Other Important Questions – Chapter 2 Electrostatic Potential and Capacitance Class 12 CBSE Physics
Q.1. Show diagrammatically the arrangement of four point electric charges of equal magnitude placed at four corners of a square such that the electric field as well as the electric potential at the centre of the square is non-zero.
Ans. For the following arrangement of four point electric charges of equal magnitude, the electric field as well as electric potential at the centre of the square will be non-zero.
Q. 2. Can electric potential at a point be zero, while the electric field is not zero?
Ans. Yes, electric potential is zero at all points on equatorial line of electric dipole, while electric field strength is not zero.
Q. 3. Can electric field at a point be zero, while electric potential is not zero?
Ans. Yes, inside a hollow charged metallic conductor, the electric field is zero, but electric potential is finite.
Q. 4. Do free electrons travel to region of higher potential or lower potential? [NCERT Exemplar]
Ans. Free electrons would travel to regions of higher potentials as they are negatively charged.
Q. 5. Can there be a potential difference between two adjacent conductors carrying the same charge? [NCERT Exemplar]
Ans. Yes.
Q. 6. Two protons are brought nearer; what will be the effect on potential energy of system?
Ans. A repulsive force acts between protons, if they are brought nearer, work must be done by external force; hence the potential energy of system increases.
Q. 7. An electron and a proton are brought nearer; how does the potential energy of system change?
Ans. There is attractive force between an electron and a proton, therefore when they come nearer, the work is done by the system itself and so the potential energy of system decreases.
Q. 8. Show that the equipotential surfaces are closed together in the regions of strong field and far apart in the regions of weak field. Draw equipotential surfaces for an electric dipole. [CBSE Sample Paper 2016]
Ans. Equipotential surfaces are closer together in the regions of strong field and farther
apart in the regions of weak field.
Q. 9. Concentric equipotential surfaces due to a charged body placed at the centre are shown. Identify the polarity of the charge and draw the electric field lines due to it. [HOTS][CBSE Sample Paper 2016]
Ans.
For a single charge the potential is given by
This shows that V is constant if r is constant. Greater the radius smaller will be the potential. In the given figure, potential is increasing. This shows that the polarity of charge is negative (– q). The direction of electric field will be radially inward. The field lines are directed from higher to lower potential.
Q. 10. A uniform electric field exists between two charged plates as shown in the fig. What should be the work done in moving a charge q along the closed rectangular path ABCDA? [HOTS]
Ans. Work done in an electric field is independent of the path and depends only on the
initial and final positions.
Here initial and final points are coincident.
Work =q × (Vfinal – Vinitial)
= q (VA – VA)
So, net work done is zero.
Q. 11. The capacitance of a charged capacitor is C and the energy stored in it is U. What is the value of charge on the capacitor?
Ans.
Q. 12. Sketch graph to show how charge Q given to a capacitor of capacitance C varies with the potential difference.
Ans. The graph of charge (Q) versus potential difference (V) is a straight line whose
slope is equal to capacitance ‘C’.
Q. 13. Name the dielectric whose molecules have
(i) Non-zero and
(ii) Zero dipole moment.
Ans. (i) The dielectric having non-zero dipole moment is water or HCl.
(ii) The dipole having zero dipole moment is CH4 or H2.
Q. 14. Define dielectric constant of a medium. What is the value of dielectric constant for a metal?
Ans. The dielectric constant of a medium is defined on the ratio of permittivity of medium to the permittivity of free space. The dielectric constant is also called the relative permittivity of medium.
The value of dielectric constant for a metal is infinity i.e., Kmetal = ∞
Q. 15. Express dielectric constant of a medium in terms of capacitance. What is its SI unit?
Ans. The dielectric constant of a medium is defined as the ratio of capacitance of capacitor when filled with a medium to capacitance of same capacitor when medium is removed.
K = Cmedium / C
i.e., It has no unit.
Q. 16. What is the function of a dielectric in a capacitor?
Ans. Dielectric reduces the effective potential on plates and hence increases the capacitance.
Q. 17. How does the electric field inside a dielectric decrease when it is placed in an external electric field?
Ans. When a dielectric is placed in an external electric field, the charges are induced on the faces of dielectric which produce opposite electric field in the dielectric. Thus net electric field inside the dielectric is reduced.
Q. 18. The graph shows the variation of voltage ‘V’ across the plates of two capacitors A and B versus increase of charge ‘Q’ stored on them. Which of the two capacitors has higher capacitance? Give reason for your answer. [HOTS]
Ans.
As slope of A is smaller, capacitance of A is higher.
Q. 19. The graph shown here shows the variation of total energy (E) stored in a capacitor against the value of the capacitance (C) itself. Which of the two: the charge on capacitor or the potential used to charge it, is kept constant for this graph? [HOTS]
Ans. The given graph represents,
This is satisfied by the expression, for constant q.
That is, the charge (q) is kept constant.
Q. 20. If a point charge is rotated in an arc of radius r around a charge q, what will be the work done? Explain. [HOTS]
Ans. All points of circle of radius r are at same potential, hence work done is zero.
Q. 21. “Gauss’s law in electrostatics is true for any closed surface, no matter what its shape or size is.” Justify this statement with the help of a suitable example. [HOTS]
Ans. According to Gauss theorem, the electric flux through a closed surface depends only on the net charge enclosed by the surface and not upon the shape or size of the surface.
For any closed arbitrary slope of the surface enclosing a charge the outward flux is the same as that due to a spherical Gaussian surface enclosing the same charge.
Justification: This is due to the fact that
(i) Electric field is radial and
(ii) The electric field