Capacitors and Capacitance:
In order to store the charge, a device which is called capacitor
is used. It consists of two thin metal plates, parallel to each
other separated by a very small distance (Fig. 13.17). The
medium between the two plates is air or a sheet of some
insulator. This medium is known as dielectric.
If +Q amount of charge is given to plate A of a parallel plate
capacitor, due to electrostatic induction, it would induce –Q
charge on the inner surface of the plate B and + Q on its outer
surface. The lines of force between the plates have been
shown in (Fig. 13.17-b).
There exists a force of attraction between the charge +Q stored
on the plate A and the charge –Q induced on the inner surface of
plate B. Due to this force of attraction, the charges are bound
within the plates and remained stored for a long periods.
Due to presence of the charges on the plates, a potential
difference V is developed across them which is directly
proportional to the charge Q deposited on the plate A.
Q = CV
where C is the constant of proportionality called the
capacitance of the capacitor and is defined as the ability of
the capacitor to store charge. It is given by the ratio of charge
and the electric potential as
SI unit of capacitance is farad (F), defined as:
If one coulomb of charge given to the plates of a capacitor
produces a potential difference of one volt between the plates
of the capacitor then its capacitance would be one farad.
farad is a large unit, usually we use a smaller unit called micro
farad (µF)and pico farad (pF) etc.
The capacitance of a parallel plate capacitor is
100 pF. If the potential difference between its plates is
50 volts, find the quantity of charge that capacitor can store.
What will be the charge on each plate?
Potential difference between the plates V = 50V
Capacitance C = 100pF = 100 × 10-12 F.
Charge Q =?
Using the formula Q = CV
Putting the values Q = 100 × 10-12 F × 50V
= 5 × 10-9 C = 5 nC
Charge on each plate will be 5 nC, because each plate has
equal amount of charge.
The volt is named after the Italian physicist Alessandro Volta (1745-1827), who developed the first practical electric battery, known as a voltaic pile. Because potential difference is measured in units of volts, it is sometimes referred to as voltage.
A voltage across a device, such as capacitor, has the same meaning as the potential difference across the device. For instance, if we suppose that the voltage across a capacitor is 12 V, it also means that the potential difference between its plates is 12 V.
Physics Insight about Capacitors and Capacitance:
Parallel plate capacitor consists of two metal plates separated with an insulator. The presence of charge Q on plate A induces a –Q charge on the inner surface of plate B
For your information
Farad is a big unit of capacitance. We generally use the following submultiples:
1 micro farad = 1μF = 1 × 10-6F
1 nano farad = 1nF = 1 × 10-9F
1 pico farad = 1pF = 1 × 10-12 F