A Beginner’s Guide to Capacitor (Symbol, Formula, Working)

A capacitor is an electronic device that can store energy in the form of an electric field and releases it into a circuit wherever possible. Capacitors are used in many electrical and electronic systems for electronic noise filtering, power conditioning, remote sensing, signal coupling or decoupling, and more. This blog post will discuss what a capacitor is?  Its symbol, equation, and uses.

What is the difference between Capacitor and Capacitance?

Capacitor
Capacitor

A Capacitor is a two-terminal device that stores charge. And its ability to store charges is known as capacitance. They are also known as condensers.

What Do Computers Actually Do?
What Do Computers Actually Do?

Capacitors are rated by their maximum voltage. Hence don’t exceed the applied voltage more than its rated voltage, or it will explode.

Capacitor exploaded due to overvoltage
Capacitor exploaded due to overvoltage(src: Wikipedia)
Note: Capacitors do not generate energy, but can store it or dissipate it. Hence capacitor is a type of passive device.
An electrolytic capacitor in a Rectifier circuit
Electrolytic capacitor as a Filter

Given above is a simple rectifier circuit. A rectifier circuit converts the AC voltage to stable DC voltage with the help of diodes and filters. Diodes used in this circuit convert the input AC waveform into fluctuating DC waveform. This fluctuating DC waveform is then made stable using the electrolytic capacitor as a filter.

Capacitor Symbol

The symbol of a capacitor contains two parallel lines with a gap between them. i.e., Flat, curved, or an arrow passes through it.

  • The flat line denotes that the capacitor is non-polarized.
  • The curved line denotes that the capacitor is polarized.
  • The arrow type denotes that it is of a variable type.

Schematic symbols used for capacitors are given below:

Polarized capacitor

Polarised Capacitor
Polarised Capacitor

These types of capacitors are called polarized because they use electrolytes as one of their electrodes. They have positive and negative terminals. Example: Electrolytic capacitor

Electrolytic capacitor exploded due to reverse polarity connection
Electrolytic capacitor exploded due to reverse polarity connection(src: Wikipedia)

Note: Don’t wire a polarized capacitor backward, as this can damage the capacitor and the circuit.

Non – polarized / fixed capacitor

Fixed capacitor
Fixed capacitor

These types of capacitors have fixed capacitance values. It can be connected in either direction. Example: Ceramic capacitor

Variable capacitor

Variable capacitor
Variable capacitor

These types of capacitors can change their capacitance values. They are used in the tuning circuits to adjust the frequency. The capacitance is varied by increasing or decreasing the effective area between the pairs of conductors that affects the capacitance of the capacitor.

Variable non-polarized / Trimmer capacitor

Trimmer capacitor
Trimmer capacitor

These types of capacitors are used to calibrate a circuit during manufacturing or troubleshooting. The capacitance is changed using trimming of the dielectric medium.

In both polarized and non-polarized cases, there are two terminals running perpendicularly into pairs of conductors.

What is the unit of capacitance?

The capacitance is measured in Farads and is denoted by (F).

1F = 1 coulomb per volt

A capacitor has the capacitance of one Farad when one coulomb of electric charge is stored in the conductor on applying a voltage of one volt. It is always positive.

Construction of a Capacitor

Construction of a Capacitor(electrolytic)
Construction of a Capacitor(electrolytic)

The physical construction of the capacitor varies. It contains two electrical conductors (metal plates)  that are separated by a distance. Positive charges (in the form of protons) get deposited on one conductor and negative charges (in the form of electrons) get deposited on the other conductor.

Parallel plate capacitor
Parallel plate capacitor

The distance between the conductors is filled by air or vacuum or by some form of a good insulating material such as waxed paper, ceramic, mica, plastic, or some form of liquid gel. This insulating material is known as di-electric.

Read also: How does a battery work? Symbol, Chemistry, & Basics

What is the role of dielectric in a capacitor?

The purpose of a dielectric is to increase the capacitance of the capacitor by reducing the amount of energy that is lost when the electric field is created. The dielectric also helps to protect the plates from being damaged by the electric field. Different types of capacitors are fabricated in many forms, styles, lengths, girths, and materials. They have different dielectric materials present in between the conductors.

Conductors allow an easy path for the electrons to pass through them when it is excited by any power supply but dielectric material present in between them blocks the path of the flow of electrons.

What is a dielectric constant?

The dielectric constant is a measure of the ability of the dielectric to store electrical energy. The higher the dielectric constant, the greater the amount of energy that can be stored in the capacitor. It is a dimensionless quantity relative to free space. A dielectric material with a high dielectric constant is a good insulator.

The term permittivity defines how well dielectric material stores an electric field. Permittivity between the conductors is equal to the product of permittivity of free space (∈o) and the relative permittivity(∈r) of the material being used as the dielectric. It is given by:

∈ = ∈o x ∈r

How to calculate the capacitance of a capacitor?

The capacitance of any capacitor can be calculated with the help of a formula:

C = εA/d

Where:

  • C is capacitance;
  • ε is permittivity, a term for how well dielectric material stores an electric field;
  • A is the parallel conductor area
  • d is the distance between the two conductors.

Let us assume a capacitor with two electrical conductors having charges Q1 and Q2 (Normally if one plate has +q the other has –q charge). The electric field (to store charges) present in between the gap of two electrical conductors depends on the charge given to the electrical conductors. As it is known that potential difference (V) is directly proportional to the electric field hence we can say,

Charge ∝ potential difference

Q ∝ V

Q = CV

This proportionality constant C is known as the capacitance of the capacitor. It is defined as the ratio of the maximum charge (Q) that can be stored in a capacitor to the applied voltage (V) across its conductors.

C = Q/V

Where:

  • C is the capacitance in Farads
  • Q is the charge held on the conductors in coulombs
  • V is the potential difference across the conductors in volts

Capacitor Specifications

A capacitor’s most basic rating is its capacitance. Capacitance specifies a capacitor’s charge-holding capability per volt. A capacitor also has some other specifications that are discussed below:

  • Working Voltage: This is the maximum voltage at which the capacitor operates without failure during its cycle life.
  • Tolerance: It is the extent to which the actual capacitance varies from its nominal value.
  • Leakage Current: It is the current that will flow through a dielectric when a capacitor discharges.
  • Equivalent Series Resistance (ESR): It is the capacitor’s impedance at high frequencies (including the dielectric resistance)
  • Working Temperature: The temperature of a capacitor at which it operates nominally.
  • Temperature Coefficient: The maximum change in the capacitance value over a specified temperature range.

Read the guide on diodes: A Beginner’s Guide to Diode (Definition, symbol & working)

How can we increase the capacitance of a capacitor?

The following ways can increase the capacitance of the capacitor:

  1. By choosing a dielectric as good as an insulator
  2. By decreasing the distance between the electrical conductors
  3. By increasing the surface area and the size of conductors

How does a capacitor work?

Consider an uncharged capacitor as shown in the figure below. The two plates (conductors) in the capacitors are electrically neutral i.e., they have an equal amount of positive and negative charge.

Charging of capacitor

Charging of capacitor

When a voltage is applied to this capacitor, the electrons (negative charges) in the upper plate get attracted by the positive terminal of the battery. While the negative terminal of the battery pushes the electrons present in the bottom plate of the capacitor.

Now because of the dielectric between the two conductors, electrons collected on the inner side of this capacitor are not able to cross this barrier.

After some time, more electrons will get accumulated at the bottom plate of this capacitor, while the top plate will have a shortage of electrons. (i.e., the top conductor will get positively charged, while the bottom plate will get negatively charged).

And because of these charged particles, a potential difference will develop across the two conductors. This will continue till the point potential difference across the two conductors is equal to the supply voltage.

Hence due to this potential difference, the electric field will develop across the two conductors. And this developed electric field is directly proportional to the potential difference and inversely proportional to the distance between two conductors.

E = V/d

In this way, the capacitor stores the energy in the form of an electric field.

Discharging of capacitor

Discharging of capacitor

Now even if we remove the voltage source, the plates of this capacitor hold these charges until the terminals are connected in a circuit.

If we connect a bulb as a load across the terminals, the electrons present in the bottom plate will get attracted by the top plate, thus a charge transfer or a current flow takes place through this conductive path and the bulb will glow.

This process will continue till both the plates will become electrically neutral again. When this happens, there will not be any electron flow through the conductive path and the bulb will be turned off. Thus the capacitor is now discharged.

Read also: A Practical Guide to Voltage Regulators (Circuits explained)

Capacitor in series

Capacitor in series
Capacitor in series

When capacitors are connected in series, the overall capacitance of the circuit is reduced. This is because the capacitors are effectively sharing the voltage across them. The formula for calculating the total capacitance of a series circuit is:

1/Ctotal = 1/C1 + 1/C2 + … + 1/Cn

Capacitors in parallel

Capacitor in parallel
Capacitor in parallel

When multiple capacitors are connected together in parallel, the overall effect is that of a single capacitor with increased capacitance. The formula for calculating the total capacitance of a parallel circuit is:

Ctotal = C1 + C2 + C3 +… + Cn

Energy of Capacitor

The energy of the capacitor is measured in joules and is calculated using the formula:

E = 1/2 CV2

Applications of a Capacitor

A capacitor is used in a number of applications. Some of them are listed below:

  • Capacitors are used for storing energy, which can be used by the device for temporary power outages whenever they need additional power.
  • To reduce voltage fluctuations in power supplies and increase the efficiency of power transmission.
  • They are used to generate electromagnetic oscillations.
  • Used in tuning circuits (with inductors) to select information in a particular frequency band.
  • They are also used in electric power distribution for power factor correction.
  • It can be used like a temporary battery as it can store energy when connected to a charging circuit and can dissipate that stored energy when disconnected.

FAQs on Capacitor

Is Capacitor an active or passive device?

A capacitor can store and release energy but can neither control the flow of current nor inject energy on its own. That’s why it is a passive device.

Is Capacitor a linear or non linear device?

Whether a capacitor is a linear or non linear device depends on the frame of reference i.e, with respect to voltage and current OR voltage and charge.

Yes an ideal capacitor is linear because voltage is proportional to the charge stored.

But on the other hand, a capacitor does not follow ohm’s law(V-I characterestics) hence it’s a non linear device. Meaning, current does not varies linearly with varying voltage in a capacitor.

So the answer depends. But theoretically, capacitor is called a linear device.

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