## Class 12th Physics Electrostatic Potential and Capacitance Important Question and PYQ

• November 11, 2020

5 Marks Questions

1. (a) Define dielectric constant in terms of the capacitance of a capacitor? On what factor does the capacitance of a parallel plate capacitor with dielectric depend?

(b) Find the ratio of the potential differences that must be applied across the

(1) parallel

(2) Series combination of two identical capacitors so that the energy stored in the two cases becomes the same.

Ans. (a) Dielectric constant is defined as the ratio of capacitance of a capacitor when the dielectric is filled in between the plates to the capacitance of a capacitor when there is vaccuum in between the plates

Therefore, the potential is zero at a distance of 40 cm from the positive charge outside the system of charges.

6. A parallel plate capacitor with air between the plates has a capacitance of 8 pF ( What will be the capacitance if the distance between the plates is reduced by half, and the space between them is filled with a substance of dielectric constant 6?

Ans. Capacitance between the parallel plates of the capacitor, C = 8 pF

Initially, distance between the parallel plates was d and it was filled with air. Dielectric constant of air, k = 1

Capacitance, C, is given by the formula,

Therefore, the capacitance between the plates is 96 pF.

7. A charge of 8 mC is located at the origin. Calculate the work done in taking a small charge of  from a point P (0, 0, 3 cm) to a point Q (0, 4 cm, 0), via a point R

(0, 6 cm, 9 cm).

Therefore, the potential at the centre of the cube is .

The electric field at the centre of the cube, due to the eight charges, gets cancelled. This is because the charges are distributed symmetrically with respect to the centre of the cube. Hence, the electric field is zero at the centre.

9. Two tiny spheres carrying charges 1.5  are located 30 cm apart. Find the potential and electric field:

1. at the mid-point of the line joining the two charges, and

2. at a point 10 cm from this midpoint in a plane normal to the line and passing through the mid-point.

Ans.

## Electrostatic Potential and Capacitance Previous Year Questions

Q.10 (a)A small sphere of radius a carrying a positive charge q, is placed concentrically inside a larger hollow conducting shell of radius b (b> a). This outer shell has charge Q on it . Show that if these spheres are connected by a conducting fire, charge will always flow from the inner sphere to the outer sphere, irrespective of the magnitude of the two charges.
[D 2009 C]

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