K.C.S.E Physics Q & A - MODEL 2016PP2QN15

K.C.S.E Physics Q & A - MODEL 2016PP2QN15

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 ac. Mains supply.
i) 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 it is 0.80A. 

answers

(a) ​(i)The galvanometer will be deflected to one side and then back to zero.
(ii)A greater deflection will be obtained in the opposite direction as the current takes less time to die off than to build up
b)(i)The changing current in the primary coil induces a current in the secondary coil due to changing magnetic field of the primary current
ii)Ns/Np = Vs/Vp hence Vs = (240 x 200)/1000 = 48V
iii)Efficiency = power output/Power input x 100       
(4.8 x 0.8 x 100)/240 x 0.2= 80%