Electromagnetic inductionAQA A-Level Physics: Subtopic test

1. 1
   In a laboratory demonstration, the north pole of a bar magnet is pushed steadily towards, and into, a coil of wire that is connected to a sensitive voltmeter.

   (a)

   What is the magnetic polarity of the end of the coil nearest the approaching magnet while the magnet is moving in?

   [1 mark]

   • ASouth pole
   • BNorth pole
   • CNo pole, since the magnet is not touching the coil
   • DAlternately north and south

   (b)

   The magnet is now pushed into the coil more quickly. What is the effect on the induced emf?

   [1 mark]

   • AIt is smaller, because the magnet is near the coil for less time
   • BIt is unchanged, because the total change of flux is the same
   • CIt is zero, because the magnet does not touch the coil
   • DIt is larger, because the flux linkage changes more quickly

   (c)

   Explain, using Lenz's law, why work must be done to push the magnet into the coil when the coil is part of a complete circuit.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A coil of 300 turns and cross-sectional area 4.0 × 10⁻³ m² is placed with its plane perpendicular to a uniform magnetic field. The flux density of the field decreases uniformly from 0.80 T to 0.20 T in 0.15 s.

   (a)

   What is the magnitude of the induced emf in the coil?

   [1 mark]

   • A0.016 V
   • B0.72 V
   • C4.8 V
   • D6.4 V

   (b)

   Which statement describes the magnetic field produced by the induced current in the coil?

   [1 mark]

   • AIt is in the same direction as the original field, opposing the decrease in flux
   • BIt is in the opposite direction to the original field
   • CIt is zero, because the original field is uniform
   • DIt is at right angles to the original field

   (c)

   Explain why the induced emf has a constant value during the 0.15 s.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An aircraft with a horizontal metal wing of span 36 m flies horizontally at a constant speed of 250 m s⁻¹. The vertical component of the Earth's magnetic flux density where it is flying is 4.5 × 10⁻⁵ T.

   (a)

   Show that the emf induced between the wing tips is Blv, where l is the span of the wing and v is the speed.

   [3 marks]

   (b)

   Calculate the emf between the wing tips at 250 m s⁻¹. The aircraft then slows to 200 m s⁻¹ in the same field. Calculate the new emf and the percentage change.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A generator has a rectangular coil of 60 turns and area 5.0 × 10⁻³ m² which is rotated at a constant 50 revolutions per second in a uniform magnetic field of flux density 0.25 T. The ends of the coil are connected to an external circuit through slip rings. Time is measured from the instant when the plane of the coil is perpendicular to the field.

   (a)

   Explain, using Faraday's and Lenz's laws, how an emf is induced as the coil rotates, and describe how it varies with time over one revolution.

   [6 marks]

   (b)

   Calculate the peak emf and the emf at t = 3.0 ms. State the effect on the peak emf and the frequency of the emf of doubling the rotation rate.

   [6 marks]

   Total for question 4: 12 marks

End of questions