Electrostatic Potential and Capacitance Class 12 Physics MCQs Pdf
51. Why is electrostatic potential energy of a pair of like point charges positive?
Answer/Explanation
Answer:
Explaination: Because the work has to be done against the electrostatic force of
repulsion.
52. In the expression W = pE (cos 0O – cos 0j), why is 0Q is taken as n/2 for obtaining expression for the potential energy of electric dipole?
Answer/Explanation
Answer:
Explaination:
When the dipole axis is perpendicular to the electric field, i.e. θO = π/2,
the work done against the external electric field \(\vec{E}\) in bringing
the charges +q and -q is equal and opposite, and cancel out, i.e. q[V(r1)
– V(r2)] = 0. Therefore, initial potential energy is zero.
53. Why should electrostatic field be zero inside a conductor? [Delhi 2012]
Answer/Explanation
Answer:
Explaination:
In the static situation, there is no excess charge. Hence, electric field is
zero everywhere inside the conductor.
54. Why can the interior of a conductor have no excess charge in the static situation?
Answer/Explanation
Answer:
Explaination:
Consider any volume element V enclosed by the Gaussian surface S inside the
conductor. According to the Gauss’s law,
Therefore, in the static situation, the charge will reside only on the outer
surface.
55. Why does the electric field inside a dielectric decrease when it is placed in an external electric field?
Answer/Explanation
Answer:
Explaination:
An electric field is developed inside the dielectric due to induction in a
direction opposite to the direction of external electric field. Thus, net
electric field decreases.
56. For what position of an electric dipole in a uniform electric field its potential
energy is
(i) minimum and
(ii) maximum?
Answer/Explanation
Answer:
Explaination:
(i) Potential energy of dipole in external field is minimum when \(\vec{p}\) and
\(\vec{E}\) are parallel, i.e. θ = 0°.
∴ U = -pE cos θ
⇒ Umin = -pE
(ii) The potential energy of dipole in the external field is maximum when
\(\vec{p}\) and \(\vec{E}\) are anti-parallel, i.e. θ = 180° and Umax
= +pE.
57. If the electric potential equals to zero at a point, must the electric field be zero at that point?
Answer/Explanation
Answer:
Explaination:
No. for example, at any point on the equitorial line of the electric dipole; the
electric potential is zero, but the electric field is not zero.
58. If electric field at a point equals to zero, must the electric potential at that point be zero?
Answer/Explanation
Answer:
Explaination:
No. Inside a charged conducting sphere, E = 0, but the potential is constant and
is not zero.
59. A hollow metal sphere of radius 5 cm is charged such that the potential on its surface is 10 V. What is the potential at the centre of the sphere? [AI 2011]
Answer/Explanation
Answer:
Explaination: 10 V, as the potential inside the hollow metal sphere is same
everywhere.
60. A point charge Q is placed at point O as shown in the figure. Is the potential
difference VA – VB positive, negative or zero, if Q is
(i) positive (ii) negative? [Foreign 2017, Delhi 2011]
Answer/Explanation
Answer:
Explaination:
As V ∝ \(\frac{1}{r}\)
(i) For Q having +ve value, VA > VB, i.e. VA
– VB is +ve.
(ii) For Q having -ve value, VA < VB, i.e. VA – VB is
negative.
61. Figure given below shows three points A, B and C in a uniform electrostatic
field. At which of the points will the electric potential be maximum?
Answer/Explanation
Answer:
Explaination:
At B, the electric potential will be maximum, because E =
–\(\frac{dV}{Dr}\).
62. Why is the potential inside a hollow spherical charged conductor constant and has the same value as on its surface? [Foreign 2012]
Answer/Explanation
Answer:
Explaination:
Electric field inside a hollow charged conductor is zero and there is no
tangential component on the surface of the conductor. So, work done is zero in
moving a charge inside and on the surface of the conductor.
E = –\(\frac{dV}{Dr}\)
From this, we conclude that potential inside the spherical charged conductor is
constant and it is same on the surface. In inside a hollow charge
conductor,
E = 0
V = constant
63. Define dielectric strength of a medium. What is its value for vacuum?
Answer/Explanation
Answer:
Explaination:
It is the maximum value of electric field that can exist inside a dielectric
without causing its electrical breakdown. For vacuum, its value is infinity.
64. A charge q is moved from a point A above dipole of dipole moment P to a point B
below the dipole in equatorial plane without acceleration. Find the work done in the
process. [AI 2016]
Answer/Explanation
Answer:
Explaination:
The work done is zero, as electrostatic potential at any point in equitorial
plane is zero.
65. If the plates of a charged capacitor be suddenly connected to each other by a wire, what will happen?
Answer/Explanation
Answer:
Explaination:
Capacitor will be discharged immediately, and the part of electrical energy will
be converted in the form of heat and light (sparks will be produced).
66. Name the physical quantity whose SI unit is F m-1 (farad/metre).
Answer/Explanation
Answer:
Explaination:
Electrical permittivity (\(\varepsilon \text { or } \varepsilon_{0}\)).
67. Where does the energy of a capacitor reside?
Answer/Explanation
Answer:
Explaination: Electrical energy resides in the space, within the plates.