Lecture #8 Electric dipoles. Dielectrics. Electric Capacitance and Capacitors

2.Capacitance and Capacitors

Any system of two conductors placed near each other but separated by an insulator can store more charge than can an isolated conductor

A capacitor is a device for storing electric charge.

The important measure of these devices is their capacity to store charge, in time they became known as capacitance.

Capacitors

Capacitors come in many shapes and sizes, made from a variety of materials.
Capacitors, which evolved from early devices for storing charge, are part of most common electronic circuits, from the tuning circuit in your radio to the ignition system in your car. Their operation can be understood by building on the ideas of electric charge and electric potential.

Capacitance

The capacitance С of a capacitor is defined as the ratio of the charge stored on one of the to the potential difference V between the conductors.

The magnitude of the capacitance is a constant for each particular capacitor and depends only on the geometrical shape of the capacitor and on the nature of the insulating material between the conductors.

When the charge is measured in coulombs and the potential in volts, the capacitance is measured in farads (F):
1 farad = 1 coulomb/volt.
The farad is a large unit.
Many practical capacitors have values of microfarads (F), nanofarads (nF), or even picofarads (pF).
The circuit symbol for a capacitor

The value of the capacitance is fixed, we can rearrange the previous equation to show the linear relationship between the charge stored and the voltage on the capacitor:

The value of the capacitance is fixed, we can rearrange the previous equation to show the linear relationship between the charge stored and the voltage on the capacitor:

q= CV

If the potential difference between the plates is increased, the charge stored on the capacitor is also increased. Similarly, if you need to store a large charge with only a small applied voltage, then you need a large capacitance.