Electronic components Article:

What is a capacitor. How does a capacitor works.



A capacitor in its simplest form consist of two parallel conducting plates seperated by a non-conducting material called Dielectric.

capacitor construction

What is a Capacitor?

A capacitor is a device which store electrical energy in the form of electric field formed when there is a built up of opposite charges on its plates as shown in the following diagram, one plate has excess electrons which cause creation of negative charges and the other has an excess of protons which cause creation of positive charges.

charged capacitor

Recall that charges are carried by electrons and protons and if an object contains an equal number of electrons and protons their charges cancel each other and the net charge of an object will be zero.

A capacitor can also be defined as a device designed to introduce a known value of capacitance into a circuit. In storing electrical energy a capacitor also offers opposition to the flow of current.

A common application of a capacitor is in smoothing of ac rectified voltage which is part of the process of converting ac to dc. In smoothing, a capacitor reduce flactuations in rectified ac voltage, where it first charges when the voltage is growing to maximum and then discharge when there is reduction in voltage.

Capacitance of a Capacitor.

Capacitance of a capacitor is the ability of a capacitor to store electrical energy. Since a capacitor offers opposition to flow of current we can also say capacitance is the ability of a capacitor to oppose the flow of current.

Capacitance of a capacitor is measured in Farads. The larger the value of capacitance of a capacitor the more energy that can be stored by a capacitor and also the more opposition that can be given to flow of current.

The total opposition given to flow of current by a capacitor is called Capacitive reactance. Capacitive reactance is measured in Ohms just like resistance.

How does a capacitor works.

When an uncharged capacitor (a capacitor which contains an equal number of electrons and protons on each of its plates) is connected to a source of voltage like a battery.

The positive side of a battery will attract electrons from one plate of a capacitor, leaving the plate with more protons than electrons.

And also at the same time the negative side of a battery repel electrons away from it causing the other plate to have more electrons than protons.

charged capacitor

Recall again, when a plate contain an equal number of electrons and protons, the charges on protons and electrons cancel each other and the plate is regarded as neutral. Electrons carries negative charges and protons carries positive charges.

If a plate contain more electrons than protons it is negatively charged and also if a plate contain more protons than electrons it is positively charged.

The creation of opposite charges on two plates of a capacitor create an electric field which is the sign of storage of energy by a capacitor. This energy will be released by a capacitor whenever certain conditions are meet in a particular circuit.

Capacitor in DC circuits.

Direct Current electricity is a type of electricity in which electrons flows in only one direction and the current and voltage does not varry in magnitude. When a capacitor is connected to a dc source like a battery.

Current will only flow while the capacitor is charging and stop to flow when the capacitor is charged to the supply voltage.

Since current only flow while the capacitor is charging and stops when it fully charges, in this way we say a capacitor blocks DC current.

A capacitor has a dielectric material between its plates, current cannot pass through it, during charging and discharging, electrons are only occupying one plate and others leaving another plate.

So in dc where current flow in only one direction, when a capacitor is fully charged or charged to supply voltage, one plate is completely occupied with excess electrons, no space for others to came in.

And the other plate all of its electrons are taken by the positive side of the battery, no electrons are are available to flow. Current is then resumed when ever certain condition is meet for the capacitor to discharge.

Capacitor in AC circuits.

Alternating Current electricity is a type of electricity in which electrons changes direction of flow and the current and voltage of ac varies in magnitude regularly.

When a capacitor is connected to an ac source voltage, the capacitor charges and discharge while the ac is changing direction and magnitude. Current always flow in an ac circuit, in this way we say capacitor allows ac current.

Factors affecting capacitance of a capacitor.

When designing a capacitor they are factors which are taken into account so that a capacitor of known value of capacitance is manufactured. We have explained these factors in detail in the article, factors affecting capacitance you can check out.

These factors are cross sectional area of plates, distance between plates and type of dielectric material used.

factors affecting capacitance of a capacitor

1.Cross sectional area of plates (A).

Capacitance of a capacitor varies directly with cross sectional area of plates, this mean that large plates area provide high value of capacitance of a capacitor than small cross sectional area plates.

2.Distance between plates (d).

Capacitance of a capacitor varies inversely with distance between plates that is if distance between plates is increased capacitance decreases

3.Type of dielectric material.

Capacitance of a capacitor depends on the dielectric material used. Dielectric material provide an easy establishment of electric field and also prevent movement of electrons directly to the other plate. Iron used as a dielectric material provide higher capacitance than air.

Dielectric material is measured based on Permittivity(Eo.Er), which is the easy with which an electric field can establish in a material. The larger the permittivity value the larger also the capacitance is.

To come out with the value of capacitance of a capacitor, the three factors are combined in the formula:

capacitance formula

After having the value of capacitance of a capacitor we can then find the capacitive reactance (Xc) of a capacitor offered by a capacitor in a particular circuit using the formula.

capacitive reactance formula

Types of Capacitors.

Capacitors are generally grouped into fixed and variable capacitors. Fixed capacitors are capacitors with a capacitance value which cannot be varied while variable capacitors are capacitors with capacitance value which can be varied or tuned between zero and its maximum value.

In Capacitors we have polarized and non polarized capacitors. Polarized capacitors are only used in dc circuit, they have definate poles which shows the positive and negative points for connection to dc.

Non-polarized capacitors are capacitors which can be used both in dc and ac circuit, they have no definate poles and can be connected into a circuit in either way.

Based on the material used as dielectric, we have various capacitors which includes air, mica, paper, ceramic, plastic, titanium, electrolytic capacitors etc.