Ohm’s Law = V = I R
In a purely capacitive
AC circuit, V = I XC

V
I=
R

V
I=
Xc

V
Xc =
I

Example 1:
A capacitor connected across a 50V AC 400Hz
supply causes a current of 10mA. What is
it’s capacitance value?
XC
=
V / I
= 50 / 10 x 10-3
=
5 kΩ

XC
1 / 2 π f C
=
∴
C
=
1 / 2 π f XC
=
1 / 2 x 3.1416 x 400 x (5 x 103)
=
7.96 x 10-8 F
=
79.6nF or 0.08 μF