The current in a 20 mh inductor is known to be i 40 ma , e 0; the voltage across the inductor (passive sign con- vention) is 28 v at t = 0. a) find the expression for the voltage across the inductor fort > 0. b) find the time, greater than zero, when the power at the terminals of the inductor is zero.

In the winter, in order to keep the classroom steadily at 68 f before 10 pm, heating with an average rate of 42,000 btu/hr is provided. assume the outdoor temperature maintains at 32°f, determine the electrical power (kw) required to (a) operate a reversible heat pump (b) operate a real heat pump with a cop (7) of 4.5 and (c) operate an electrical-resistance heater.