CLASS-XII: April 2020

CHAPTER 1: ELECTRIC CHARGE & FIELD

2 Marks questions:-

1. State vector form of Coulomb Law.

2. Three equal charges Q are placed at corners of a equilateral triangle of side l . Find out electric force at charge placed at one corner of triangle.

3. Write 4 properties of electric lines of force.

4. State Gauss law & write its mathematical expression.

5. Define electric dipole moment & write its SI unit.

6. Find the number of field lines originating from a point charge q = 8.854 μc .

7. Derive an expression for the work done in rotating an electric dipole from its equilibrium position to an angle θ with uniform electrostatic field.

8. An electron & a proton fall through a distance in an uniform electric field E .Compare the time of fall.

9. If q is the positive charge on each molecule of water, what is the total positive charge in a mug of water (360 gm)?

10. What is the ratio of electric field intensity at a point on the equatorial line to the field at a point on the axial line, when the points are at same distance from the centre of the dipole?

3 marks questions:

1. Explain why charge given to a hollow conductor is transferred immediately to outer surface of the conductor.

2. Using Gauss’s theorem in electrostatics, deduce an expression for electric field intensity due to charged spherical shell at a point (i)inside (ii)on it’s surface (iii)out sided it.

3. Derive an expression of work done in rotating an electric dipole from stable equilibrium position to unstable equilibrium.

4.25µC & 9 µC charges are separated by a distance of 100 cm .At what distance from 25μC the net electric field intensity will be zero.

5. Derive an expression of electric field intensity at axial point due to a dipole.

6. Derive an expression of electric field intensity due to linear charge distribution of infinite long wire using Gauss law.

7. An electric dipole with dipole moment 4x10^-9C-m is aligned at 30⁰with the direction of uniform electric field of magnitude 5x10⁴ NC^-1 .Calculate the magnitude of the torque acting on the dipole.

8. A particle of mass m and charge q is released from rest in a uniform electric field E..Calculate the kinetic energy, it attains after moving a distance S between the plates.

9. A polythene piece rubbed with wool is found to have a negative charge of 3x10^-7C.

(a)Estimate the number of electrons transferred (from which to which)

(b)Is there a transfer of mass from wool to polythene?

10. A point charge of 2µC is at the center of a cubic Gaussian surface 9 cm on edge. What is the net electric flux through the surface.

ANSWER KEY:

2 Marks questions

1	Correct answer
2	KQ²/l²
3	Correct answer
4	Correct answer
5	cxm

6	10¹²
7	Correct answer
8	Correct answer
9	1.92x10⁷C
10	Correct answer

3 Marks questions

1	Correct answer
2	Correct answer
3	W=2pE
4	62.5cm
5	Correct answer

6	Correct answer
7	10^-4Nm
8	qES
9	(a)1.87x10¹² from wool to polythene
	(b)Yes, because of transfer of electrons
10	3.76x10⁴vxm