Saturday, July 30, 2016

Working of Capacitor and Dielectric effect on Capacity

Capacitor is a device used to store the charge and its corresponding energy. Capacity is the ability of the device to store the charge. The charge stored in a device is directly proportional to the potential difference it has with the other parts from where the charge happens to flow. To eliminate the proportionality, a constant called capacity is used. And of course  it does not depend on the charge and potential difference and depends on the area of the plates and inversely proportional to the distance of separation between the plates.

Capacitors can be of different shapes and one of the popular one is parallel plate capacitor. It consists of two thin plates kept parallel to each other and there may be air or any non conducting materiel between them.It is measured with a SI unit called farad. 

Principle of working of a capacitor

Capacitor is a device which is used to store the charge in between the two parallel metallic plates. On one plate charge is allowed to accumulate and other plate is kept just next to it. The second plate is under the electric field influence of the first plate and hence, charges separation start happening in the second plate. This phenomenon is called electric charge induction.

Let the first plate is having positive charges. Because of induction, on the inner side of the second plate, negative charges are accumulated as the positive charges of the first plate attracts them.The positive charges on the second plate are repelled and hence they reach the outer side of the second plate.

Because of the inner charges of the second plate, it becomes easy for the new charges accumulation on the first plate and hence they help in storing more charge on the plate. But the second plates positive charges oppose any further loading of new positive charges on the first plate so that storing more charges turns more difficult.

Hence if we eliminate the outer positive charges of the second plate keeping the inner negative charges of it intact, it will help storing the more charge between the plates of the capacitor. That is the reason why the outer part of the second plate of the capacitor is connected to the earth. Thus all outer positive charges of the second plate goes to the earth and make the surface neutral. Hence we will be able to store more charge between the two plates at lower potential. This is how the capacitor is able to store charge between parallel plate capacitor.

Effect of dielectric material on capacity of a capacitor

In general the space between the two plates of capacitor is filled with air. If we place any non conducting materiel in between the plates, charges polarization happens in the atoms of it and hence generates a reverse potential in between. This further helps to store more charge in between the plates of the capacitor.

As the potential decreases, it automatically increases the capacity of the capacitor as  shown in the diagram below.

Basing on the definition of potential and capacity, we can prove that the capacity of a spherical capacitor is directly proportional to its radius as shown below.

Taking the same concept into consideration, we can measure the capacity of the earth capacitor assuming that the earth has a known radius 6400 KM. It is as shown below.

Effect  of dielectric on capacity of capacitor when it is partially filled

When a capacitor is filled with a dielectric partially such that it is filling the entire area but part of separation, it devises the capacitor into two capacitors connected in series. Thus we need to measure each capacity of the capacitor and need to add them basing on the rule of adding the capacitors in series. It is as shown in the diagram below.

Thus the effective capacity increases in this case also.

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