REVIEW OF FUNDAMENTALS OF ELECTRICITY WITH TELECOMMUNICATION APPLICATIONS
An ac circuit with inductance only.
Capacitive Reactance. Capacitive reactive has the opposite behavior of inductive reac-
tance. In this case, the current lags the voltage by 90
. Also, as the frequency increases,
the capacitive reactance decreases, whereas with inductive reactance, as the frequency
increases, the reactance increases. The following is an expression to calculate capaci-
= -1/2f C
f is in hertz and C is in farads.
The more customary capacitance unit is the microfarad
(µF). When we use this unit
of capacitance, the formula in Eq. (A.33a) becomes:
= -1 × 10
Example. Figure A.22 illustrates a capacitive reactance circuit with a standard capacitor
.16 µF and an emf of 20 V at 1020 Hz. Calculate the current in amperes flowing in
the circuit. Use formula (A.33b):
= -1 × 10
/2 × 3.14159 × 1020 × 2.16
= -72.24 ,
I = E/X
I = -20/72.24 = -0.277 A
(minus sign means leading current).
Circuits with Combined Inductive and Capacitive Reactance. To calculate the combined
or total reactance when an inductance and a capacitance are in series, the following
formula is applicable:
X = X
X = 2f L - 1 × 10
A word about signs: If the calculated value of
X is positive, inductive reactance predom-
inates, and if negative, capacitive reactance predominates.