CAPACITOR

CAPACITOR

Figure 1: Examples of capacitor

Capacitor is a device that stores electric potential energy by storing separated positive and negative charges.

Figure 2: Visual of two charged plate

Work done to separate the positive charge from negative charge since there is an attractive force between the two charges. The work done to separate the charges is called electric potential energy.

An electric field lines arises between the two conductors. The field lines beginning on the conductor with positive charges and ending on the conductor with negative charge.

Electric potential energy is the energy stored in an electric field.

A parallel plate capacitor is one of the simplest form of capacitor. It consisting of two metal plates each of the same area, A and separated by distance, d. A charge +Q are on one plate and another charge -Q on the other. (Assume that the plates are separated by air gap.

If charge magnitude Q us evenly spread over each plate with surface area A, then the surface charge density is denoted by

The magnitude of the electric field outside the conductor is

With

Since the field between the plates is uniform, the magnitude of the potential difference is

The constant of proportionality between charge and potential difference depends only on geometric factors, size and shapes of the plates and also the material between the plates.

Where

     Q  is the magnitude of the charge on each plate

   is the magnitude of the potential difference between the plates

     C  is the constant proportionality or capacitance

CAPACITANCE, C can be thought as the capacity to hold charge, Q for a given potential difference .

The SI unite of capacitance are Coulombs per volts, C/V (which is called farad, F)

(commonly microfarads, nano-farads, or picofarads)

To find capacitance for parallel plate capacitor, the electric field is

Where A is the inner surface area of each plate. If the distance of each plate is d, then the magnitude of the potential difference is

Q can be written as:

Thus capacitance of a parallel plate capacitor is

A large charge is important to store in a capacitor. In order, to store the charge without making the potential difference between the plate excessively large, we need large capacitance.

• Capacitance ix inversely proportional to the spacing d between the plates, the air between the capacitor is not appropriate.
• Air can easily breakdown when the plate spacing is small
• Breakdown allows the spark to jump across the gap causing the stored charge lost.

One way to overcome this problem is to put insulator between the plates. Some insulating material also called dielectrics, can with stand electric field larger than those that cause air to breakdown. In addition the capacitance is also increase by placing the dielectrics between the plates.

For a parallel plate capacitor with dielectric fills between the plates , the capacitance is

K is the dielectric constant, it is the effect of the dielectric which increase the capacitance by a factor K.(K= 1, Air is slight larger than 1)

Dielectric constant determines how much charge can be stored for a given potential difference.

Dielectric strength determines how large a potential difference,  can be applied to a capacitor before dielectric breakdown occurs

POLARIZATION IN A DIELECTRIC

Polarization is a separation of the charge in an atom or molecule. There is two type which in polarized and unpolarized atom. The unpolarized atom with a central positive charge is surrounded by a cloud of electrons. When a positively charged, rod is brought near the atom, it repels the positive charge and attracts the negative charge in the atom.

Throughout the dielectrics, there a still equal amount of positive and negative charges. The net effect of polarization of the dielectric is a layer of positive charge on one side and negative side on another. The dielectric constant of a material is a measure of the ease it can be polarized. A larger dielectric constant indicates a more easily polarized material.