Fig. 1 shows a bicycle dynamo beside a wheel. As the wheel turns, a magnet rotates close to a fixed coil. The lamp receives an alternating current and produ...
Question 1 Report
Fig. 1 shows a bicycle dynamo beside a wheel. As the wheel turns, a magnet rotates close to a fixed coil. The lamp receives an alternating current and produces light. The faster the cyclist travels, the brighter the lamp becomes.
(a) What is the name of the process that produces a current in the coil? [1]
(b) When is the induced potential difference greatest: when the wheel is stationary or when it is turning fastest? [1]
(c) Describe the change in the magnetic field at the coil as the magnet rotates. [2]
This circuit is used by students to investigate the motor effect in a science club. Fig. 1 shows a straight wire between the poles of two magnets. The current in the wire is into the page, shown by the cross. When the switch is closed, the wire moves upwards.
(a) What provides the magnetic field in this investigation? [1]
(b) Which change would make the wire move downwards instead? [1]
(c) Describe the energy transfer when the wire starts moving. [2]
Answer Details
Bicycle dynamo
- The process is electromagnetic induction: a changing magnetic field through a coil induces a potential difference and current. [1]
- The induced potential difference is greatest when the wheel is turning fastest, because the magnetic field changes most rapidly. [1]
- As the magnet rotates, the magnetic field through the coil changes. Its direction and/or strength changes repeatedly as different magnetic poles pass the fixed coil, producing alternating current. [2]
Motor effect investigation
- The permanent magnets provide the magnetic field. [1]
- To make the wire move downwards, reverse the current or reverse the magnetic field by swapping the magnet poles. Reversing one, but not both, reverses the force. [1]
- When the wire begins moving, electrical energy is transferred to the kinetic energy of the moving wire. [2]
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