Question
Fig. 7.1 represents an alternating current (a.c.) generator.
(a) A student rotates the handle H, as shown in Fig. 7.1.
(i) On Fig. 7.2, sketch a graph to show how the electromotive force (e.m.f.) between terminals X and Y varies with time during two complete revolutions of the coil.
(ii) On Fig. 7.2, mark and label a point P, for the e.m.f. when the coil is horizontal, as shown in Fig. 7.1.
(iii) The student turns the handle more quickly.
State two ways in which the e.m.f. between terminals X and Y changes.
1. ………………………………………………………………………………………………………………………
2. ………………………………………………………………………………………………………………………
(b) Terminals X and Y are connected to the primary coil of a transformer.
State and explain what happens in the transformer as the student turns the handle of the a.c. generator.
(c) Explain why the power losses in transmission cables are lower when electrical energy is transmitted at higher voltages.
Answer/Explanation
Answer:
(a) any three from:
• y-axis labelled e.m.f. and x-axis labelled time
• at least one cycle of a sinusoidal wave
• only two complete cycles of a sinusoidal wave
• constant amplitude and constant period for first two periods of a sinusoidal wave
(a)(ii) peak or trough or corresponding time labelled P
(a)(iii) (amplitude / maximum e.m.f.) increases
(e.m.f.) changes direction more often or greater frequency
(b) alternating current in primary coil
alternating / changing magnetic field
or magnetic field cuts secondary coil (continuously)
(alternating) e.m.f. induced in the secondary coil
(c) smaller current (and same resistance when the power is transmitted and an equal rate)
less thermal energy loss / produced (in cables)
Question
A student turns the handle of an alternating current (a.c.) generator and the coil rotates.
Fig. 8.1 represents the structure of the a.c. generator.
(a) There is an alternating voltage output between the two terminals.
(i) Explain why rotating the coil produces an output voltage.
(ii) State the position of the rotating coil when the alternating output voltage is at a maximum value and explain why the maximum output occurs at this position.
(b) A lamp and an open switch are connected in series to the output terminals of the a.c. generator.
The switch is closed and the lamp lights up. The student has to apply a greater force on the handle.
Explain why a greater force is needed to keep the lamp lit.
▶️Answer/Explanation
Answer:
a)(i) magnetic field mentioned
coil / wire cuts (magnetic) field OR changing (magnetic) field (through coil)
e.m.f. / voltage induced OR produced by electromagnetic induction
(a)(ii) (plane of coil) horizontal OR in position shown in diagram
coil cutting magnetic field the fastest
b)
