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Capacitors are essentially storage devices for electrons. They consist of two conductive materials called plates, separated by a thin insulator called a dielectric. Think sandwich. The plates are the bread, and the dielectric is the filling. The plates can't touch each other. Waxed paper sandwiched between two aluminium foil plates can be used.
Figure Parts of a Capacitor
When connected to a battery, electrons begin to leave one plate of the capacitor, and collect on the other one. The plate getting electrons becomes negatively charged, and the plate losing electrons becomes positively charged. This happens very fast at the beginning, but begins to go more and more slowly as the difference in voltage on the two plates gets closer to the voltage of the battery.
Figure Capacitor Charging
Figure Graph of Capacitor Charging
Capacitors can store electrons for a short period of time, and release them later. If, for example, you hook a capacitor to a 3 volt battery, it will charge up to 3 volts. If you then disconnect the battery, and connect a light bulb to the capacitor, the bulb will light. It will fade out quickly as the electrons move from the negatively charged plate of the capacitor, through the bulb and to the positively charged plate.
Figure Capacitor Used to Light a Bulb
Capacitance, Or How many Electrons Can it Store?
Capacitance is a measure of how many electrons a capacitor can store. Several things affect capacitance
- the size of the plates (bigger stores more)
- the type of material the plates are made from
- the distance between the plates (closer is more effective, but is related to the dielectric)
- type of dielectric (a dielectric that offers more resistance to the flow of electrons is better)
Capacitance is measured in Farads (symbol F). A capacitor that is 1 F could hold 1 coulomb of electrons at an electromotive force of 1 Volt. A farad is a very large unit. Most capacitors are measured in millionths of a farad. Capacitors, particularly large ones used in TVs, can be very dangerous as they can hold the charge for long periods of time and will discharge it across anything that connects the two terminals (including your hands).
Uses for Capacitors
Capacitors have a lot of applications in electronic circuits. You may be familiar with electronic flash units used with cameras. When you turn them on it takes a bit of time for the flash to be ready. This is caused by the time required to charge the capacitor. When the flash is 'fired', all the electrons in the capacitor are released immediately across the flash bulb, triggering it. Old style flashes had bulbs that burned out when they were fired. Electronic flashes use Xenon tubes, which require a lot of electrons to set off. It can be fired many times.
Capacitors are used to help control timing (it takes a while to charge them, and the exact time can be designed into the circuit), and to control the flow of current. A capacitor stops charging when it is full, stopping electron flow in the circuit. A capacitor in a DC circuit will only allow current flow when it is being charged, then stops. With alternating current this is different. The continuous change in direction causes the capacitor to continuously charge and discharge. It does have an effect on the circuit though, related to the time it takes to charge/discharge the capacitor. This property is employed in power supplies to help convert AC into DC that looks like battery DC. They smooth out the DC.
Capacitors are also used with transistors to create computer memory chips (DRAM).
Radios used two sets of metal plates as capacitors to 'tune-in' the station. As the figure below shows, more plate surface creates more capacitance, and less plate surface creates less capacitance. This type of variable capacitor uses air as the dielectric. The quality of air as a dielectric depends on humidity. The dryer the air, the better the dielectric effect.
Figure Radio Tuner Variable Capacitor. Rotate the knob, change the station.
For More Information
To get more information on capacitors, try these sites
- How Capacitors Work
- Interactive Java Tutorial Factors Affecting Capacitance
- Interactive Java Tutorial Charging and Discharging a Capacitor
- Interactive Java Tutorial Variable Capacitor
- Interactive Java Tutorial Tuning a Radio Receiver
- Capacitor
- Identifying Electronic Components: Passive Devices
- Lesson 3 - Capacitors
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