1. In a very practical way, how might one charge or discharge a capacitor? 2. Re

Question

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?

Explanation / Answer

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

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