1. 1
   A cyclist fits a small dynamo to her bicycle wheel so that, as the wheel turns, a magnet inside the dynamo spins near a coil of wire, lighting the bicycle's front lamp.

   (a)

   Which effect explains why a potential difference is produced across the coil as the magnet spins?

   [1 mark]

   • AThe generator effect: a potential difference is induced across a conductor when it experiences a changing magnetic field
   • BThe motor effect: a current-carrying conductor experiences a force in a magnetic field
   • CInduced magnetism: the coil itself becomes a permanent magnet
   • DStatic discharge between the magnet and the coil

   (b)

   The cyclist notices that when she pedals faster, the bicycle lamp glows more brightly. Which statement correctly explains this observation?

   [1 mark]

   • APedalling faster has no effect on the potential difference induced across the coil
   • BPedalling faster increases the speed at which the magnet moves relative to the coil, increasing the induced potential difference
   • CPedalling faster reduces the number of turns on the coil, decreasing the induced potential difference
   • DPedalling faster reverses the polarity of the induced potential difference, making the lamp brighter

   (c)

   Suggest two other changes (other than pedalling faster) the cyclist could make to the dynamo's design that would increase the size of the potential difference induced across the coil, explaining why each would have this effect.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A wind farm engineer connects an oscilloscope across the output terminals of a turbine's alternator while the turbine is turning at a steady rate.

   (a)

   Which statement correctly describes the type of current generated by an alternator, and the shape of the trace expected on the oscilloscope?

   [1 mark]

   • AAn alternator generates alternating current, but the oscilloscope trace shows a constant pd of one polarity only
   • BAn alternator generates direct current (dc); the oscilloscope trace shows a constant, unchanging pd
   • CAn alternator generates alternating current (ac); the oscilloscope trace shows a pd that regularly reverses direction, forming a repeating wave pattern
   • DAn alternator produces no measurable potential difference at all

   (b)

   The engineer then examines the output of a different generator fitted with a split-ring commutator instead of slip rings. Which type of current does a generator with a split-ring commutator (a dynamo) produce?

   [1 mark]

   • AA current that reverses direction only once per hour
   • BAlternating current, regularly reversing direction
   • CNo current, since a commutator blocks all current flow
   • DDirect current, always flowing in the same direction

   (c)

   Sketch in words the difference between a graph of potential difference against time for the alternator and one for the dynamo, and explain how the split-ring commutator produces this difference in the dynamo's output compared with the alternator's slip rings.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A tidal power station uses the movement of sea water to turn large turbines connected to generators, in a similar way to a wind farm.

   (a)

   Explain, using the generator effect, how moving the turbine coil relative to a magnetic field produces a potential difference, and explain what would happen to the current if the coil and circuit formed a complete loop.

   [3 marks]

   (b)

   During a change of tide, the direction of the water flow reverses, causing the turbine to spin in the opposite direction. Explain the effect this has on the potential difference induced across the generator's coil, and suggest one design change engineers could make to the generator to increase the pd it produces for a given turbine speed.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A power station burns natural gas to boil water into high-pressure steam, which is directed onto the blades of a turbine, causing it to spin. The turbine shaft is connected directly to the rotor (coil) of a large alternator housed within a strong magnetic field.

   (a)

   Explain, step by step, how the chemical energy in the gas is ultimately converted into electricity delivered to the National Grid, referring to the role of the boiler, turbine and generator in your answer.

   [6 marks]

   (b)

   Engineers plan to upgrade the power station by fitting a more powerful alternator with a stronger magnetic field and more turns on the rotor coil, while keeping the turbine's rotational speed the same. Explain what effect these changes would have on the size of the potential difference induced by the generator, and evaluate one advantage and one drawback of using natural gas, rather than a renewable resource such as wind, to drive the turbine in the first place.

   [6 marks]

   Total for question 4: 12 marks

End of questions