Capacitors in Everyday Life

Capacitors are not confined to physics problems -- devices built directly on the principle of storing charge (and energy) in a capacitor are used constantly in everyday life. This closing section surveys a few familiar examples purely QUALITATIVELY, as the WBCHSE syllabus specifically asks, with no formulas or calculations attached.

Camera flash. The bright, brief flash produced by a camera's flash unit is a direct, everyday demonstration of capacitor charging and discharging. A capacitor inside the flash unit is charged up slowly, over a second or two, by the camera's battery; when the shutter is triggered, this stored charge is released almost instantaneously through the flash bulb, delivering a burst of energy far more powerful, for that brief instant, than the battery alone could ever supply directly -- illustrating how a capacitor can accumulate energy slowly and then release it very rapidly, something an ordinary battery cannot do nearly as effectively.

Touch-sensitive screens and keypads. Many modern touchscreens and keyboard keys work by detecting a tiny, localised CHANGE in capacitance when a finger (or a conductive stylus) comes close to or presses down on a sensing surface -- the body's own conductivity alters the local capacitance in a way built-in circuitry can detect and register as a touch or a keypress.

Smoothing and filtering in electronic circuits. Inside power supplies and many everyday electronic devices, capacitors are used to smooth out unwanted fluctuations in voltage, charging up when the supplied voltage rises slightly and discharging when it dips, keeping the voltage delivered to sensitive downstream circuitry noticeably steadier than it would otherwise be.

Tuning circuits in radio receivers. A variable capacitor, whose capacitance can be adjusted mechanically or electronically, is used together with an inductor in the tuning circuit of a radio receiver; changing the capacitance shifts the circuit's resonant frequency, allowing a listener to select which broadcast station's frequency the receiver responds to. …