1. 1
   A hand-cranked emergency radio contains a simple a.c. generator in which turning a handle rotates a coil inside a fixed magnetic field, producing the electricity needed to power the radio.

   (a)

   What causes the e.m.f. to be induced in the coil as the handle is turned?

   [1 mark]

   • AThe coil becomes permanently magnetised as it turns
   • BThe handle directly supplies charge to the coil
   • CThe magnetic field itself is being physically turned by the handle
   • DThe coil is moving relative to the magnetic field as it rotates, changing the field linking the coil

   (b)

   The user turns the handle at a constant, steady speed. What type of current does this simple generator produce?

   [1 mark]

   • AA direct current that flows in one direction only
   • BNo current at all, since the coil is not connected to a battery
   • CAn alternating current that periodically reverses direction
   • DA current that only flows during the first half rotation of the handle

   (c)

   Describe how sketching a graph of the generator's e.m.f. against time, for the handle being turned at a constant speed, would show that the output is alternating current.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A power station uses a large rotating coil generator connected to the external circuit and the electricity grid via slip rings and brushes, and an engineer explains their function to a trainee.

   (a)

   A power station uses a large rotating coil generator, in which slip rings and brushes are used to connect the rotating coil to the external circuit. What is the purpose of the slip rings and brushes in this arrangement?

   [1 mark]

   • AThey store electrical energy generated during peak output for later release
   • BThey convert the a.c. output directly into d.c. before it leaves the generator
   • CThey increase the strength of the magnetic field inside the generator
   • DThey allow a continuous electrical connection to be maintained between the rotating coil and the stationary external circuit

   (b)

   The trainee asks how the position of the coil relates to the shape of the e.m.f.–time graph. At the instant the plane of the coil is parallel to the magnetic field lines (so its sides are moving at right angles to the field lines, cutting through them at the greatest rate), what is happening to the induced e.m.f.?

   [1 mark]

   • AThe e.m.f. is negative and at its most negative value
   • BThe e.m.f. is exactly zero
   • CThe e.m.f. is at its peak (maximum) value
   • DThe e.m.f. cannot be determined from the coil's position

   (c)

   State what is happening to the induced e.m.f., and to the position of the coil relative to the magnetic field, at the point where the e.m.f.–time graph crosses zero.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student uses a hand-cranked model a.c. generator and a data logger to sketch the e.m.f.–time graph produced as the coil is turned at a constant speed over exactly two full rotations.

   (a)

   A student sketches the e.m.f.–time graph for a simple a.c. generator being turned at a constant speed over exactly two full rotations of the coil. Describe the overall pattern of this graph, including the number of complete positive-to-negative cycles shown and the general shape between the peaks and the zero crossings.

   [3 marks]

   (b)

   The student then turns the handle at a much faster, but still constant, speed and sketches a new e.m.f.–time graph over the same total time as before. Explain two ways in which this new graph would differ from the original graph, referring to the peak e.m.f. and the number of cycles shown in the same time period.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the late 19th century, engineers built early rotating-coil a.c. generators using slip rings and carbon brushes to connect the rotating coil to external wiring, a design that is still used in the generators of modern power stations today.

   (a)

   Explain how the slip ring and brush arrangement allows a continuous electrical connection to a rotating coil, why brushes gradually wear down and require maintenance, and how the resulting e.m.f.–time graph relates to the coil's rotation throughout a full cycle.

   [6 marks]

   (b)

   An engineer proposes modifying the power station generator to rotate the magnet instead of the coil, keeping the coil fixed in place and mounting the electromagnet on the rotating shaft instead, supplying the electromagnet's current through slip rings and brushes rather than taking the generated a.c. output through them. Explain why this alternative arrangement can still produce the same alternating output from the fixed coil, and explain one practical advantage this arrangement might have over rotating the main output coil itself, in terms of the currents that the slip rings and brushes must carry.

   [6 marks]

   Total for question 4: 12 marks

End of questions