Electromagnetic Induction (OCR A Level Physics): Structured Questions

A student conducts an experiment to confirm that the uniform magnetic flux density B between the poles of a magnet is 30 mT.

A current-carrying wire of length 5.0 cm is placed perpendicular to the magnetic field.

The current I in the wire is changed and the force F experienced by the wire is measured. Fig. 22.1 shows the graph plotted by the student.

Fig. 22.1

The student’s analysis is shown on the graph of Fig. 22.1 and in the space below.

Evaluate the information from Fig. 22.1 and the analysis of the data from the experiment. No further calculations are necessary.

Fig. 22.2 shows a transformer circuit.

Fig. 22.2

The primary coil is connected to an alternating voltage supply. A filament lamp is connected to the output of the secondary coil.

(i) Use Faraday’s law of electromagnetic induction to explain why the filament lamp is lit.

[3]

(ii) The primary coil has 400 turns and the secondary coil has 20 turns. The potential difference across the lamp is 12 V and it dissipates 24 W. The transformer is 100% efficient.

1. Calculate the current in the primary coil.

current = ................................. A [2]

2. The alternating voltage supply is replaced by a battery and an open switch in series. The switch is closed. The lamp is lit for a short period of time and then remains off.

Explain this observation.

[2]

Fig. 21.1 shows a coil of a simple generator rotating in a uniform magnetic field.

Fig. 21.1

The coil has 85 turns of insulated wire. The cross-sectional area of the coil is 14cm². Fig. 21.2 shows the variation of magnetic flux density B through the plane of the coil with time t as it rotates.

Fig. 21.2

i) Explain why the electromotive force (e.m.f.) induced across the ends of the coil is a maximum at the times when B = 0.

 [1]

ii) Draw a tangent to the curve in Fig. 21.2 when B = 0, and hence determine the maximum e.m.f. induced across the ends of the coil.

maximum e.m.f. = ...................................................... V [3]

Fig. 21.3 shows the variation of the e.m.f. induced across the ends of the coil with time t.

Fig. 21.3

The magnitude of the magnetic flux density of the uniform field is now halved and the coil is rotated at twice its previous frequency. On Fig. 21.3 sketch the new variation of the e.m.f. induced with time t.

[2]

State Faraday’s law of electromagnetic induction.

A rectangular coil of wire has turns and a cross-sectional area of .

The coil is placed in a uniform magnetic field of flux density .

The coil rotates about an axis perpendicular to the magnetic field.

Calculate the maximum magnetic flux linkage through the coil.

Fig. 21.1 shows the variation of magnetic flux linkage with time for the rotating coil.

Fig. 21.1

(i) Determine the maximum electromotive force (e.m.f.) induced in the coil.

[2]

(ii) On the axes provided in Fig. 21.2, sketch the corresponding variation of the induced e.m.f. with time .

Fig. 21.2

[3]

A magnet rotates inside a shaped soft iron core. A coil is wrapped around the iron core as shown in Fig. 5.1. The coil is connected to an oscilloscope.

Fig. 5.1	Fig. 5.2

The spinning magnet induces an e.m.f. in the coil. A graph of the e.m.f. displayed on the oscilloscope screen is shown in Fig. 5.2.

(i) Explain the shape of the graph in terms of the magnetic flux linking the coil.

[2]

(ii) On Fig. 5.3 sketch a graph of the magnetic flux linkage of the coil against time. The variation of the induced e.m.f. across the coil is shown as a dotted line.

[1]

Fig. 5.3

(iii) The coil shown in Fig. 5.1 has 150 turns. The maximum induced e.m.f. V₀ across the coil is 1.2 V when the magnet is rotating at 24 revolutions per second.

Calculate the maximum magnetic flux through the coil using the equation

Give a unit with your answer.

maximum flux = ..................................... unit ..............

[2]

A student is given a transformer with coils X and Y, as shown in Fig. 5.4.

Fig. 5.4

The student is intending to investigate how the maximum induced e.m.f. V₀ in coil Y depends on the frequency f of the alternating current in coil X.

The changing magnetic flux density in coil X induces an e.m.f. in coil Y. Faraday’s law indicates that the maximum induced e.m.f. V₀ should be directly proportional to f.

Describe how you would investigate the suggested relationship between V₀ and f in the laboratory using these coils. In your description include all of the equipment used and how you would analyse the data collected.

Use the space below to draw a suitable diagram.

Fig. 3.1 shows the design of a ‘mechanical’ torch.

Fig. 3.1

There is no battery in the torch. Instead, when the torch is inverted, the magnet falls a short vertical distance h through the coil of wire, as shown in Fig. 3.2. This induces an electromotive force (e.m.f.) across the ends of the coil. The e.m.f. is used to store charge in a capacitor, which lights a light-emitting diode (LED) when it discharges.

Fig. 3.2

Fig. 3.3 shows the variation with time of the e.m.f. generated as the magnet falls the distance h.

Fig. 3.3

Explain the shape of the curve in Fig. 3.3.

When the torch is inverted, the pulses of e.m.f. shown in Fig. 3.3 cause a capacitor of capacitance 0.12 F to become charged.

Each positive and each negative pulse adds 9.0 × 10⁻³ C to the charge stored in the capacitor.

(i) The torch is inverted 80 times.

Calculate the total energy stored in the capacitor.

total energy = ...................................................... J

[3]

(ii) When the torch is switched on, the energy stored in the capacitor lights a 50 mW LED.

Estimate the time for which the LED lights.

time = ...................................................... s

[1]

In the torch, the gravitational potential energy of the magnet is converted into electrical energy supplied to the 50 mW LED.

You are asked to investigate whether the efficiency of this energy conversion depends on the number of inversions of the torch.

• Describe how you will make accurate measurements to collect your data. Assume that both the torch and the tube can be opened.

• Explain how you will use the data to reach a conclusion.