Series Circuits - Capacitance

Capacitors are essentially "voltage storage devices" commonly found in controls, motors and welding circuits, and many other places. They are "charged up" by another source and hold that charge even when the circuit is open, unlike typical resistance loads that lose their voltage as soon as the current goes to zero. Capacitors are essentially "static electricity storage devices" just as you are when you walk across a rug dragging your feet on a dry winter day. You discharge your capacitance when you touch a conductor such as a door knob.

The simplest capacitor consists of two plates separated by a dielectric or non-conductive material. 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.

Current flow in a circuit "sees" a capacitor in an AC circuit similar 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

Ohm's law can be applied in the form E = IXC for capacitors.

EXAMPLE ac Capacitor Current

Find the current flow in the circuit shown.

Find the capacitive reactance.

Applying Ohm's Law

The sine wave for the current will lead the sine wave for the voltage by 90 degrees in an ac circuit with pure capacitance loading because of the time required to charge and discharge the capacitor.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. Therefore, the capacitor is an energy storing element and not a dissipative one like a resistor.

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

The total capacitive reactance of 3 capacitors in series with capacitive reactances of 10, 20 and 30 ohms would be calculated by: