1. 1
   A scrapyard uses an electromagnetic crane to lift scrap metal. The crane's electromagnet is made from a coil of insulated wire wound around a soft iron core.

   (a)

   Which change would increase the strength of the magnetic field produced by the electromagnet?

   [1 mark]

   • AIncreasing the current flowing through the coil
   • BReplacing the iron core with a wooden core
   • CSpacing the turns of the coil further apart
   • DSwitching from direct current to a smaller alternating current

   (b)

   The operator switches off the current to the electromagnet so that the crane releases the scrap metal. What happens to the magnetism of the soft iron core immediately after the current is switched off?

   [1 mark]

   • AThe core keeps its full magnetic strength permanently
   • BThe core loses almost all of its magnetism very quickly
   • CThe core becomes a permanent magnet with reversed polarity
   • DThe core's magnetism gradually increases over several hours

   (c)

   Explain why shaping the coil of wire into a solenoid, rather than using a single straight wire, produces a much stronger magnetic field for the crane's electromagnet.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   At a science museum, a straight copper rod is placed between the poles of a strong horseshoe magnet so that the rod is at right angles to the magnetic field. A visitor presses a button and current flows through the rod.

   (a)

   Which rule can be used to predict the direction of the force on the rod?

   [1 mark]

   • AThe right-hand grip rule
   • BNewton's third law
   • CFleming's left-hand rule
   • DLenz's law of conservation

   (b)

   The visitor then presses a second button that reverses the direction of the current through the rod, while the magnet's poles stay the same. What happens to the force on the rod?

   [1 mark]

   • AThe force doubles in the same direction
   • BThe force disappears completely
   • CThe force stays exactly the same
   • DThe direction of the force reverses

   (c)

   State two factors, other than the direction of the current, that affect the size of the force produced on the rod.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An engineer is testing a straight conducting rod of length 0.40 m, carrying a current of 5.0 A, held at right angles to a magnetic field of flux density 0.25 T as part of a factory braking mechanism.

   (a)

   Calculate the force exerted on the rod.

   [3 marks]

   (b)

   The engineer then doubles the current in the rod to 10.0 A but replaces the magnet with one of half the original flux density (0.125 T), keeping the rod length the same. Calculate the new force on the rod, and state how this new force compares with the force calculated in part (a).

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The motor effect is used in many devices. A simple electric motor consists of a rectangular coil of wire mounted on an axle, placed between the poles of a permanent magnet, connected to a direct current supply via a split-ring commutator. A moving-coil loudspeaker contains a coil of wire attached to a paper cone, placed in the magnetic field of a permanent magnet.

   (a)

   Explain, in terms of forces on the current-carrying sides of the coil, how the motor produces continuous rotation, including the role of the split-ring commutator.

   [6 marks]

   (b)

   A moving-coil loudspeaker contains a coil of wire attached to a paper cone, placed in the magnetic field of a permanent magnet. Explain how the loudspeaker uses the motor effect to convert a varying electrical current from an audio amplifier into sound waves.

   [6 marks]

   Total for question 4: 12 marks

End of questions