the resistance and inductance of series circuit are `5Omega` and `20H` respectively. At the instant of closing the switch, the current is increasing at the rate `4A-s`. The supply voltage is

Options

A. 20 V
B. 80 V
C. 120 V
D. 100 V

Answer

B

Explanation

For growth of current in L-R circuit, current is given by
`i=i_(0)(1-e^(-Rt//L))`
`therefore" "(di)/(dt)=-i_(0)(-(R)/(L))e^(-Rt//L)=(i_(0)R)/(L)e^(-Rt//L)`
At `(di)/(dt)=(i_(0)R)/(L)=(E)/(L)rArr4=(E)/(20)`
`therefore" "E=20xx4=80V`