The time constant of an inductance coil is `2 xx 10^(-3) s`. When a `90 Omega` resistance is joined in series, the same constant becomes `0.5 xx 10^(-3) s`. The inductance and resistance of the coil are

Options

A. 30mH,`30Omega`
B. 30mH,`60Omega`
C. 60mH,`30Omega`
D. 60mH,`60Omega`

Answer

C

Explanation

Time constant, `tau=(L)/(R)=2.0xx10^(-3)s`
and `(L)/(R+90)=0.5xx10^(-3)s`
Solving these two equations, we get
L = 60 mH and R = 30 `Omega`