Fig. 34.1 shows a simple experimental setup to investigate the factors affecting the induced e.m.f. in electromagnetic induction. A coil of wire is connecte...
Question 1 Report
Fig. 34.1 shows a simple experimental setup to investigate the factors affecting the induced e.m.f. in electromagnetic induction. A coil of wire is connected to a data logger which measures the peak e.m.f. A bar magnet is plunged into the coil at a controlled speed using a mechanical arm.
The student records the following results.
| experiment | speed v | number of turns | magnet strength | peak e.m.f. / mV |
|---|---|---|---|---|
| 1 | slow | 50 | weak | 4 |
| 2 | fast | 50 | weak | 12 |
| 3 | slow | 100 | weak | 8 |
| 4 | slow | 50 | strong | 10 |
(a) Compare experiments 1 and 2. State the factor being investigated and the conclusion. [2]
(b) Compare experiments 1 and 3. State the conclusion. [1]
(c) Compare experiments 1 and 4. State the conclusion. [1]
(d) Explain why a faster speed produces a larger e.m.f. [2]
(e) Predict the peak e.m.f. for a fast speed with 100 turns and a weak magnet. [1]
(f) Name the law which states that the induced e.m.f. opposes the change producing it. [1]
(g) State the energy transfer that occurs during this experiment. [1]
Answer Details
(a) The factor being investigated is the speed of the magnet (experiments 1 and 2 differ only in speed while turns and magnet strength are kept constant). [1] Increasing the speed increases the induced e.m.f.: from 4 mV (slow) to 12 mV (fast), a threefold increase. [1]
This is a fair test because only one variable (speed) is changed while the others (50 turns, weak magnet) are controlled. The result shows that faster movement of the magnet through the coil produces a larger e.m.f.
(b) Doubling the number of turns doubles the induced e.m.f.: from 4 mV (50 turns) to 8 mV (100 turns), with speed and magnet strength kept the same. [1]
Each turn of wire acts as an independent source of e.m.f. When the number of turns doubles, the total e.m.f. doubles because the individual contributions add together.
(c) Using a stronger magnet increases the induced e.m.f.: from 4 mV (weak) to 10 mV (strong), with speed and number of turns kept the same. [1]
A stronger magnet provides more magnetic flux through the coil. For the same rate of movement, the rate of change of flux is greater, producing a larger induced e.m.f.
(d) A faster speed means the magnetic flux through the coil changes more rapidly. [1] The induced e.m.f. is proportional to the rate of change of flux (Faraday's law), so a faster change produces a larger e.m.f. [1]
When the magnet moves quickly, the coil experiences the same total change in flux but over a shorter time. Since e.m.f. = rate of change of flux, compressing the same flux change into less time gives a proportionally larger e.m.f.
(e) Approximately 24 mV. [1]
From experiment 2, fast speed with 50 turns and a weak magnet gives 12 mV. Doubling the turns from 50 to 100 (as shown by comparing experiments 1 and 3) doubles the e.m.f. Therefore: \(12 \times \frac{100}{50} = 24\) mV.
(f) Lenz's law. [1]
Lenz's law states that the induced e.m.f. is always in such a direction as to oppose the change producing it. It is a consequence of the conservation of energy: the induced current must create effects that resist the motion of the magnet, otherwise energy would be created from nothing.
(g) Kinetic energy (of the moving magnet) is transferred to electrical energy (in the coil circuit). [1]
The mechanical arm does work pushing the magnet into the coil against the opposing force predicted by Lenz's law. This mechanical/kinetic energy is converted into electrical energy as the induced e.m.f. drives a current through the circuit.
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