12th Physics Chapter 1 Important Questions With Answers

 UNIT 1 - ELECTROSTATICS

2mark questions with answers

1. Define electric dipole moment. Give its unit.

Electric dipole moment:

The magnitude of the electric dipole moment (p) is equal to the product of the magnitude of one of the charges (q) and the distance (2a) between them. (i.e) |p| = q.2a. Its unit is Cm.

2. Define action of point or Corona Discharge.

The electric field near this sharp edge is very high and it ionized the surrounding air. The positive ions are repelled and negative ions are attracted towards the sharp edge. This reduces the total charge of the conductor near the sharp edge. This is called action of points or corona discharge.

3. State Coulomb’s law in electrostatics.

Coulomb's law: The electrostatic force is directly proportional to the product of the magnitude of the two point charges and is inversely proportional to the square of the distance between the two point charges.

F ∝q1q2/r²

4. Define electric flux with SI unit.

The number of electric field lines crossing a given area kept normal to the electric field lines is called electric flux (ΦE). Its S.I unit is

Nm²C-¹. It is a scalar quantity.

5. What is called electric dipole? Give an example.

Electric dipole moment and its unit:

The magnitude of the electric dipole moment (p) is equal to the product of the magnitude of one of the charges (q) and the distance (2a) between them. (i.e) |p| = q.2a. Its unit is Cm.

6. State Gauss law.

Gauss law: states that if a charge ‘Q’ is enclosed by an arbitrary closed surface, then the total electric flux through the closed surface is equal to 1/e0 times the net charge enclosed by the surface.

1/e0 = ∮Ē.dĀ = Qinside/e0

7. A sample of HCl gas is placed in a uniform electric field of magnitude 3×104 NC-1. The dipole moment of each HCl molecule is 3.4×10-30 Cm. Calculate the maximum torque experienced by each HCl molecule.

The maximum torque experienced by the dipole is when it is aligned perpendicular to the applied field.

8. Define one coulomb (1 C).

The S.I unit of charge is Coulomb (C) One Coulomb is that charge which when placed in free space or air at a distance 1 m from an equal and similar charge repels with a force of 9x109 N.

9. Give the vector form of Coulomb’s law.

The force on the point charge q2 exerted by another point charge q1

is F21 =1/4πε0 × q1q2/r2 × r̂12

Similarly the force on the point charge q1 exerted by another point charge

q2 is F12 =1/4πε0 × q1q2/r² × r̂21

Here, r̂12 → unit vector directed from q1 to q2

r̂21 → Unit vector directed from q2 to q1

10. Define superposition principle.

According to Superposition principle, the total force acting on a given charge is equal to the vector sum of forces exerted on it by all the other charges.

3 Marks Question & Answer

11. Discuss the basic properties of electric charge.

Basic properties of electric charge:

(i) Electric charge:

Like mass, the electric charge is also an intrinsic and fundamental property of particles. The unit of electric charge is coulomb

(ii) Conservation of electric charge:

The total electric charge in the universe is constant and charge can neither be created nor be destroyed. In any physical process, the net change in charge will be zero. This is called conservation of charges.

(iii) Quantization of charge:

The charge ‘q’ of any object is equal to an integral multiple of this fundamental unit of charge ‘e’ (i.e) q = ne.

Where n → integer and1.6 x 10-19 C

12. A parallel plate capacitor has square plates of side 5cm and separated by a

distance of 1mm. (a) Calculate the capacitance of this capacitor.

(b) If a 10V battery is connected to the capacitor, what is the charge

stored in any one of the plates? (The value of εo=8.85x10-12 Nm²C-²)

(a) The capacitance of the capacitor is 

C = ϵ0A/d ; 8.854 x 10-¹² x 25 x 10-⁴ / 1 x 10-³

= 221.2 x 10-¹³F; ; C = 22.12 x 10-¹² F; = 22.12 pF

(b) The charge stored in any one of the plates is Q = CV, Then

Q = 22.12 x 10-¹² x 10 = 221.2 x 10-¹²C ; Q = 221.2 pC

13. Derive an expression for energy stored in capacitor.

Energy stored in capacitor:

Capacitor is a device used to store charges and energy. When a battery is connected to the capacitor, electrons of total charge ‘-Q’

are transferred from one plate to other plate. For this work is done by the battery. This work done is stored as electrostatic energy in capacitor. To transfer ′dQ′ for a potential difference ‘V’, the work done is

dW = VdQ = Q/C × dQ  [because, V=Q/C]

The total work done to charge a capacitor, 

14. Derive an expression for capacitance of parallel plate capacitor.

Capacitance of parallel plate capacitor:

Consider a capacitor consists of two parallel plates each of area ‘A’ separated by a distance ‘d’

Let ‘o~′ be the surface charge dennsity of the plates.

The electric field between the plates,

15. Give the applications and disadvantage of capacitors

Applications of capacitor:

1. Flash capacitors are used in digital camera to take photographs

2. During cardiac arrest, a device called heart defibrillator is used to give

a sudden surge of a large amount of electrical energy to the patient’s

chest to retrieve the normal heart function.

3. Capacitors are used in the ignition system of automobile engines to eliminate sparking.

4. Capacitors are used to reduce power fluctuations in power supplies

and to increase the efficiency of power transmission.

Disadvantages:

1. Even after the battery or power supply is removed, the capacitor stores

charges and energy for some time. It caused unwanted shock.

16. Derive an expression for torque experienced by an electric dipole placed

in the uniform electric field.

Torque experienced by the dipole in electric field: