Capacitance

The ability of a capacitor to store electric charge is called capacitance. Capacitors with high capacitance will store large amount of electric charge whereas the capacitors with low capacitance will store small amount of electric charge.

The capacitance of a capacitor can be compared with the size of a water tank: the larger the water tank, the more water it can store. In the similar way, the larger the capacitance, the more electric charge or electricity it can store.

Capacitors with different physical characteristics (such as shape and size of their plates) store different amounts of charge for the same applied voltage (V) across their plates. The capacitance (C) of a capacitor is defined as is the largest amount of charge per volt that can be stored on the device:

C=Q/V.  The SI unit of capacitance is the farad (F). 

Combining capacitors

Capacitors can be connected to achieve different values of capacitance.

When capacitors are connected in series and to a voltage supply:

• no matter what the value of its capacitance, each capacitor in the combination stores the same amount of charge.
• the total charge stored by a series of capacitors is the charge on each of the two outer plates and is equal to the charge stored on each individual capacitor
•  the total charge stored is less than the charge that would be stored by any one of the capacitors connected individually to the power supply.

The effect of adding capacitors in series is to reduce the capacitance. 

When you connect capacitors in parallel, then the total capacitance will be equal to the sum of all the capacitors capacitance.

A variable air capacitor has two sets of parallel plates. One set of plates is fixed (indicated as “stator”), and the other set of plates is attached to a shaft that can be rotated (indicated as “rotor”). By turning the shaft, the cross-sectional area in the overlap of the plates can be changed; therefore, the capacitance of this system can be tuned to a desired value. Capacitor tuning has applications in any type of radio transmission and in receiving radio signals from electronic devices. Any time you tune your car radio to your favorite station, think of capacitance.

Figure 3: In a variable air capacitor, capacitance can be tuned by changing the effective area of the plates. 

Energy is needed from a power supply or other source to charge a capacitor. A charged capacitor can supply the energy needed to maintain the memory in a calculator or the current in a circuit when the supply voltage is too low.

The amount of energy stored in a capacitor depends on:

• the amount of charge on the capacitor plates
• the power required to place this charge on the capacitor plates, i.e. the capacitance of the capacitor.