Magnetic flux density and force on a conductorAQA A-Level Physics: Mind map

Exam questions on Magnetic flux density and force on a conductor

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.
   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
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.
   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
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.
   Calculate the current required for the magnetic force on the conductor to balance its weight.3 marks