Capacitance

When two conducting surfaces are close enough that they are influenced by each other's electric fields, there will be a tendency for equal magnitude and opposite sign charges to occupy the facing surfaces. This is true no matter how much net charge is on either of the conducting objects to begin with, since the electrons are very mobile and will redistribute to allow for this (electric fields will charge the facing surfaces by induction). The combination of the two surfaces, the opposite charges, and the intervening electric field is called a capacitor.

Capacitors are formed all of the time in everyday situations: when a charged thunderstorm cloud induces an opposite charge in the ground below, when you put your hand near the monitor screen of this computer, when you interfere with the reception on your portable TV by walking near the antenna. In this course, we are working with the simplest of situations by studying the parallel-plate capacitor, but the concepts we are learning for the parallel-plate capacitor can be applied to many more-complicated systems.

Capacitance is a measure of how much opposite charge, per unit of electric field strength, can be induced to reside on the facing conducting surfaces of a capacitor. Capacitance is largest when the facing surfaces are large and/or close together, as this allows for best communication of the electric field and therefore best induction of opposite charge. Capacitance can also be increased when the gap between the capacitor "plates" is filled up with a dielectric (a polarizable material that helps to communicate the electric field from one surface to the other). A capacitor is a charge storing device; the higher the capacitance, the more charge it can store.

A capacitor is also an energy storing device. This is most-easily understood in the charging of a capacitor with an external chemical battery: the battery does mechanical work in moving electrons from the positive plate of the capacitor (a place of low potential energy) to the negative plate (a place of high potential energy). Once the capacitor is charged, the battery can be removed and the charged capacitor retains the energy expended by the battery. Then if the capacitor is connected to a circuit, a conducting path is provided for the electrons to return to the positive plate thereby neutralizing it, and the electrons will expend the "stored" energy as they move. A charged capacitor acts like a temporary battery when it is used in this way.

We use the equations of capacitance to study the concept of energy storage in a charged capacitor. For more information, click here.

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