If the voltage and current have a phase difference of 90 degree in an A.C. circuit, the power will be :

12) Which one of the following frequencies has the longest time period ?

13) The total voltage in a series R - L circuit leads the current by an angle :

14) The total voltage in a series R - C circuit lags the current by an angle :

15) In a series R - L - C circuit the impedance is equal to _____ and current equal to _____ at resonance.