| Intermediate Technology Education | Energy and Power Technology | Pre-Design | Topic 3 | Activity 1 |

The Capacitor

The purpose of this activity is to investigate the energy storage ability of the capacitor
  • Learn how the capacitor is constructed
  • Learn how the capacitor can store electric charges
  • Use capacitors to store and release electrical energy

The Capacitor

Figure. Selection of Capacitors

The capacitor was invented in 1745 and was first used to store static electricity charges that were produced by friction.

The capacitor is a basic device consisting of two metal plates separated by an insulator. In its simplest form, the insulator can be air. Capacitors are found in just about every electronic circuit. They store the high voltage charge used in heart defibrillators and electronic camera flashes (the whine you hear in these devices is the capacitor charging). In other high energy applications they store the huge charge needed to fire high energy lasers. In lower energy applications, they help computers "remember" and filter power supplied of pulsing DC current. Modern capacitors can hold enough energy to power small devices such as LEDs and liquid crystal displays.

The following illustrations explain how a capacitor can be "charged" thus storing potential energy and then "discharged" though a light where the energy is converted to light and heat.

Figure. Left—Capacitor Discharged.  Right—Capacitor Charging

Figure. Left—Capacitor Charged.  Right—Capacitor Discharging

Figure. Capacitor Discharged

Capacitor Properties

The unit of capacitance is the Farad (named for Michael Faraday). The Farad has to do with two basic electrical properties, charge and potential difference between the two charged plates. (If you really want to know, the Farad is a Coulomb of charge per volt. That is, if there is a charge on the plates of 1 Coulomb and this causes a potential difference between them of 1 volt, the capacitance is 1 Farad). A Farad is a large unit and most capacitors are rated in micro-Farads or millionths of a Farad. Typically the Greek letter μ (mu) is used to represent the prefix micro which is one millionth or 1 x 10-6 .

The larger the plate area, the larger the charge a capacitor can hold. The closer the plates are together the more charge they can hold. Large capacitors consist of two sheets of very thin aluminum foil folded up in a metal can with a plastic or paper material (called a dielectric) between them.

Capacitors have two parameters that must be taken into consideration when choosing one for an application. The first is the capacitance and the second is the working voltage.

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