost

• Putting the voltmeter in series or the ammeter in parallel.
• Choosing a fuse just below the normal current, or far above it.
• Rounding up instead of down when finding the maximum number of lamps.
• Saying the commutator “changes the direction of the coil” instead of reversing the current every half turn.
• Cancelling the two forces on a motor coil instead of adding their moments.