# Physics Class 12 NCERT Solutions: Chapter 2 Electrostatic Potential and Capacitance Part 12

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Q: 26. The plates of a parallel plate capacitor have an area of each and are separated by . The capacitor is charged by connecting it to a supply.

(A) How much electrostatic energy is stored by the capacitor?

(B) View this energy as stored in the electrostatic field between the plates, and obtain the energy per unit volume. Hence arrive at a relation between and the magnitude of electric field between the plates.

Answer:

Area of the plates of a parallel plate capacitor,

Distance between the plates,

Potential difference across the plates,

(A) Capacitance of the capacitor is given by the relation,

Electrostatic energy stored in the capacitor is given by the relation,

Where,

Permittivity of free space

Hence, the electrostatic energy stored by the capacitor is .

(B) Volume of the given capacitor,

Energy stored in the capacitor per unit volume is given by,

Again,

Where,

Electric intensity

Q: 27. A capacitor is charged by a supply. It is then disconnected from the supply, and is connected to another uncharged capacitor. How much electrostatic energy of the first capacitor is lost in the form of heat and electromagnetic radiation?

Answer:

Capacitance of a charged capacitor,

Supply voltage,

Electrostatic energy stored in is given by,

Capacitance of an uncharged capacitor,

When is connected to the circuit, the potential acquired by it is

According to the conservation of charge, initial charge on capacitor is equal to the final charge on capacitors, and .

Electrostatic energy for the combination of two capacitors is given by,

Hence, amount of electrostatic energy lost by capacitor