Edexcel IGCSE Physics: Magnetism and Electromagnetism — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Magnetism and Electromagnetism revision notes

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Section A

1. State Fleming’s left-hand rule and say what it applies to. [3]

2. Explain why a transformer cannot work with direct current. [2]

3. State two ways of increasing the strength of an electromagnet. [2]

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Section B

4. A current-carrying wire is held perpendicular to a magnetic field and experiences a force.

(a) Name the rule used to predict the direction of the force, and state what each of its three fingers/thumb represents. [2]

(b) State the force if the wire is rotated to lie parallel to the field, with a reason. [2]

(c) State two ways of reversing the direction of the force. [2]

5. A simple d.c. motor uses a split-ring commutator.

(a) Explain the function of the split-ring commutator. [3]

(b) Explain what would happen without it. [2]

(c) State two ways of increasing the turning effect. [2]

6. A transformer has 800 primary turns and 40 secondary turns. The primary voltage is 240 V.

(a) Calculate the secondary voltage. [2]

(b) State whether it is a step-up or step-down transformer. [1]

(c) Assuming 100% efficiency, calculate the primary current when the secondary current is 5.0 A. [3]

(d) Describe the structure of a transformer, and explain what happens to the secondary (output) voltage if the number of turns on the secondary coil is increased while the number of primary turns stays the same. [4]

7. Explain why electricity is transmitted across the National Grid at very high voltage. [3]

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Section C

8. A bar magnet is made from one type of material; an electromagnet’s core is made from another.

(a) Distinguish between magnetically hard and magnetically soft materials. [2]

(b) Explain why each material is suited to its respective use. [2]

9. A straight wire and a solenoid both carry a current.

(a) Describe the difference between the magnetic field pattern around a straight current-carrying wire and the field pattern inside a solenoid. [2]

(b) A coil generates electricity as a magnet rotates inside it. State two factors, other than the strength of the magnet, that increase the size of the induced e.m.f. [2]

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Answers

1. First finger = field, seCond finger = current, thuMb = motion [1] [1]. It applies to the motor effect — the force on a current-carrying conductor in a magnetic field [1].

2. Direct current produces a constant magnetic flux in the core [1], so there is no change of flux to induce an e.m.f. in the secondary coil [1].

3. Any two: increase the current [1]; increase the number of turns; add a soft iron core [1].

4. (a) Fleming’s left-hand rule [1] — First finger = Field, seCond finger = Current, thuMb = Motion [1].

(b) Zero [1], because there is no component of the current perpendicular to the field — the force is maximum when they are perpendicular and zero when parallel [1].

(c) Reverse the current [1]; reverse the magnetic field [1]. Reversing both leaves the force unchanged.

5. (a) It reverses the direction of the current in the coil [1] every half turn [1], so the force on each side of the coil always acts in the same rotational direction and the coil continues to rotate [1].

(b) After half a turn the force on each side would act in the opposite rotational sense [1], so the coil would oscillate back and forth rather than rotate continuously [1].

(c) Any two: increase the current [1]; increase the number of turns; use a stronger magnet; increase the area of the coil [1].

6. (a) V_s = V_p × (N_s ÷ N_p) = 240 × (40 ÷ 800) [1] = 12 V [1].

(b) Step-down [1].

(c) I_pV_p = I_sV_s [1] I_p = (5.0 × 12) ÷ 240 [1] = 0.25 A [1].

(d) A transformer has a primary coil and a secondary coil, each wound around a common soft iron core, with no direct electrical connection between the two coils [1]. The alternating current in the primary coil produces a changing magnetic flux in the core [1], which links the secondary coil and induces an alternating e.m.f. (voltage) in it [1]. Increasing the number of secondary turns (while the primary turns stay the same) increases the secondary voltage, since Vₛ ÷ Vₚ = Nₛ ÷ Nₚ — more secondary turns relative to primary turns gives a larger output voltage [1].

7. Power loss in the cables is I²R [1]. Transmitting at high voltage means a much lower current for the same power [1], and since loss depends on the square of the current, the energy wasted is greatly reduced [1]. The explanation must be in terms of current, not voltage.

8. (a) A magnetically hard material retains its magnetism once magnetised [1]; a magnetically soft material loses its magnetism easily [1].

(b) Hard materials suit permanent magnets, since they must stay magnetised without a continuing current [1]; soft materials suit electromagnet cores, since they must be magnetised and demagnetised rapidly as the current switches on and off [1].

9. (a) A straight wire produces concentric circular field lines around it [1]; a solenoid produces a strong, mostly uniform field inside it, resembling that of a bar magnet [1].

(b) Any two: increasing the speed of relative motion between the magnet and coil [1]; increasing the number of turns on the coil [1].

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Where marks are usually lost

• Using the right-hand rule for the motor effect.
• Forgetting the force is zero when the conductor is parallel to the field.
• Not knowing the commutator’s function.
• Forgetting that the turns-ratio equation links voltage and number of turns, not current, and mixing up which side is primary/secondary.
• Explaining transmission losses in terms of voltage rather than I²R.
• Confusing magnetically hard materials (permanent magnets) with magnetically soft materials (electromagnet cores).
• Describing the field inside a solenoid as circular rather than uniform.