What is a Capacitor?
A capacitor stores electric charge onto its plates. The amount of electrical charge that a capacitor can store on its plate is called its capacitance. The S.I unit of capacitance is Farad (F). Amount of charge stored by a capacitor per unit of potential difference.
C = Q/V
The capacitance of a Capacitor
It is the electrical property of a capacitor and is the measure of a capacitor’s ability to store an electric charge onto its two plates with the unit of capacitance being the Farad (F) named after the British physicist Michael Faraday.
It has the capacitance of one Farad when a charge of one Coulomb is stored on the plates by a voltage of one volt.
However, Farad is a large unit of measurement. Therefore sub-multiples of the Farad are generally used such as micro-farads, nano-farads, and pico-farads.
1 Microfarad (1μF) = 10-6 F
1 Nanofarad (1μF) = 10-9 F
1 Picofarad (1μF) = 10-12 F
Factors affecting the value of capacitance of a capacitor
- Surface Area: All other factors being equal, the grater plate area gives grater capacitance, less plate area gives less capacitance.
- Dielectric Material: The types of material that separate the two plates called the dielectric; the higher the permittivity of the dielectric, the greater is the capacitance.
- Distance: The distance, d between the plates; the smaller the distance, the greater is the capacitance.
The dielectric material is a material that separates the metal plates of a capacitor. It can be air or even vacuum but generally a non-conducting insulating material, such as waxed paper, glass, mica different types of plastics, etc.
The dielectric constant, property of the dielectric material, varies from one material to another increasing the capacitance by a factor of k. It is of importance because:
- Dielectric material provides mechanical support between the two plates allowing the plates to be closer together without touching.
- It increases the maximum operating voltage compared to air.
- Its permittivity increases the capacitance.
Dielectric constant for common materials are:
Pure vacuum = 1
Air = 1.0006
Paper = 2.5 to 3.5
Glass = 3 to 10
Mica = 5 to 7
Wood = 3 to 8
Metal oxide powders = 6 to 20
Advantage of Capacitor
A capacitor has an insulating layer such that DC current cannot flow through it.
- It is used to adjust the frequency response of an audio circuit or coupling together separate amplifier stages that must be protected from the transmission of DC current.
- It is used to block DC current while passing audio signals, alternating current, or pulses, or other time-varying waveforms. The ability to block DC currents enables capacitors to be used for smoothening the output voltages of power supplies.
Working characteristics of a capacitor
On DC, a capacitor has an infinite impedance (open-circuit), at very high frequencies a capacitor has zero impedance (short-circuit). So selecting a capacitor with a rating of at least 50 percent more than the supply voltage is significant.
A parallel plate capacitor is the simplest form of a capacitor. It can be constructed using two metal or metalized foil plates at a distance parallel to each other, with its capacitance value in Farads, being fixed by the surface area of the conductive plates, and the distance of separation between them.
Combination of Capacitor
- Series-connected capacitors: Total capacitance of series-connected capacitors is equal to the reciprocal of the sum of the reciprocals of individual capacitances.
1/C = 1/C1 + 1/ C2 +…. + 1/Cn
- Parallel-connected Capacitors: The capacitance of parallel-connected capacitors is equal to the sum of the individual capacitances.
C = C1 + C2 +….. + CN
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