June 1989 P2 Q12
(a) The figure below shows an arrangement of apparatus which can be used to demonstrate electromagnetic induction. Coil A is connected to a battery as shown and coil B is connected to a sensitive centre-zero galvanometer. Coils A and B are placed close together.
State and explain what is observed
(i) when switch S is closed and remains closed,
(ii) when S is opened again.
(b)
(i) The experiment in (a) is repeated with a soft-iron rod placed in both coils. State and explain one difference in the observations which would be made.
(ii) In what way, if any, would the observations change if the soft iron rod is removed and a large sheet of soft iron is placed between the coils at right angles to the line joining coils A and B?
(c)
(i) Draw a labelled diagram to illustrate the structure of a simple transformer.
(ii) State and explain one reason why the design of the transformer you have drawn is better than the arrangement of coils shown above.
(d) An ideal transformer is to have an output of 12 V. Assuming that the input voltage is 240 V and that there are 3000 turns on the primary coil, calculate the number of turns required on the secondary coil.
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Answer:
(a)
(i) When the switch is closed a momentary deflection of the galvanometer pointer is noticed. A magnetic field is caused by the current in the coil A, causing a change in the magnetic flux cutting the coil B. This change induces an e.m.f. across B for a while.
When S remains closed, the pointer remains at O of the galvanometer scale as there is no induced current in B because the magnetic flux cutting the coil does not change.
(ii) When S is opened, the magnetic flux cutting the coil B decreases. This change of magnetic flux causes an e.m.f. in the opposite direction. Hence a momentary deflection in the opposite direction is observed in the galvanometer.
(b)
(i) When a soft iron rod is placed through the coils, greater momentary deflections are recorded in the galvanometer. Since magnetic lines of force pass through soft iron more easily than through air, they are concentrated from the coil A to the coil B. The rate of change of flux cutting the coil increases.
(ii) If a large sheet of iron is placed between coils, it acts as a magnetic screen. There will not be any deflection of the pointer in the galvanometer.
(c)
(i)
(ii) The closed soft iron core concentrates almost all the magnetic lines of force from the primary into the secondary coil. The rate of change of flux cutting the secondary will be greater than in the coil without the soft iron rod. Hence a greater e.m.f. is induced in the secondary.
(d)
Number of turns in secondary / number of turns in primary = output e.m.f. / input e.m.f.
n / 3000 = 12 / 240
n = 150
Hence, number of turns in the secondary coil = 150.
