Capacitive reactance

- the capacitive reactance is the effect a capacitor has on the flow of an alternating current.

It is important to know what effect a capacitor will have on any circuit in which it operates. Not only does it prevent the direct current component of a signal from passing through, but also has an effect on any alternating signal that may appear.

Reactance

In a direct current circuit where there may be a battery and a resistor, it is the resistor that resists the flow of current in the circuit. This is basic Ohms Law. The same is true for an alternating current circuit with a capacitor. A capacitor with a small plate area will only be able to store a small amount of charge, and this will impede the flow of current. A larger capacitor will allow a greater flow of current. A capacitor is said to have a certain reactance. This name is chosen to be different to that of a resistor, but it is measured in Ohms just the same. The reactance of a capacitor is dependent upon the value of the capacitor and also the frequency of operation. The higher the frequency the smaller the reactance.

The actual reactance can be calculated from the formula:

X_c     =    1     / (2 pi f C)

where

X_c is the capacitive reactance in ohms
f is the frequency in Hertz
C is the capacitance in Farads

Current calculations

The reactance of the capacitor that is calculated from the formula above is measured in Ohms. The current flowing in the circuit can then be calculated in the normal way using Ohms Law:

V     =     I     X_c

Adding resistance and reactance

Although resistance and reactance are very similar, and the values of both are measured in Ohms, they are not exactly the same. As a result it is not possible to add them together directly. Instead they have to be summed "vectorially". In other words it is necessary to square each value, and then add these together and take the square root of this figure. Put in a more mathematical format:

X_tot²     =     X_c²     +     R²