Basics / Capacitance / Capacitor

Capacitor What is a Capacitor? How does it work?

We also call a capacitor as a condenser.

What is a Capacitor?

A capacitor consists of two conducting surfaces. In addition, there is an insulating medium between these conducting surfaces. We call this medium as the dielectric. The conducting surfaces may be of different shapes. For example, they may be circular, rectangular, spherical or cylindrical. The purpose of a capacitor is to store electrical energy. A capacitor does it with the help of electrostatic stress in its dielectric medium.

How does a Capacitor Work?

 

"Parallel

Now, we are showing a parallel plate capacitor in the figure above.

charging a capacitor

There are two plates. We have named these plates as A and B. Then we connect plate A to the positive end of a battery. Also, we connect the negative end of the battery to plate B. Due to the electrostatic attraction free electrons in the metallic plate A move away toward the battery. As a result, this plate gets a positive charge. Due to the effect of this positive charge in plate A, plate B becomes negatively charged. That is, plate B accumulates free electrons from the battery through its negative terminal. Therefore, this transient flow of electrons gives rise to a charging current of the capacitor. Also, it establishes a potential difference between these two plates.

charged capacitor

The charging current is maximum at the initial condition. Because at the initial condition the two plates are in uncharged condition. Then gradually the charging current decreases.  Finally, the charging becomes zero. Since, after a certain time, the condenser voltage becomes exactly the same and opposite of the battery voltage. At that time, the battery can not supply any current to the capacitor.

Current Through Capacitor

Actually, the current does not cross the dielectric. The battery only causes the transfer of electrons from and to the plates. The battery also creates a potential difference between the plates. So, when we connect a condenser across a battery, there is no continuous flow of current through it. But, there is an only momentary movement of electrons.

Let us connect a suitable lamp to the circuit as shown in the figure above. The lamp will glow as long as the capacitor is in the charging condition.

It will not glow when the condenser becomes fully charged. That is the condenser voltage becomes equal to the battery voltage.

Altogether we can say, a capacitor blocks steady direct current to flow through it. This is due to the insulating or dielectric medium between its conducting surfaces. But there may be an initial transient current in the capacitor.

Breakdown of Capacitor

Suppose, the potential difference across the plates increases. Therefore, the potential stress in the dielectric increases. After certain potential stress, an electric breakdown occurs in the dielectric medium. Hence, there may be a spark between the plates. The maximum voltage that a dielectric medium can withstand without rupture or breakdown is the dielectric strength.

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