Any conducting object that carries a charge is characterized by an electric potential that is constant everywhere on and within that object. If 2 such conductors have a potential difference between them then as any potential difference is able to accelerate charges, the system effectively stores energy. Such a device that can maintain a potential difference, storing energy by storing charge is called a capacitor. Capacitance is an amount of charge required to raise the potential of a conductor by 1 volt. The relation between the amount of the charge the capacitor stores and the potential difference it maintains depends on the geometry of the conductor. The storage of charge is also affected by the presence of dielectric material.
DEFINITION OF CAPACITOR:
Capacitance is defined as the amount of charge required to raise the potential of a conductor by 1 volt.
PRINCIPLE OF CONDUCTOR:
An insulated metal plate is considered. Let it be conducted to a voltage source and charged to the maximum and that is no more flow of charge occurs from the cell to the conductor.
Now an uncharged plate b is brought near it. Due to electrostatic induction, a negative charge is introduced on the left face and positive charge on the right face. The negative charge reduces the potential on the plate a and the positive charge increase4s the potential of the plate a. as a negative charge is closer to the plate, its effect is more and the potential of the plate is reduced. Thus, more charges can be given to an increasing its capacitance. If the other face of plate b is connected to the earth, the positive charge flows to earth, thus the potential of the plate a is decreased even more making it possible to store more charge. Thus, we see that if an earthed conductor brought near to a conductor, its capacitance increases.
PARALLEL PLATE CAPACITOR:
This arrangement consists of 2 thin conducting plates, each of the areas a and separated by d distance. When charge q is given to 1st plate, a charge -q is induced on the inner face of the other plate and positive on the outer face of the plate. As this face is connected to the earth, a net negative charge is left on this plate. Thus, the arrangement is equivalent to 2 thin sheets of charge.
EFFECT OF DIELECTRIC ON CAPACITANCE:
A dielectric constant k is inserted between the plates filling the entire space between the plates. The plates of the capacitor are given charges +q and -q and hence induced charges-q of p and +q of p appear on the surface of the slab.
The energy stored in the large capacitance can do harm, giving us a burn or a shock. One reason we are warned not to touch a circuit, or inside of electronic devices, is because the capacitors may still be carrying charge even if the external power has been turned off. On the other hand, the basis of the heart defibrillators is a capacitor charged to a high voltage. A heart attack can be characterized by 1st irregular beating of the heart, known as cardiac fibrillation. The heart then does not pump blood to the rest of the body properly, and if it lasts for long then it can also give a death result. A sudden brief jolt of charge through the heart from the defibrillator can cause3 complete heart stoppage, sometimes followed by a resumption of normal beating. The defibrillator capacitor is charged to a voltage typically of a few thousand volts and is allowed to discharge very rapidly through the heart via the pair of wide contacts known as paddles that spread out the current over the chest.
Polarised dielectric is equivalent to 2 charged surfaces having induced surface charge densities say sigma and – sigma. The external field is thus opposed by the field produced by these surface charge densities. The capacitance depends only on the geometrical configuration of the capacitor and is independent of the charge or potential of the capacitor. In the series combination, charges are the same on each capacitor. In parallel combination, the same potential difference is applied across all the capacitors. The potential energy of the capacitor is stored in the electric field between the plates. Irrespective of the charge and the field configuration outside, any cavity inside a conductor remains shielded from outside electric influence. But the exterior of the conductor is not shielded from the fields by the inside charges. All the charges reside on the outer surface of the conductor.
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