Capacitive Reactance

Author: N.H. Crowhurst

The ratio between the voltage across and the current through a capacitor at a specified frequency is called its reactance. This is valued in ohms like a resistance. But where voltage and current in a resistance are simultaneous, there is a time (or phase) difference (of 90°) in a reactance.

The effects of capacitive reactance

When the reactance of a capacitor is equal to the circuit resistance, the voltage across it is equal to that across the circuit resistance. At this frequency the phase between the input and output will be 45°, and the voltage output will be 0.707 of the voltage input or 3 db down.

Output and input voltages are 45° out of phase, and output voltage is 0.707*input voltage, or 3 dB down.

Notice that making the two voltages (E_c and E_out) equal does not mean that either of them is equal to half the input voltage (E_in), because there is a 90° phase angle between the voltage across the capacitance and that across the resistance. For this reason, the two voltage drops cannot be added arithmetically.