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Q2) A d.c. relay coil having an inductance of 300 mH and negligible resistance is connected in series with a 5 Ω resistor across a 20 V d.c. supply.

(a) Calculate the value of resistor voltage 30 ms after switch-on. (4)

(b) State the expression for the instantaneous current and determine its value 30 ms after switch-on. (4)

(c) When steady state conditions have been reached, the supply is replaced instantaneously by a short-circuit.

(i) Determine the time taken for the current to fall to 10 % of the steady state value. (6)

(ii) Sketch a clearly labelled graph with approximately scaled axes to show the growth of circuit current after switch-on. (2)