Parallel Circuits - Capacitive

Capacitors are a basic electrical element found in controls, motors and welders circuits, and many other places. A capacitor consists of two plates the dielectric. Materials such as air, paper, mica, and oil can be used as dielectrics.

Capacitance is the amount of electrical charge that a capacitor can store for each volt of applied potential. Capacitance is measured in units of farads. The farad is a very large unit of capacitance. Practical devices are more often rated in terms of microfarads, where micro means 1 millionth.

A capacitor in an ac circuit limits the current flow in a similar manner to a resistor. The amount of opposition to current flow is quantified as the capacitive reactance. Capacitive reactance is measured in units of ohms and can be calculated from the frequency of the source and capacitance using the following formula:


f = frequency in hertz
C = capacitance in microfarads

EXAMPLE ac Capacitor Current

Find the current flow in the circuit shown.

Find the capacitive reactance.

Because of the time required to charge and discharge the capacitor, in an ac circuit with pure capacitance loading the current will lead the voltage by 90 degrees. This can be shown by the current leading the voltage in waveform and in phasor form. The ideal capacitor does not dissipate energy in the form of heat but simply stores it in the form of the electric field and can return it to the system. The capacitor is an energy storing element and not a dissipative one like a resistor.

Total capacitance of a number of capacitors in parallel can be found using the following rule:

Parallel: CT = C1 + C2 + C3 + ...

The total capacitive reactance of a 120 volt circuit of 3 capacitors in parallel with capacitive reactances of 10, 20 and 30 ohms would be calculated by:

T = C1 + C2 + C3 = 10 ohms + 20 ohms + 30 ohms = 60 ohms
The total current flowing in the parallel capacitive circuit is: