Capacitor Basic Questions and Answers

What is a capacitor?

A capacitor is a device that stores electrical energy in the form of electric field. It consists of two conducting surfaces or plates separated by an insulating material called a dielectric.

By Eric Schrader from San Francisco, CA, United States – 12739s, CC BY-SA 2.0, Link

How the capacitors store charge?

It stores charge by detaching electrons from one plate and accumulating it at another plate using an external supply. The plate that loses electron attains positive charge and that gains electron becomes negatively charged. This charge built up is stored electrostatically in an electric field.

What is capacitance?

Capacitance is the ability of a body or a capacitor to hold the charge. Capacitance is denoted by C and it is measured in the unit called Farad.

Capacitance C = Q/V, Q – charge stored in the capacitor, V – voltage across the plates.

Hence, the capacitance of a capacitor is the ratio of stored charge to the potential difference across the plates.

Example: – If a capacitor has a rated capacitance of 1000uf and voltage of 10V. If we apply a 10V supply across it, then the charge it has when the capacitor is fully charged will be, Q = CV = 1000uf x 10 = 0.01 Coulomb

What are the factors affecting the capacitance of a capacitor?

The capacitance depends upon the area of the plates, dielectric material, and distance between the plates. The capacitance is directly proportional to the relative permittivity of the dielectric medium and the area of the capacitor plate, but inversely proportional to the distance between the plates.

The Capacitance in a parallel plate capacitor with the uniform medium, C = ε A / d

If the dielectric medium is air then C = ε_o A / d

ε_r – Relative permittivity

ε_o – Permittivity of free space or vaccum

ε – Permittivity of medium

A – Area in square meters

d – The distance between plates or dielectric thickness

What is called a dielectric constant?

Dielectric constant or relative permittivity is the property of the material that affects the ability to concentrate coulomb force between the two charged plates.

The dielectric constant of a material can be expressed as the ratio of its permittivity ε to the permittivity ε_o when air as dielectric.

Dielectric constant or relative permittivity, κ or ε_r = ε / ε_o.

How to calculate the energy stored in a capacitor?

Energy stored in a capacitor is equal to the work done to build a potential across the capacitor,

W (Work done) = E (Energy stored) = CV²/2 = QV/2, it is measured in joules.

List a few types of capacitors based on the dielectric material.

• Air capacitors
• Paper capacitors
• Polyester capacitor
• Ceramic Capacitor
• Electrolytic capacitors
• Mica Capacitors

How to calculate the value of Capacitors connected in series and parallel?

Capacitors connected in Series

In capacitors connected in series, the charge Q stored by each capacitor is equal to the total charge Q, but the voltage across each is different.

V = V1 + V2 + V3, equation V = Q/C

V = Q/C_T = Q/C1 + Q/C2 + Q/C3

Q/C_T = Q(1/C1 + 1/C2 + 1/C3)

Total capacitance of 3 capacitors connected in series, 1/C_T = 1/C1 + 1/C2 + 1/C3

Capacitors connected in Parallel

Just opposite to series here the voltage across each capacitor is equal but the charge stored by each capacitor is different.

Total charge, Q = Q1 + Q2 + Q3

Q = V C_T = VC1 + VC2 + VC3

VC_T = V(C1 + C2 + C3)

Total capacitance of 3 capacitors connected in parallel, C_T = C1 + C2 + C3

Refer: series & parallel capacitance

Brief the Capacitor behavior in AC and DC circuits.

If a DC voltage is applied across a capacitor, the capacitor charges and voltage across it reaches the maximum value which is equal to the supply voltage. At the initial time the current will be maximum and once the capacitor is fully charged the current flow will be approximately equal to zero.

The instantaneous voltage across a capacitor, v = V(1-e^-t/RC)

Refer: voltage across a charging capacitor

In an AC circuit, a capacitor offers a reactance called capacitive reactance which is inversely proportional to the frequency of the supply, and the value of capacitive reactance can be calculated as,

Xc = 1/2πfC, f-frequency, C- capacitance

Capacitive reactance has a leading power factor, that is in a pure capacitive reactive circuit without resistance the current leads the voltage by 90 degrees.

List a few applications of capacitors.

It is very hard to find an electronic circuit without a capacitor because it has that much of applications.

DC filter circuits – Used in rectifier outputs to filter AC component in DC.

Capacitors in AC filter circuits – Circuits using the combination of a capacitor with a resistor or inductor like LC, RC circuit can be used to build Low pass, High pass, and bandpass filter circuits.

Multivibrator, Timer circuits– The capacitor is a key component in multivibrator circuits because most circuits work by taking the charging and discharging time of capacitors.

Oscillator circuits or Tank circuits – Works by the electrical resonance of combination circuit including capacitors.

Improve power factor – Large capacitor banks are used along with inductive loads to improve the power factor.