Magnetic flux density and force on a conductorAQA A-Level Physics: Subtopic test

1. 1
   A technician places a straight horizontal copper rod between the poles of a magnet. The rod carries a conventional current of 6.0 A towards the north. The magnet provides a uniform vertical magnetic field, directed downwards, with flux density 0.24 T over a 0.050 m length of the rod; the rod is at right angles to the field.

   (a)

   What is the direction of the magnetic force on the rod?

   [1 mark]

   • ANorth
   • BEast
   • CWest
   • DVertically upwards

   (b)

   What is the magnitude of the force on the rod?

   [1 mark]

   • A0.80 N
   • B0.072 N
   • C29 N
   • D0.0072 N

   (c)

   The current is halved and the length of rod in the field is doubled, with the flux density unchanged. Deduce the new force on the rod and its direction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student investigates the force on a wire using a top pan balance. A magnet assembly rests on the pan, with a rigid horizontal wire clamped so that 0.040 m of it lies at right angles to the uniform field between the poles, without touching the magnet. The balance is set to zero with no current. When a current of 2.5 A is switched on, the balance reading changes. The flux density between the poles is 0.095 T. Take g = 9.81 N kg⁻¹.

   (a)

   Why does the balance reading change when the current is switched on?

   [1 mark]

   • AThe weight of the wire increases
   • BThe wire has become a permanent magnet
   • CThe current heats the magnet assembly, expanding it
   • DThe wire exerts an equal and opposite force on the magnet assembly

   (b)

   What change in balance reading, in grams, does the force on the wire cause?

   [1 mark]

   • A0.97 g
   • B9.5 g
   • C0.0095 g
   • D97 g

   (c)

   The student repeats the experiment for several currents with the length of wire in the field fixed, and plots the balance reading in kilograms against current. State the shape of the graph and explain how the flux density can be found from it.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A straight horizontal conductor of length 0.15 m and mass 8.0 g hangs from two very flexible leads in a uniform horizontal magnetic field of flux density 0.060 T, which is at right angles to the conductor. Current in the conductor is arranged so that the magnetic force on it acts vertically upwards. Take g = 9.81 N kg⁻¹.

   (a)

   Calculate the current required for the magnetic force on the conductor to balance its weight.

   [3 marks]

   (b)

   The current is now increased by 20% from its balancing value. Calculate the initial acceleration of the conductor and state its direction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A light, smooth conducting rod of length 0.20 m and mass 0.050 kg rests across two long, horizontal, parallel conducting rails and can slide along them. The rails lie in a uniform vertical magnetic field of flux density 0.080 T. A battery connected to the rails supplies a constant current of 4.0 A through the rod, which is initially at rest.

   (a)

   Describe how the direction and size of the force on the rod are determined, and state how the tesla is defined in terms of this force.

   [6 marks]

   (b)

   Calculate the speed of the rod 1.5 s after the current is switched on, and suggest two reasons why the speed measured in practice would be less than this.

   [6 marks]

   Total for question 4: 12 marks

End of questions