- Figure 15 shows two coils A and B placed close to each other. A is connected to a steady D.C supply and a switch, B is connected to a sensitive galvanometer.
(i) The switch is closed . State the observations made on the galvanometer (2 marks)
(ii) Explain what would be observed if the switch is then opened. (2 marks).
2. The primary coil of a transformer has 1000 turns and the secondary coil has 200 turns. The primary coil is connected to a 240V a.c mains, supply.
(i) Explain how an e.m.f is induced in the secondary coil (2 marks)
(ii) Determine the secondary voltage (3 marks)
(iii) Determine the efficiency of the transformer given that the current in the primary coil is 0.20 A and in the secondary coil it is 0.80 A
Question 3
3 .(a)(I) State Faraday’s law of electromagnetic induction (1 mark)
(ii) State the Lenz’s law of electromagnetic induction (1 mark)
(b) A transformer is used on a 240 a.c supply to deliver a 7.5 A at 90 V to a heating coil. if the transformer is 95% efficient, what is the current in the primary winding? (2 marks).
( c) Hysteresis losses are a source of inefficiency in a transformer. explain
(I) What is meant by hysteresis losses (2 marks)
(ii) How these hysteresis losses can be minimized. (1 mark)
( d) An a.c supply lights a lamp with the same brightness as a 12 V battery. Determine
(i)The r.m.s voltage (1 mark)
(ii) The peak voltage of the a.c supply (2 marks)
Question 4
4. (a) State the lenz’s law (1 mark)
(b) The diagram in figure 7 below shows a coil of wire next to a magnet. A voltmeter is connected to the coil of the wire.
Describe two ways of inducing a voltage in the coil of the wire. (2 marks)
(c) State the three factors that affects the size of the induced voltage (3 marks)
(d) A 6v, 24w lamp shines at a full brightness when it is connected to the output of a mains transformer as shown in figure 8 below.
Assuming that the transformer is ideal, calculate
(i) The number of turns in the secondary coil if the lamp is to work at it’s normal brightness (2 marks)
(ii) The current which flows in the mains cables (2 marks)
Question 5
5. Figure 14 shows an E shaped steel block being magnetised by a current through two coils in series.
on the figure, indicate
(i) the north and south poles of the resulting magnet (1 mark)
(ii) the complete magnetic field pattern between the poles (1 mark)
(b) Figure 15 shows the permanent magnet made in part (a) above
A coil wound loosely on the middle limb is connected in series with low voltage a.c and a switch. State and explain the observations made on the coil when the switch is closed (2 marks)
Question 6
6.A piece of metal AB was magnetised using the method shown below.
(a) By what method is metal AB being magnetised? (1 mark)
(b) what is the polarity of end B (1 mark)
Question 7
7. Figure 2 shows a soft iron bar AB placed in a coil near a freely suspended magnet.
Explain the observation made when the switch is closed (2 marks)
Question 8
8. When a transformer is connected to an ac source, the output voltage is found to be 24 V. If the power input is 200 W, determine the output current. (assume the transformer is 100% efficient). (3 marks)
9. (a) State what is meant by the term “electromagnetic induction”. (1 mark).
9 (b) Figure 9, shows a simple electric generator.
(i) Name the parts labelled P and Q. (2 marks)
P ……………………………………………
Q …………………………………………..
(ii) Sketch on the axes provided a graph to show how the magnitude of the potential difference across R, changes with time t. (1 mark)
(iii) State two ways in which the potential difference produced by such a generator can be increased. (2 marks)
(c) In a transformer, the ratio of primary turns to the secondary turns is 1:10. A current of 500mA flows through a 200 ohms resistor in the secondary circuit. Assuming that the transformer is 100% efficient, determine:
(i) the secondary Voltage; (1 mark)
(ii) the primary voltage; (2 marks)
(iii) the primary current. (2 marks)