How does a capacitor work?
A capacitor is a passive electrical component that stores the electrical charge and has the property of capacitance. The capacitor has a unique working principle.
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Basic construction
In its simplest form, a capacitor is an electrical device that stores the electrical charge and is constructed of two parallel conductive plates separated by an insulating material called the dielectric. Connecting leads are attached to the parallel plates.
How does a capacitor store charge?
In the neutral state, both plates of a capacitor have an equal number of free electrons.
When the capacitor is connected to a voltage source through a resistor, electrons (negative charge) are removed from plate A, and an equal number is deposited on plate B. As plate A loses electrons and plate B gains electrons, plate A becomes positive with respect to plate B.
During this charging process, electrons flow only through the connecting leads. No electrons flow through the dielectric of the capacitor because it is an insulator; however, the dielectric becomes polarized. The movement of electrons ceases when the voltage across the capacitor equals the source voltage.
If the capacitor is disconnected from the source, it retains the stored charge for a long period of time (the length of time depends on the type of capacitor) and still has the voltage across it. A charged capacitor can act as a temporary battery.
How does a capacitor stores energy?
A capacitor stores energy in the form of an electric field that is established by the opposite charges stored on the two plates. The electric field is represented by lines of force between the positive and negative charges and is concentrated within the dielectric. Although current does not flow through the dielectric, molecules in the dielectric orient themselves with the electric field, creating a polarized region within the dielectric. The polarized molecules within the dielectric create an electric field that reduces the overall field within the dielectric. A good dielectric is one that is easily polarized.
The plates in the figure have acquired a charge because they are connected to a battery. This creates an electric field between the plates, which stores energy. The energy stored in the electric field is directly related to the size of the capacitor and to the square of the voltage as given by the following equation for the energy stored:
When capacitance (C) is in farads and voltage (V) is in volts, the energy (W) is in joules.
Physical characteristics of a capacitor
The following parameters are important in establishing the capacitance and the voltage rating of a capacitor: plate area, plate separation, and dielectric constant.
Plate area
Capacitance is directly proportional to the physical size of the plates as determined by the plate area, A. A larger plate area produces more capacitance, and a smaller plate area produces less capacitance. The below figure shows that the plate area of a parallel plate capacitor is the area of one of the plates. If the plates are moved in relation to each other, the overlapping area determines the effective plate area. This variation in effective plate area is the basis for a certain type of variable capacitor.
Plate separation
Capacitance is inversely proportional to the distance between the plates. The plate separation is designated “d”. Greater separation of the plates produces a smaller capacitance.
The breakdown voltage is directly proportional to the plate separation. The further the plates are separated, the greater the breakdown voltage.
Dielectric constant
the insulating material between the plates of a capacitor is called the dielectric. Dielectric materials tend to reduce the voltage between plates for a given charge and thus increase the capacitance. If the voltage is fixed, more charge can be stored due to the presence of a dielectric than can be stored without a dielectric. The measure of a material’s ability to establish an electric field is called the dielectric constant or relative permittivity.
Capacitance is directly proportional to the dielectric constant. The dielectric constant of a vacuum is defined as 1 and that of air is very close to 1.
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