Electromagnetic inductionEdexcel A-Level Physics: Subtopic test

1. 1
   A bar magnet is pushed, north pole first, into one end of a long solenoid that is connected to a sensitive centre-zero voltmeter.

   (a)

   Which change would increase the peak reading on the voltmeter?

   [1 mark]

   • APushing the magnet in more slowly
   • BPushing the magnet in more quickly
   • CUsing a solenoid with fewer turns
   • DHolding the magnet stationary inside the solenoid

   (b)

   Which statement describes the magnetic pole produced at the end of the solenoid nearest the approaching north pole, according to Lenz's law?

   [1 mark]

   • AA south pole, because it attracts the magnet and helps it in
   • BNo pole, because the induced current is too small
   • CA north pole, because the induced current must oppose the motion of the magnet
   • DA pole that alternates between north and south as the magnet moves

   (c)

   Explain, using the principle of conservation of energy, why the induced current must produce a magnetic field that opposes the movement of the magnet.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A flat coil of 400 turns and cross-sectional area 2.5 × 10⁻³ m² is placed with its plane perpendicular to a uniform magnetic field. The flux density falls uniformly from 0.80 T to 0.20 T in 0.30 s.

   (a)

   What is the magnitude of the average e.m.f. induced in the coil?

   [1 mark]

   • A2.0 V
   • B5.0 × 10⁻³ V
   • C3.3 V
   • D0.60 V

   (b)

   What does the minus sign in Faraday's law, E = −d(NΦ)/dt, represent?

   [1 mark]

   • AThe e.m.f. is always negative when the flux linkage decreases
   • BThe induced e.m.f. always acts in a clockwise direction
   • CEnergy is lost as heat in the coil
   • DThe induced e.m.f. acts in a direction that opposes the change in flux linkage that produces it

   (c)

   The same fall in flux density, from 0.80 T to 0.20 T, now takes 0.60 s instead of 0.30 s. Calculate the new average induced e.m.f. and explain your answer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A flat rectangular coil of 50 turns and area 6.0 × 10⁻³ m² is held with its plane perpendicular to a uniform magnetic field of flux density 0.40 T. It is rotated through 90° about an axis lying in its plane, in a time of 0.050 s, so that its plane finishes parallel to the field lines.

   (a)

   Calculate the initial flux linkage of the coil and the average e.m.f. induced in it during the rotation.

   [3 marks]

   (b)

   The coil is connected to a resistor so that the total resistance of the circuit is 8.0 Ω. Calculate the average current during the 0.050 s rotation. The rotation is then repeated in 0.025 s. Explain, with calculation, why the total charge passing round the circuit is unchanged.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two coils, P and S, are wound on opposite sides of a closed soft iron ring. Coil P is connected in series with a battery and a switch. Coil S is connected to a sensitive centre-zero voltmeter. The switch is initially open.

   (a)

   Describe and explain what the voltmeter shows when the switch is closed, while it remains closed, and when it is opened again.

   [6 marks]

   (b)

   The student wants a larger induced e.m.f. in coil S. Evaluate three changes that could be made, and conclude which is the most practical.

   [6 marks]

   Total for question 4: 12 marks

End of questions