Capacitor is defined as an electrical component which has capacity to store electrical energy, in form of electric charge, thus producing potential difference across plates of capacitor.

Basically, a capacitor consists of two (or more) parallel conductive plates which are not connected to each other and are separated by air or some insulating material (waxed paper, ceramic or plastic). This insulating layer is called dielectric. This dielectric material is non-conductive even an insulator. Due to separation, electric current cannot flow, so charge remains present on metal plates. These metal plates can be circular, rectangular, cylindrical or spherical in shape.

When capacitor is connected in a DC circuit, it charges up and blocks the current through itself because of non-conductive behavior of dielectric. But if it is connected in AC circuit then current passes through circuit with zero resistance.

When DC voltage is applied, positive charge accumulates on one plate and negative charge on other. For every positive charge arriving on one plate, a negative charge will depart from other plate, thus overall system is neutral and a potential difference established b/w plates.

It keeps itself charging until voltage across its plates is equal to the applied voltage, at this point capacitor is completely charged. The potential difference across it depends upon, that how much charge was deposited on plates by source voltage? and secondly, what is the capacitance of capacitor?

A capacitor will have greater capacitance if it has larger in size and less distant plates. To measure capacitance of a capacitor to store charge “Q”, voltage “V” is applied across it, then

C = Q/V or Q = CV

Actually electrical energy present in electrical charges is stored in its plates, not charge, now by applying current, these charges speed up and store more energy. Unit of capacitance is farad which is defined as, capacitance will be one farad, if charge of 1 coulomb is stored by applying 1 volt.

Dielectric of a capacitor

Dielectric is a non- conductive material placed between the plates of capacitor. The plates are composed of metal foil which allow the passage of charges. The factor by which the dielectric increases the capacitance as compared to air is known as dielectric constant. A dielectric material having high dielectric constant is a better insulator. Dielectric constant is dimensionless quantity.

Types of capacitor

Capacitors can be of various kinds like

  • Electrolytic Capacitor: they are used when large value of capacitor is required.
  • Mica Capacitor: they are very stable chemically and electrically. They are used at high frequencies and are made of natural minerals.
  • Paper Capacitor: In it, two tin foil sheets are separated by paper or thin wax. They require very high voltage about 2000 V.
  • Film Cap: they use thin plastic as dielectric. Polyester, metallized, polypropylene and polystyrene film capacitor are its types.
  • Non-Polarized Cap: they require AC application in series/parallel with power supply.
  • Ceramic Cap: they use ceramic material as dielectric.

Charging and discharging of capacitor

During charging , a battery is connected to the capacitor then current flows and potential difference increases across it. Value of current and P. D across it, both falls as more charge builds up on plates and finally no current when P. D across it and battery are same. At this time it is fully charged.

During discharging, battery is disconnected from circuit and a large value of current flows and P. D starts falling across it. After some time charge on plates become zero and current and P. D are also at zero value, this is the time when it is fully discharged.

Why does a fan can’t work without capacitor?

Ceiling fan is an induction motor which is single phase. This motor can’t get started itself, it needs external force to run it at the time when it starts. To start any kind of motor, there should be net magnetic field to create torque to rotate rotor. In a single phase motor, without capacitor, field created by stator winding has half positive and half negative cycle, resulting zero net field, so torque is also zero. To overcome this problem, capacitor is used. When it is connected in starting and running winding, then it creates a phase difference b/w windings thus a resultant field is produced. Capacitor is not only necessary to start fan even fan can’t run without it.

Uses of capacitor

They are used for

  • Storage of energy
  • Power conditioning
  • As sensors
  • Signal processing
  • To sooth the signals

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