7.7 Generators: alternators and dynamos
10 exam questions worth 24 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is the energy transfer in a generator?
Choose one answer.
Correct Not quite: the answer is Kinetic to electrical
Mark scheme
B: Kinetic to electrical
Model answer
Kinetic to electrical (B).
Which part of a generator makes its output direct current?
Choose one answer.
Correct Not quite: the answer is A split-ring commutator
Mark scheme
B: A split-ring commutator
Model answer
A split-ring commutator (B).
Explain why a potential difference is induced across the coil of a generator when it turns.
Mark scheme
the coil rotates in a magnetic field (1); the sides of the coil cut the magnetic field lines (1); so a potential difference is induced (electromagnetic induction) (1)
Model answer
The coil turns in the magnetic field, so its sides cut the field lines. By electromagnetic induction, a potential difference is induced.
Describe how an alternator produces an alternating current.
Mark scheme
as the coil rotates the induced p.d. reverses every half turn (1); slip rings keep each brush connected to the same end of the coil (1); so the output current alternates in direction (1)
Model answer
The induced p.d. reverses every half turn. Slip rings keep each brush connected to the same end of the coil, so the output current alternates.
State the difference between the output of an alternator and a dynamo.
Mark scheme
alternator: a.c. / changes direction (1); dynamo: d.c. / one direction (1)
Model answer
An alternator gives alternating current, while a dynamo gives direct current.
State what the p.d. is at the point when the coil is at right angles to the magnetic field, and explain why.
Mark scheme
zero (1); the sides of the coil are moving parallel to the field lines, so they do not cut any field lines (1)
Model answer
It is zero, because the sides of the coil are moving parallel to the field lines and so are not cutting them.
A student doubles the speed at which the coil of an alternator rotates. Describe two changes to the graph of the output.
Mark scheme
the peak p.d. is bigger / doubles (1); the frequency is higher / doubles / more cycles in the same time (1)
Model answer
The peak potential difference is bigger (doubled), and the frequency doubles.
The coil of an alternator rotates 50 times per second. State the frequency of the output and calculate the period.
Mark scheme
frequency 50 Hz (1); T = 1 ÷ f = 1 ÷ 50 (1); 0.020 s (1)
Model answer
The frequency is 50 Hz. T = 1 / f = 1 / 50 = 0.020 s.
Give three ways of increasing the potential difference produced by a generator.
Mark scheme
rotate the coil faster (1); use stronger magnets (1); use more turns on the coil / a bigger coil (1)
Model answer
Rotate the coil faster, use stronger magnets, or use more turns on the coil.
Describe how a bicycle dynamo lights a lamp and explain why the lamp gets brighter when the cyclist goes faster.
Mark scheme
the wheel turns a magnet / coil in a magnetic field (1); electromagnetic induction produces a potential difference (1); the current lights the lamp (1); faster rotation cuts the field lines at a greater rate, so the p.d. is larger and the lamp is brighter (1)
Model answer
The wheel turns a magnet near a coil, so electromagnetic induction produces a p.d. that lights the lamp. When the cyclist goes faster, the field lines are cut at a higher rate, so the p.d. is bigger and the lamp is brighter.
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