How Capacitor Works

How Capacitor Works?

In this post we will understand how the capacitor stores electrical energy.

Capacitor - Introduction:

  • A capacitor is an electrical device which is used to store electrical energy.
  • It is also refered as Condenser.
  • They are used extensively in electrical and electronic circuits.
  • Capacitors are used to smooth rectified a.c. outputs, used in radio receivers for tuning to the required frequency, used in time delay circuits, in electrical filters, in oscillator circuits etc.

Capacitor Symbol:

The symbols of fixed capacitor and a variable capacitor are shown below:





Capacitor Unit:

The unit of capacitance is called as "farad" F. (Usually μF=10−6F or pF =10−12F).
A Farad is defined as the capacitance when a potential difference of one volt appears across the plates when charged with one coulomb.

Capacitor Charge Storing Operation:

We know that two parallel plates forms a simple capacitor. When the capacitor is connected to the DC voltage supply,

  1. Electrons are pushed onto one plate by the negative terminal of the  dc supply, and
  2. Electrons are pulled from the other plate by the positive terminal of the dc supply.

If the charge difference between the two plates increases beyond certain limit, discharge will happen by forming a spark across the gap between the plates. 

  • To increase the amount of  stored charge in the parallel plates, a dielectric (non conducting material) is placed between the plates.
  • This dielectric material behaves like a spark blocker across the plates.
  • Thus it increases the charge storing capacity of the capacitor.
  • Some of dielectric material used in commercial capacitors are paper, plastic film, mica, glass, ceramic and air.
  • Based on the type of dielectric material used the capacitors are classified into various types.
    Click here to know more about the types of capacitors. 
  • The capacitor’s plate surface area and the distance between the parallel plates are some of  other factors that affect capacitance levels.

The electrical energy is stored in the capacitor in the form of electrostatic field.

C = Q/V

C = Capacitance in F
Q = Charge in Coulomb
V = Voltage applied across the capacitor in Volts

The capacitor current equation is given below

I = C.dV/dt

C =  Capacitance in F

dV/dt = Change in voltage across the capacitor
I = Displacement current. It is not the current flows through the capacitor

It is very common that students will confuse with this equation by assuming that I is the conventional current flows through the capacitor. It is not the conventional current flows through the device, like current flows through the resistor or wire. It is the displacement current.

What is meant by Displacement current?

It is an apparent current through the capacitor that results from the act of the plates charging up with time and produces magnetic fields that induce electrons to move in the opposite plates.

  • Actually current never goes across the parallel plates.
  • Instead of that, charges on one plate are shoved by the changing magnetic fields produced by the opposing plate.The overall effect makes things appear as if current is flowing across the gap.

Click Here to know about Capacitor Types and Uses of Capacitor

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