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Capacitors

A capacitor or condenser is an electrical or electronic device that can store energy.

It stores the energy within the electric field between a pair of conductors (called "plates"). The process of storing energy in the capacitor is known as "charging", and involves electric charges of equal size, but opposite charge, building up on each plate.

Once charged the plates have a uniform electric field between them. Within the main body of the plates this field is truly uniform - but at the edges the uniformity is disrupted due to 'edge effects'. Therefore any practical work should take account of this.

Capacitors are often used in electric and electronic circuits as energy-storage devices. There are many different types but in physics questions you are usually asked about a simple parallel plate capacitor. In electronics you will learn about the many types and their uses.

Capacitors are of two types: electrolytic and non-electrolytic.

They have two values stamped on them - their capacitance and a working voltage. If the capacitor exceeds this voltage, the insulating layer will break down and the component will short out. 

If we connect up circuit A and then close the switch we would observe the bulb lighting up brightly and then getting gradually dimmer until it went out. (This would happen quickly so we would just observe a flash).

This is because the brightness of the bulb will depend upon the size of the current flowing. Initially charge would flow quickly onto the plates of the capacitor (brightly lit bulb) then, as the plates began to fill with charge, the rate of charge flow would exponetially decrease (bulb would grow dimmer) until finally the capacitor would be fully charged and no more charge woul flow (bulb would be dark).

With circuit B, however we would not even notice that the capacitor was there! the bulb would remain lit all of the time. The capacitor would never be fully charged it would be in the process of charging 50 times a second on opposite plates - therefore there would be a good rate of charge transfer (current) to keep the bulb brightly lit up.

The capacitor therefore blocks d.c. current but allows a.c. current through.

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