GENERATORS
4 marks · NormalA small dynamo attached to a bicycle wheel generates electricity for the bike’s lights. The speed of the bicycle and the voltage output were recorded:
| Speed (m/s) | Voltage (V) |
|---|---|
| 2.0 | 0.5 |
| 3.0 | 0.9 |
| 4.0 | 1.2 |
| 5.0 | 1.5 |
Describe the trend shown by the data.
If the cyclist increases her speed from 2.0 m/s to 5.0 m/s, by what factor does the voltage across the dynamo increase?
Explain why increasing the speed of the bicycle causes the voltage to increase.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) As speed increases, voltage increases.
(b) 3 (from 0.5 V to 1.5 V, the voltage triples)
(c) Increasing speed means the dynamo cuts magnetic field lines more quickly, so a greater voltage is induced (rate of field line cutting is greater).
Common mistakes: Incorrect ratio calculation; Incomplete explanation of line cutting
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states that as speed increases, voltage increases.
B2 (b) · 1 mark · Correctly calculates or states that the voltage increases by a factor of 3.
M1 (c) · 1 mark · Explains that more field lines are cut per unit time at higher speed.
A1 (c) · 1 mark · Concludes that this increases the induced voltage.
Worked solution
a. The voltage increases as the speed increases. b. At 2.0 m/s: V = 0.5; at 5.0 m/s: V = 1.5. Factor = 1.5 / 0.5 = 3. c. At a higher speed, the dynamo moves through the magnetic field faster, cutting more field lines each second. This increases the rate of electromagnetic induction, so the voltage is greater.