In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy.In How Batteries Work page,we discussed that a battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons on the other terminal. A capacitor is much simpler than a battery, as it can't produce new electrons,it only stores the energy that will liberate electrons when in contact with a conductor.
WORKING PRINCIPLE OF A CAPACITOR (WATER ANALOGY)
When we "charge" a conventional metal-plate capacitor, the power supply pushes electrons into one plate, and the fields from these extra electrons reach across the gap between the plates, forcing an equal number of electrons to flow out of the other plate and into the power supply. This creates opposite areas of imbalanced charge i.e one plate has less electrons and excess protons, and the other plate has more electrons than protons. Each plate does store charge.
However, if we consider the capacitor as a whole, no electrons have been put into the capacitor. None have been removed. The same number of electrons are in a "charged" capacitor as in a capacitor which has been totally "discharged." Yes, a certain amount of charge has been forced to flow momentarily during "charging," and a rising potential difference has appeared. But the current is directed THROUGH the capacitor, and the incoming electrons force other electrons to leave at the same time. Every bit of charge that's injected into one terminal must be forced out of the other terminal at the same time. The amount of charge inside the capacitor never changes. The net charge on each plate is cancelled by the opposite charge on the other plate. Capacitors are never "charged" with electric charge!
Reasoning :
When "charging" a capacitor, a momentary current causes the voltage to rise. Volts times electron-flow equals energy-flow ( V x I = P). Therefore during a momentary current through a capacitor, there is a joules-per-second transfer of energy from the power supply into the capacitor.Therefore, during the "charging" process, ENERGY is placed in the capacitor. Capacitors store energy, not charge. When we "charge" a capacitor, we give it a charge of energy. Because we use the word "charge" to refer both to electric charges and ALSO to quantities of energy, capacitor explanations are impossible to understand. "Charging" a capacitor means injecting electrical energy into the device. Similar trouble is caused when we say that we "charge" a battery. We charge a battery with some energy in the form of stored chemical fuel, but we pump electric charge THROUGH the battery and none of it builds up inside.
It's all terribly confusing. What are students to think if we tell them that "charging a battery" does not store any charge, yet charge must flow through the battery if we want to charge it! Ugh. The word "charge" has far too many meanings. In science this is always a Very Bad Thing.
Another, less misleading situation is similar: think of the word "charge" as applied to gunpowder. A charge is placed in an old cannon, followed by a cannonball. It would be silly to assume that, because we've "charged" the cannon, the cannon now has an electrical charge. But whenever we state that we've "charged" a capacitor, we DO assume that an electrical charge has been stored inside. This is just as silly as mistaking gunpowder for electrostatic charges. Charging a capacitor is like charging a cannon; in both situations we are inserting energy, not electrical charge.
Here's yet another way to visualize it. Whenever we "charge" a capacitor, the path for current is THROUGH the capacitor and back out again. The extra electrons on one plate force electrons to leave the other plate, and vice versa. Visualize a capacitor as being like a belt-driven wind-up motor. If we shove the rubber belt along, the spring-motor inside the capacitor winds up. If next we let the rubber belt go free, the wound-up spring inside the motor drives the belt in the other direction, and the spring becomes "discharged." But no quantity of "belt" is stored inside this motor. The belt flows THROUGHT it, and we wouldn't want to label this motor as a "machine which accumulates rubber." Yet this is exactly what we say whenever we state that a capacitor "stores charge."
My favorite capacitor analogy is a heavy hollow iron sphere which is completely full of water and is divided in half with a flexible rubber plate through its middle. Hoses are connected to the two halves of the sphere, where they act as connecting wires. The rubber plate is an analogy for the dielectric. The two regions of water symbolize the capacitor plates.
Imagine that the rubber plate is flat and undistorted at the start. If I connect a pump to the two hoses and turn it on for a moment, the pump will pull water from one half of the iron sphere and force it into the other. This will bend the rubber divider plate more and more. The more the plate bends, the higher the back-pressure the plate exerts, and finally the pressure will grow strong enough that the pump will stall. Next I seal off the hose connections and remove the pump. I now have created a "charged" hydraulic capacitor.
It would be misleading to say "this iron sphere is a device for accumulating water", or "this sphere can be charged with water, and the stored water can be retrieved during discharge." Both statements are wrong. No water was injected into the sphere while it was being "charged."
Imagine that I now connect a single length of pre-filled hose between the two halves of the capacitor. As soon as the last connection is complete, the forces created by the bent rubber plate will drive a sudden immense spurt of water through this hose. Water from one side will be pushed into the other side, and the rubber plate will relax. I've discharged my hydraulic capacitor. How much water has been discharged? None! A momentary current has flowed through the sphere device, and the rubber plate is back to the middle again, and the water has become a bit warm through friction against the surfaces of the hose. The stored energy has been "discharged," but no water has escaped. The hydraulic capacitor has lost its energy, but still has the same amount of water.
FUNCTIONS OF CAPACITOR
As I said earlier the main function of a capacitor is to:
i) store electrical energy
other functions:
ii) as a filter i.e passing alternating current a.c and blocking direct current d.c.
iii) used with inductor to make an oscillator i.e radio tuning.
iv)capacitor is used in regulator to smooth regulated power.
