For the circuit shown in the figure, the switch has been open for a very long time. (a) What is the potential drop across the 15.0-mH inductor just after closing the switch? (b) What is the potential drop across the 70.0-?F capacitor after the switch has been closed for a very long time?
An inductor is an open circuit at t0 and a short circuit at t∞. A capacitor is a short circuit at t0 and an open circuit at t∞. That makes things very easy. (a) At t0, all you have is a 75? in series with a 25? resistor. So the voltage on the bottom side of the 15mH inductor is +200V????????? +50V. There is no current in the 50? resistor, so no voltage drop, so the other side of the 15mH inductor is at +200V. So the voltage across the inductor is 150V at t0. (b) At t∞, all you have now is a 50? in series with a 25? resistor. So the voltage on the bottom side of the 70μF capacitor (there is no current in the 38? resistor) is +200V????????? +66?V. The voltage on the other side of the 70μF capacitor is +200V. So the voltage across the capacitor is 133? V at t∞.
