O-LevelPhysicsMagnetism5054/22 · Oct/Nov 2014 · Q87 Marks

Fig. 8.1 shows the structure of a simple alternating current (a.c.) generator. [Figure 8.1]

(a)On Fig. 8.1, label • the coil of the generator with the letter C, • a slip ring with the letter S, • a carbon brush with the letter B.[2]
(b)The a.c. generator is operating and the arrows on Fig. 8.1 show the direction of rotation. Explain why there is an electromotive force (e.m.f.) between the two output terminals.[3]
(c)The output terminals of the a.c. generator are connected to a cathode-ray oscilloscope (c.r.o.). Fig. 8.2 shows the trace on the screen of the c.r.o. [Figure 8.2] Describe how the trace and a setting on the c.r.o. are used to find the time for one revolution of the coil of the a.c. generator. You may draw on Fig. 8.2 if you wish.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[1] one label correct and not contradicted
[1] C, 1S and 1B all correct and clear and none contradicted
[3] any three from: magnetic field (between poles) (coil/wire) cuts field/flux or field/flux cuts (coil/wire) or field/flux changes (electromagnetic) induction brushes rub against/in contact with rings
[1] (half) distance across screen or count divisions of/measure wavelength or the wavelength corresponds to one rotation
[1] half distance multiplied by time base setting

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