Capacitor, << kuh PAS uh tuhr, >> is a device that can store electric charge. One main use of capacitors is a smoothing out of alternating current produced by electric generators. Alternating current regularly reverses its direction of flow, and equipment that uses alternating current operates most efficiently if the reversals occur smoothly. Capacitors also store data in computer chips, work with other components to tune radios and television sets, and supply bursts of electric energy to certain lasers.
The simplest capacitor consists of two metal plates that are held parallel to each other with a small space between them. Occupying this space is a substance called the dielectric medium. This substance can be any electric insulator (material that blocks the flow of electric current). Examples include oil, paper, glass, ceramics, mica, plastic, and even air.
In a modification of this design, two sets of metal sheets are alternately stacked together, with dielectric medium between all the sheets. The order of stacking is: sheet in Set A, dielectric medium, sheet in Set B, dielectric medium, sheet in set A, dielectric medium, and so on. All the sheets in Set A are then electrically connected to one another, and all the sheets in Set B are electrically connected to one another. Each set of sheets is therefore electrically equivalent to one plate of a two-plate capacitor. But each sheet has a much smaller area than that of an equivalent plate. Thus, a stacked capacitor can store a large amount of charge in a small area.
A wound capacitor can store the most charge in a given volume of space. This kind of capacitor consists of two metal foils and two plastic sheets that are alternately stacked and then wound together to form a cylinder. The foils function as capacitor plates, and the plastic sheets are the dielectric medium.
Capacitors come in various sizes. A capacitor used with a gas laser has two square sheets of metal, each 16 inches (40 centimeters) on a side. Capacitors in computer chips are microscopic. One square centimeter of chip area can contain several million capacitors.
How capacitors work.
In an electric circuit, two wires connect the capacitor plates to the opposite terminals of a power source. Charges of equal strength but opposite sign—positive and negative—build up on the plates. The medium prevents current from flowing easily between the plates, though some leakage does occur.
If the power supply is a battery, a voltage builds up between the plates that is equal to the battery voltage. The battery also supplies energy to the capacitor—the energy necessary to charge the capacitor minus losses due to leakage of current between the plates.
A separate pair of wires and an electric switch enable the capacitor to deliver this energy to a device, such as a laser or an electronic flash on a camera. One of these wires connects a capacitor plate to an electric terminal on the device. The second wire connects the device’s other terminal to the switch, which is also connected to the capacitor’s opposite plate. Normally, the switch is open, so no current flows through the device and no energy is delivered. Closing the switch delivers the energy.
A capacitor can “pass” alternating current almost as if the capacitor were a conductor, even though charge does not flow between the plates. Charge surges into and out of each plate as the current reverses direction. The amount of charge on the plates increases and decreases smoothly, even when the alternating current does not reverse direction smoothly. Thus, a capacitor can smooth out alternating current.
Capacitance
is a measure of a capacitor’s ability to store charge. The unit of capacitance is the farad, which is named after English chemist and physicist Michael Faraday. But 1 farad is a huge amount of capacitance. Most capacitors have capacitances measured in microfarads or picofarads. One microfarad equals one-millionth of a farad. One picofarad equals one-millionth of a microfarad. Capacitance is defined as the charge on either plate divided by the voltage between the plates. The standard unit of charge is the coulomb, and so 1 farad equals 1 coulomb per volt.
