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1. In a very practical way, how might one charge or discharge a capacitor?

2. Real capacitors are always rated by capacitance and a maximum voltage. What is the significance of the voltage value? What governs this voltage?

3. Until a few years ago, one-farad capacitors were an abstract notion. Now they are readily available and a push is on to use them to replace batteries in certain applications. How is it possible to have large values of capacitance in about the same size as a few stacked coins?
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1) Charge

With the switch at A, the capacitor is charging. Current flows from the battery through the capacitor. The electrons move to one plate, but they do not jump the insulating gap inside the capacitor. They collect on the surface of the plate.

Meanwhile, electrons are removed from the other plate from the abundance that is always there in metals. That gives the plate a net positive charge. And removing the charge completes the path around which current flows.

Discharge

Example: Suppose your capacitor is charged to 9 volts, and at time t = 0 the switch is connected to a one ohm resistor. The discharge time is regulated by the resistance.

The initial current (t = 0) is I = V/R = (9 volts)/(1 ohm) = 9 amps.

2) Q= C* V

Where: Q (Charge, in Coulombs) = C (Capacitance, in Farads) x V (Voltage, in Volts)

V = Q/C

So any of these quantities can be found provided the other two are known. The formulae can easily be re-arranged using a simple triangle similar to the one used for calculating Ohm�s Law when carrying out resistor calculations.

V = IR

3)

The comparisons between the the different types of capacitor is generally made with regards to the dielectric used between the plates. Like resistors, there are also variable types of capacitors which allow us to vary their capacitance value for use in radio or �frequency tuning� type circuits.

Commercial types of Capacitor are made from metallic foil interlaced with thin sheets of either paraffin-impregnated paper or Mylar as the dielectric material. Some capacitors look like tubes, this is because the metal foil plates are rolled up into a cylinder to form a small package with the insulating dielectric material sandwiched in between them.

There are a very, very large variety of different types of capacitor available in the market place and each one has its own set of characteristics and applications, from very small delicate trimming capacitors up to large power metal-can type capacitors used in high voltage power correction and smoothing circuits.

As capacitance represents the capacitors ability (capacity) to store an electrical charge on its plates we can define one Farad as the �capacitance of a capacitor which requires a charge of one coulomb to establish a potential difference of one volt between its plates� as firstly described by Michael Faraday. So the larger the capacitance, the higher is the amount of charge stored on a capacitor for the same amount of voltage..
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