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A d.c. voltage of 80 V is applied to a circuit containing a resistance of 80 Ω in series with an inductance of 20 H. Calculate the growth of current at the instant (i) of completing the circuit (ii) when the current is 0.5 A and (iii) when the current is 1 A. |
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Answer» The voltage equation for an R-L circuit is V = iR + L(di/dt) or L(di/dt) = V - iR or di/dt = 1/L(V - iR) (i) when i = 0; di/dt = 1/L(V - 0 x R) = V/L = 80/20 = 4A/s (ii) when i = 0.5A; di/dt = (80 - 0.5 x 80)/20 = 2A/s (iii) when i = 1A; di/dt = (80 - 80 x1)/20 = 0. In other words, the current has become steady at 1 ampere. |
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