Capacitor and Capacitance

What is a Capacitor - Definition

The capacitor is an electric component that has the ability to store energy in the form of electrical charges that creates a potential difference, which is a static voltage, much like a small rechargeable battery. 

The most basic design of a capacitor consists of two parallel conductors (Metallic plate), separated with a dielectric material.  When a voltage source is attached across the capacitor, the capacitor plate gets charged up. The metallic plate attached to the positive terminal will be positively charged, and the plate attached to the negative terminal will be negatively charged. 

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Capacitor Symbols

Types of Capacitors

  1. Film Capacitors: Film capacitors are the ones that use plastic film as the dielectric medium. They are available in nearly any value and voltages up to 1500 volts. They range from 10% to 0.01% in any tolerance. Additionally, film condensers arrive in a combination of shapes and case styles. There are two types of film condensers, radial type lead, and axial type lead.

  2. Ceramic Capacitors: Ceramic capacitors are the ones that use ceramic as the dielectric material. It is used in high-frequency circuits such as audio to RF. In ceramic capacitors, one can develop both high capacitance and low capacitance by altering the thickness of the ceramic disc.

  3. Electrolytic Capacitors: Electrolytic capacitors are the ones that use the oxide layer as the dielectric material. It has a wide tolerance capacity. There are mainly two types of electrolytic capacitors, tantalum, and aluminum. They are available with working voltages of up to approximately 500V, but the maximum capacitance values are not available at high voltage, and higher temperature units are available but are rare.

  4. Variable capacitor: Variable capacitors mostly use air as the dielectric medium. A Variable Capacitor is one whose capacitance can be mechanically adjusted several times. For example, this form of the capacitor is used to set the resonance frequency in LC circuits to change the radio to match impedance in antenna tuner devices.

Define The Capacitance of a Capacitor

The accumulation of charges in the conductors causes a potential difference across the capacitor. The amount of charge accumulated is called the charge holding capacity of the capacitor. This charge holding capacity is what is known as capacitance. The accumulated charge in the capacitor is directly proportional to the voltage developed across the capacitor:

Q ∝V 

Q = C/V

C = Q/V 

C is the constant of proportionality, also called the capacitance of a capacitor. The unit of capacitance is Farad(F) - 1 coulomb per volt. 

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The value of capacitance depends upon the physical features, area of the capacitor plates ‘A’, distance between the plates ‘d’, the permittivity of the dielectric medium ‘ε’.

C = ε x \[\frac{A}{d}\]

Energy of Capacitor

The energy is stored in joules and is equal to the half of capacitance times the square of the capacitor’s voltage. 

E = C × V2 / 2

Capacitor in Series

The total capacitance of the capacitors connected in series C1, C2, C3,.. :

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\[\frac{1}{C_{Total}}\] = \[\frac{1}{C_{1}}\] + \[\frac{1}{C_{2}}\] + \[\frac{1}{C_{3}}\] + ...

Capacitor in Parallel

The total capacitance of the capacitors connected in parallel  C1, C2, C3,.. :

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CTotal = C1+C2+C3+...

Factors Affecting Capacitance

  • Surface Area: The surface area of the two plates affects the capacitance value. Higher the value of the surface area, the higher is the capacitance. 

  • Distance: The distance between the plates affects the value of the capacitance. Lower the value of distance, the higher is the capacitance. 

  • Dielectric Medium: The type of material separating the two plates called "the dielectric." The higher the dielectric's permittivity, the higher the capacitance value.

Uses of A Capacitor

The capacitors have both electrical and electronic applications. They are used for several things such as filters, energy storage systems, engine starters, signal processing devices, etc. 

  • Capacitors are used for storing energy, which can be used by the device for temporary power outages whenever they need additional power. 

  • Capacitors are used for blocking DC current after getting fully charged and yet allow the AC current to pass through the certain of a circuit. 

  • Capacitors are used as the sensor for several things like measuring humidity, fuel levels, mechanical strain, etc. 

  • Capacitors can be used in a time-dependent circuit. This could be connected to any LED or loudspeaker system, and it’s likely that any flashing light/regular beeping uses a timing capacitor.

Fun Facts

  • Capacitors with high capacitance are made up of material with high dielectric constant. 

  • A Capacitor can take up and temporarily store energy from a circuit. Then, the capacitor will return the energy to the circuit later.

FAQ (Frequently Asked Questions)

1. Find the expression for the capacitance of a parallel plate capacitor having a surface area of the plate ‘A’ and distance between the plate is d?

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Suppose the plate 1 is given with a charge +q. Via induction, -q charge will be induced on the inner section of plate 2 and +q on the outer surface, but since it's earthed, it will go into the earth.  Suppose surface charge density of charge on each plate is ρ. The intensity of the electric field at a point between the plates is ρ/ε0

ρ = q/A

E = q/ε0A

Let the potential difference between the two plates is V. 

E  = V/d

V = qD/ε0A

Substituting the values, we get capacitance as below:

C = Aε0/d

2. What is a variable capacitor?

Variable capacitors use air as the dielectric medium. It generally consists of interwoven sets of metal plates in which one is fixed, and the other is variable. The capacitance values of these capacitors may vary from 10pF to 500pF. This type of capacitor has the capability to change its capacitance values, either "Mechanically" or "Electrically." Instead of determining values during manufacturing like Fixed Capacitors, certain ranges of values are given in Variable Capacitors. For example, this form of the capacitor is used to set the resonance frequency in LC circuits to change the radio to match impedance in antenna tuner devices.