OCR A Physics · Module 6: Particles and medical physics
Electromagnetism: practice questions with answers
6 exam-style questions on electromagnetism. Try each one on paper first, then open the answer to see the full working and the common slips that cost marks.
- Resolving vectors3 marks
A straight wire of length 0.35 m carries a current of 4.5 A in a uniform magnetic field of flux density 0.12 T. The wire makes an angle of 40° to the field. Calculate the force on the wire, in mN.
F = mN
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Answer F = 122 mN
F = BIL sin θ = 0.12 × 4.5 × 0.35 × sin 40° = 0.121 N = 121 mN.
- Got 145 mN? You used cos 40°. F = BIL sin θ, where θ is the angle between the wire and the field: sin 40° = 0.643, so F = 121 mN.
- Got 189 mN? You ignored the angle. F = BIL sin θ = 0.35 × 4.5 × 0.12 × sin 40° = 121 mN.
- Unit prefixes3 marks
An electron moves at 3.0 × 10⁶ m s⁻¹ perpendicular to a uniform magnetic field of flux density 25 mT. Calculate the radius of its circular path, in mm.
r = mm
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Answer r = 0.683 mm
Bev = mv²/r so r = mv ÷ Be = (9.11 × 10⁻³¹ × 3.0 × 10⁶) ÷ (25 × 10⁻³ × 1.6 × 10⁻¹⁹) = 0.683 mm.
- Got 6.83 × 10⁻⁴ mm? You used 25 T instead of 25 mT. B = 25 × 10⁻³ T, so r = mv ÷ Be = 0.683 mm.
- Got 1.37 mm? That is the diameter. Setting Bev = mv²/r gives the radius r = mv ÷ Be = 0.683 mm.
- Unit prefixes3 marks
A coil of 250 turns has area 35 cm². A uniform magnetic field of 0.40 T through the coil, perpendicular to its plane, is switched off in 0.050 s. Calculate the average induced e.m.f.
ε = V
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Answer ε = 7 V
ΔΦ = BA = 0.40 × 3.5 × 10⁻³ = 1.4 × 10⁻³ Wb. ε = NΔΦ ÷ Δt = 250 × 1.4 × 10⁻³ ÷ 0.050 = 7 V.
- Got 70 V? You converted cm² using 10⁻². 1 cm² = 10⁻⁴ m², so A = 3.5 × 10⁻³ m² and ε = NΔΦ ÷ Δt = 7 V.
- Got 0.028 V? You left out the number of turns. Flux linkage is NΦ, so ε = 250 × 0.40 × 3.5 × 10⁻³ ÷ 0.050 = 7 V.
- Rearranging equations2 marks
An ideal transformer has 1200 turns on its primary coil, connected to a 230 V supply. The secondary p.d. is 60 V. Calculate the number of turns on the secondary coil.
Ns = turns
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Answer Ns = 313 turns
Vs/Vp = Ns/Np, so Ns = 1200 × 60 ÷ 230 = 313, which is about 313 turns.
- Got 4600 turns? You inverted the ratio. Vs/Vp = Ns/Np, so Ns = 1200 × 60 ÷ 230 = 313 turns.
- Got 424 turns? You used the difference in p.d. in the denominator. The turns ratio equals the p.d. ratio: Ns = 1200 × 60 ÷ 230 = 313 turns.
- Using the right equation1 mark
What does Lenz's law state?
- A The induced e.m.f. is proportional to the magnetic flux.
- B The induced current flows in a direction that opposes the change causing it.
- C The induced current always flows in the same direction as the original current.
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Answer B: The induced current flows in a direction that opposes the change causing it.
Lenz's law, a consequence of conservation of energy, says the induced current opposes the change that produces it.
- Why not A? The e.m.f. is proportional to the rate of change of flux linkage, not the flux itself.
- Why not C? The induced current opposes the change, so it is not necessarily in the same direction.
- Using the right equation1 mark
A coil rotates at constant speed in a uniform magnetic field. When is the induced e.m.f. at its maximum?
- A When the plane of the coil is perpendicular to the field.
- B When the coil is at 45° to the field.
- C When the plane of the coil is parallel to the field.
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Answer C: When the plane of the coil is parallel to the field.
With the plane parallel to the field the flux linkage is zero but changing fastest, so the e.m.f. is a maximum.
- Why not A? Flux linkage is maximum here but its rate of change is zero, so the e.m.f. is zero.
- Why not B? The rate of change of flux linkage at 45° is less than the maximum, so the e.m.f. is not at its peak.