How Does a Capacitor Work? Storing Charge in a Gap

Connect a capacitor across a voltage source and electrons are pushed onto one plate and pulled off the other, leaving one negatively charged and the other positively charged, with no charge crossing the gap. The separated charges create an electric field between the plates, and the energy supplied by the source is stored in that field. Capacitance measures how much charge is stored per volt applied, and it depends on three things: the area of the plates, since larger plates hold more charge at the same voltage; the distance between them, since a narrower gap makes the attraction between opposite charges stronger and allows more charge for the same voltage; and the material filling the gap. That material, the dielectric, does real work rather than merely insulating, because its molecules polarise in the applied field and partly cancel it, which allows more charge to be stored, with the multiplier called the dielectric constant. The limit is breakdown, the voltage at which the dielectric conducts and the device fails, usually permanently.

The characteristic behaviours all follow from the fact that current flows only while the charge is changing:

• •A capacitor blocks steady direct current once charged, since no more charge can be added at that voltage, and passes alternating current, because the voltage is constantly changing so charge is constantly moving
• •Its opposition to alternating current falls as frequency rises, which is what makes it a frequency-selective component and the basis of filters
• •Charging through a resistor follows a curve set by their product, the time constant, so a capacitor reaches roughly two thirds of the applied voltage in one time constant and is effectively full after about five, which is how timing circuits are built
• •Voltage across a capacitor cannot change instantly, which makes it useful for smoothing, absorbing spikes and holding a supply steady during brief current demands
• •Energy stored rises with the square of the voltage, so doubling the voltage quadruples the energy
• •A charged capacitor remains charged after power is removed, which is why large ones in equipment carry warnings and bleed resistors, since they can deliver a dangerous shock long after unplugging

Capacitors are specialised by dielectric and each type occupies a niche. Ceramic capacitors are cheap, small and stable at high frequencies, dominating in signal work. Electrolytic capacitors use a very thin oxide layer grown on a metal foil as the dielectric, which gives enormous capacitance in a small volume at the cost of being polarised, so they are destroyed by reverse voltage, and of degrading over years as their electrolyte dries out, which is a common failure in older equipment. Film capacitors are stable and reliable and are used where accuracy matters. Supercapacitors, which store charge in an electrochemical double layer at a huge internal surface area rather than across a conventional dielectric, reach capacitances thousands of times higher and bridge the gap towards batteries, charging and discharging in seconds and tolerating a very large number of cycles, at the cost of storing far less energy per kilogram than a battery does. That combination suits them to regenerative braking, brief backup power and applications where rapid cycling would destroy a battery.

Capacitors are among the most numerous components in existence and most of their work is invisible. Power supplies use them to smooth rectified alternating current into something usable and to decouple, meaning to sit close to each integrated circuit supplying the brief surges of current it demands so that the main supply does not sag. Filters in audio, radio and every communication system use them to separate frequencies. Timing circuits use their charging curve. Motor start circuits use them to produce a phase shift. Touchscreens detect a finger by the change it makes to capacitance at each point of a grid, which is why they respond to skin and not to a gloved hand or a pencil. Camera flashes charge a capacitor slowly and dump it into the tube in a millisecond, which is exactly the property batteries cannot supply. Condenser microphones use a capacitor whose plate is the diaphragm. And in energy research, capacitors of enormous size supply the brief high power pulses used in fusion experiments and in electromagnetic launchers.

A capacitor stores energy in the electric field between two plates separated by an insulator, holding more charge per volt when the plates are larger, the gap is narrower and the dielectric polarises more readily. It blocks steady direct current and passes alternating current more easily at higher frequencies, and its voltage cannot change instantly, which makes it a smoothing and timing component. Electrolytics give large capacitance and dry out, and supercapacitors bridge towards batteries by charging in seconds.