K.C.S.E Physics Q & A - MODEL 2012PP2QN016

K.C.S.E Physics Q & A - MODEL 2012PP2QN016

​(a) 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 now closed. State the observation made on the galvanometer. 
(ii) Explain what would be observed if the switch is then opened. 
(b) 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.
(1) Explain how an e.m.f is induced in the secondary coil. 
(ii) Determine the secondary voltage. 
(iii) Determine the efficiency of the transformer given that the current in the primary coil is 0.20A and in the secondary coil is 0.80A. 

answers

​(a) (i) Pointer deflects upto a certain; maximum value and then returns to zero; 
(ii) There is a deflection in the opposite direction then back to zero; As Flux in A falls, flux in B also falls and causes induced e.m.f in the opposite directions; 
(b) (i) Current in the primary is constantly changing its direction; so that the resulting flux (which link coils) is constantly changing its “
direction. Therefore alternating e.m.f is induced in the secondary coil;