CBSE Class 12 important electrostatic questions

Important electrostatic questions

Questions having one mark each:
1. Write the S.I unit of
i. electric field intensity
ii. Electric dipole moment.
iii electric Potential
2. How does the coulomb force between two point charges depend upon the dielectric constant of the intervening medium?
3. Draw an equipotential surface for a system, consisting of two charges Q, -Q separated by a distance ‘r’ apart.
4. Show graphically the variation of charge ‘q’ with time’t’ when a condenser is charged.
5. What orientation of an electric dipole in a uniform electric field would correspond to stable equilibrium?
6. The potential energy of an electric dipole placed in a uniform electric field is minimum . What is the orientation of vectors p and E ?
7. State one situation in where potential is not zero but electric field intensity is zero .
8.  Does the electric potential increase or decrease along an electric line of force ?
9. A Gaussian surface encloses no charge . Does it mean that E = 0 on its surface ?
10. At what point is the electric dipole field intensity (i) parallel and (ii) antiparallel to dipole moment of the dipole ?
11. A 10µC charge is at the centre of a square of side 10 cm . Find the work done in moving the charge of 4 C between two diagonally opposite points on the square .
12. A charged air capacitor has stored energy U . What will be the energy stored when air is replaced by a dielectric of dielectic constant K , charge Q remains the same ?
13. The sequence of bands marked on carbon resistor are Red , Red , Red , Silver . Write the value of resistance with tolerance .
14. Define electric field intensity . Write its S.I. unit . Write the magnitude and direction of electric field intensity due to an electric dipole of length 2a at the mid point of the line joining the two charges.
15.. The force of attraction between two point charges placed at distance d apart in a medium is F. What should be the distance apart in the same medium so that the force of attraction between them becomes F/4 ?
16.Name the physical quantity whose SI unit is (i) coulomb/ volt (ii) newton/coulomb(iii) joule/coulomb.
17. What is the amount of work done in moving a 56 nC charge between two points 1 cm apart on an equipotential surface ?
18.What would be the work done if a point charge +q, is taken from a point A to the point B on the circumstance of a circle drawn with another point charge +q at the center ?

19.Two test charges are brought separately near a charge +Q.
Case 1: Test charge +q are brought to within a distance r from the charge +Q (i.e. to point A).
Case 2: Test charge +2q are brought to within a distance 2r from the charge +Q (i.e. to point B).
1. In which situation is the Electric Potential Energy of the system the greatest?
2. At which position is the Electric Potential the greatest

Questions having two mark each

1. Two point charges, q =8×10^-8 C and Q= -2×10^-8 C are separated by a distance of 10 cm in air.
a) Find at what distance from the Q, would the electric potential be zero.
b) What is the net electric field at the mid point between the charges
2). Two capacitors of capacitance 10µF and 12µF are connected in series with a battery. The voltage across the 12µF capacitor is 10V. Compute the total battery voltage..

3) Electric charge is distributed uniformly on the surface of a spherical shell. Explain the variation Of Electric potential and electric Field intensity
i. on the surface
ii. inside and
iii. outside

4)Derive an expression for the energy stored in a parallel plate capacitor C , charged to potential difference V .
5)Two equally charged identical metal spheres X and Y repel each other with a force of F N. Another identical uncharged sphere Z is touched to X and then placed at the mid point between X and Y. What is the net force on Z?
6)The total capacitance of two capacitors is 4µF when connected in series and 18µF
when connected in parallel. Find the capacitance of each capacitor ?
7)A parallel plate capacitor is charged to a certain potential difference. When a  3 mm thick slab is slipped between the capacitor plates then to maintain the same Potential difference between the plates, the plate separation has to be increased by 2.4 mm. Find the dielectric constant of the slab

8) Charges (+q) and (–q) are placed at the points A and B respectively which are a distance 2L apart. C is the midpoint between A and B. What is the work done in
moving a charge +Q along the semicircle CRD

9)Two large, thin metal plates are parallel and close to each other. On their inner faces, the plates have surface charge densities of opposite
signs and of magnitude $17.7 \times 10^{-22} \[ C/m^2$ . What is electric field intensity E:
(a) in the outer region of the first plate, and
(b) between the plates?

Questions Having 5 marks each

1. Explain the principle on which Van-de-Graaff generator operates. Draw a labeled schematic sketch and write briefly its working.
2. An electric dipole is held in a uniform electric field.
i. Show that no translatory force acts on it.
ii. Derive an expression for the torque acting on it.
iii. The dipole is aligned parallel to the field. Calculate the work done in rotating it through 180 degree

3)A parallel plate capacitor is charged by a battery. After sometime the battery is disconnected and a dielectric slab with its thickness equal to the plate separation is inserted between the plates.
What is the impact on these quantities?
(i) The capacitance of the capacitor
(ii) Potential difference between the plates
(iii) The energy stored in the capacitor

4. Use Gauss theorem to derive an expression for the electric field at a point due to

a. an infinite plane sheet of charge of uniform charge density ?
b. a thin infinitely long straight line  of charge of uniform charge density ?

5.There are two identical particles each of mass m and carrying charge Q. Initially one of them is at rest and another charge moves with velocity v directly towards the particle at rest. Find the distance of closest approach.

MCQ questions

1)Which of the following is not the property of an equipotential surface?
(a) They do not cross each other.
(b) The work done in carrying a charge from one point to another on an equipotential surface is zero.
(c ) For a uniform electric field, they are concentric spheres.
(d) They can be imaginary spheres.

2) An electric dipole placed in an electric field of intensity 2 × 105 N/C at an angle of 30° experiences a torque equal to 4 Nm. The charge on the dipole of dipole length
2 cm is
(a) 7 µC
(b) 8 mC
(c) 2 mC
(d) 5 mC

3) The electric potential on the axis of an electric dipole at a distance ‘r from it’s center is V. Then the potential at a point at the same
distance on its equatorial line will be

(i) 2V
(ii) -V
(iii) V/2
(iv) Zero

4) According to Coulomb’s law, which is the correct relation for the following figure?

(i) q1 q2> 0
(ii) q1 q2<0
(iii) q1 q2 =0
(iv) 1> q1/ q2 > 0

5)An electric dipole of moment p is placed parallel to the uniform electric field. The amount of work done in rotating the dipole by $90^0$ is-
(i) 2pE
(ii) pE
(iii) pE/2
(iv) Zero

6) Three capacitors 2µF, 3µF and 6µF are joined in series with each other.
The equivalent capacitance is-
(i) 1/2µF
(ii) 1µF
(iii) 2µF
(iv) 11µF

7)Two point charges placed in a medium of dielectric constant 5 are at a distance r between them, experience an electrostatic force ‘F’. The electrostatic force between them in vacuum at the same distance r will be-
(i) 5F
(ii) F
(iii) F/2
(iv) F/5

8) Which statement is true for Gauss law-
(i) All the charges whether inside or outside the gaussian surface contribute to the electric flux.
(ii) Electric flux depends upon the geometry of the gaussian surface.
(iii) Gauss theorem can be applied to non-uniform electric field.
(iv) The electric field over the gaussian surface remains continuous and uniform at every point.

9) A capacitor plates are charged by a battery with ‘V’ volts. After charging battery is disconnected and a dielectric slab with dielectric constant ‘K’
is inserted between its plates, the potential across the plates of a capacitor will become

(i) Zero
(ii) V/2
(iii) V/K
(iv) KV

Assertion and Reason Questions

Here two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

1)Assertion(A) :In a nonuniform electric field, a dipole will have translatory as well as rotatory motion.
Reason(R):In a nonuniform electric field, a dipole experiences a force as well as torque.

2) Assertion(A):Electric field is always normal to equipotential surfaces and along the direction of decreasing order of potential
Reason(R): Negative gradient of electric potential is electric field.

Case Study Questions

A Faraday cage or Faraday shield is an enclosure made of a conducting material. The fields within a conductor cancel out with any external fields, so the electric field within the enclosure is zero. These Faraday cages act as big hollow conductors you can put things in to shield them from electrical fields. Any electrical shocks the cage receives, pass harmlessly around the outside of the cage.

1. Which of the following material can be used to make a Faraday cage?
   (i)
   a)Plastic
   b)Glass
   c) Copper
   d)Wood
2. Example of a real-world Faraday cage is
   a) car
   b) plastic box
   c) lightning rod
   d) metal rod
   a)
3. What is the electrical force inside a Faraday cage when it is struck by lightning?
   a) The same as the lightning
   b) Half that of the lightning
   c) Zero
   d) A quarter of the lightning
4. An isolated point charge +q is placed inside the Faraday cage. Its
   surface must have charge equal toa) Zero
   b) +q
   c) –q
   d) +2q
5. A point charge of 2C is placed at center of Faraday cage in the shape of cube with surface of 9 cm edge. The number of electric field lines passing through the cube normally will be
6. a) 1.9105 Nm2/C entering the surface
   b) 1.9105 Nm2/C leaving the surface
   c) 2.0105 Nm2/C leaving the surface
   d) 2.0105 Nm2/C entering the surface