Figure 8.3.1 8.3. 1: (a) Three capacitors are connected in series. The magnitude of the charge on each plate is Q. (b) The network of capacitors in (a) is equivalent to one capacitor that has a smaller capacitance than any of the individual capacitances in (a), and the charge on its plates is Q.
As the capacitor plates have equal amounts of charge of the opposite sign, the total charge is actually zero. However, because the charges are separated they have energy and can do work when they are brought together. One farad is a very large value of capacitance.
Charge on this equivalent capacitor is the same as the charge on any capacitor in a series combination: That is, all capacitors of a series combination have the same charge. This occurs due to the conservation of charge in the circuit.
When a capacitor is charged, the amount of charge stored depends on: its capacitance: i.e. the greater the capacitance, the more charge is stored at a given voltage. KEY POINT - The capacitance of a capacitor, C, is defined as:
The capacitance C of a capacitor is defined as the ratio of the maximum charge Q that can be stored in a capacitor to the applied voltage V across its plates. In other words, capacitance is the largest amount of charge per volt that can be stored on the device: C = Q V
As charge flows from one plate to the other through the resistor the charge is neutralised and so the current falls and the rate of decrease of potential difference also falls. Eventually the charge on the plates is zero and the current and potential difference are also zero - the capacitor is fully discharged.
At this point the capacitor is said to be "fully charged" with electrons. The strength or rate of this charging current is at its maximum value when the plates are fully discharged (initial condition) and slowly reduces in value to zero as the plates …