30-03-2012, 02:38 PM
Capacitors
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The combination of any two conductors separated by an insulator is called a capacitor. A capacitor is a device that can be made to store electric charge and you can compare it with a bucket used to store water.
In general a bigger capacitor can store more charge than a smaller one. The two conductors usually carry an equal and opposite charge such that the net charge on the capacitor as a whole is zero. If a capacitor is state as having a charge Q it means that the conductor at the higher potential has a charge + Q and that at the lower poten¬tial a charge - Q.
Eventually a charge Q is stored on each plate and the capacitor is said to be fully charged.
Notice that both plates have the same size of charge although they are of opposite sign.
The addition of a resistor ® in the circuit (Figure 3) does not affect the final potential difference across the capacitor. However it will slow down the time it takes the capacitor to become fully charged because the current in the circuit during charging will be less.
We will return to charging later to look at the factors which affect the rate at which capacitors can be charged and discharged in much greater detail including a mathematical treatment.