## Charging a Capacitor

In most practical applications, each conductor of a capacitor initially has zero net charge. If we connect a capacitor, a resistor, and a voltage source in series, the capacitor will be charged up until its voltage value is equal to the voltage source. A capacitor can store energy, and a resistor placed in series with it will control the rate at which it charges or discharges. This produces a characteristic time dependence and a crucial parameter that describes a capacitor’s rate of charge and discharge:

The time constant, or circuit time delay, represents the time response of the circuit when an input step voltage or signal is applied.

The charging current asymptotically approaches zero as the capacitor becomes charged up to the battery voltage. The time required for the capacitor to be fully charged is equivalent to about 5 time constants or 5T. Thus, the transient response or a series RC circuit is equivalent to 5 time constants.

Capacitors together with resistors, form so-called RC circuits, and they are a common element in electronic devices and play an important role in the transmission of electrical signals. They are used to control the speed of a car’s windshield wipers and the timing of traffic lights; they are used in camera flashes and in many other electronic devices, such as audio filters and oscillators.

## Discharging a Capacitor

Discharging a capacitor means releasing the charge stored within the capacitor. RC discharging circuits use the inherent RC time constant of the resistor-capacitor combination to discharge a capacitor at an exponential rate of decay.