# Electrostatic Potential and Capacitance Class 12 Physics Important Questions

Please refer to Electrostatic Potential and Capacitance Class 12 Physics Important Questions with solutions provided below. These questions and answers have been provided for Class 12 Physics based on the latest syllabus and examination guidelines issued by CBSE, NCERT, and KVS. Students should learn these problem solutions as it will help them to gain more marks in examinations. We have provided Important Questions for Class 12 Physics for all chapters in your book. These Board exam questions have been designed by expert teachers of Standard 12.

## Class 12 Physics Important Questions Electrostatic Potential and Capacitance

Case Based Questions :

A dielectric is a substance which does not allow the flow of charges through it but permits them to exert forces on one another. Some common examples of dielectric materials are paper, mica and water.
When a dielectric slab is placed between the plates of a parallel plate capacitor, then the electric field between the plates of the capacitor Eo polarizes the dielectric. This induces charges on the surface of the dielectric and an electric field is induced inside the dielectric (Ep). Ep is opposite to the direction of Eo. Hence, the net electric field inside a dielectric is (Eo – Ep). The dielectric constant of a dielectric is defined as the ratio of electric field outside the dielectric to the electric field inside the dielectric.
𝐾 = 𝐸𝑜/𝐸𝑜 − 𝐸𝑝
The value of K is more than 1.

Question. A parallel plate capacitor is charged by a battery. After it is fully charged, the battery is disconnected, and a dielectric medium of dielectric constant 20 is filled between the plates of the capacitor. Identify the correct statement among the following :
(a) Energy stored by the capacitor remains constant.
(b) Potential difference across the plates of capacitor remains constant.
(c) Charge stored by the capacitor remains constant.
(d) Energy density of the capacitor remains constant.

C

Question. For a polar dielectric material, which of the following statements is correct?
(a) The centres of gravity of positive and negative charges coincide
(b) The charge distribution is always symmetrical
(c) The dipole moment of each molecule is zero and the dipole moment of the specimen is also zero in the absence of external electric field
(d) The dipole moment of each molecule is non-zero but the dipole moment of the specimen is zero in the absence of external electric field

D

Question. In a parallel plate capacitor, the capacitance increases from 4μF to 80μF on introducing a dielectric between the plates. The dielectric constant of the medium is
(a) 40
(b) 20
(c) 50
(d) 76

B

Question. When a dielectric material is placed between the plates of a charged capacitor then
(a) Electric field between the plates increases
(b) Capacitance of the parallel plate capacitor decreases
(c) Electric field between the plates decreases
(d) Electric field remains the same, only capacitance is increased.

C

Electric Potential Energy
Consider a system of two charges q1 and q2 with position vector r1 and r2 relative to some origin. Let us calculate the work done (externally) in building up this configuration. Suppose, first the charge q1 is brought from infinity to the point r1. There is no external field against which work needs to be done, so work done in bringing q1 from infinity to r1 is zero. This charge produces a potential in space given by :
WHERE ARE R1 AND R2

V = kQ/r
where r is the distance of a point P in space from the location of q1. Now, work done in bringing charge q2 from infinity to the point r2 is q2 times the potential at r2 due to q1
W = kq1 q2 /r12
where r 12 is the distance between points 1 and 2. Since electrostatic force is conservative, this work gets stored in the form of potential energy of the system. Thus, the potential energy of a system of two charges q1 and q2 is U = Kq1q2/r12
The potential energy expression is unaltered whatever way the charges are brought to the specified locations, because of path-independence of work for electrostatic force.

Question. Four charges Q, -Q, Q and -Q are placed at four corners of a square of side a. Work done in moving a charge-q from its centre to infinity is
(a) zero
(b) kQ2 /a
(c) 4kQ2/ a
(d) kQ2/4a

A

Question. A system consists of twoPOINT charges 7 μC and –2 μC placed at (–9 cm, 0, 0) and (9 cm, 0, 0) respectively in an external electric field E = A (1/r 2 ); A = 9 × 105 NC–1 m2 . The electrostatic energy of this system is
(a) 0.7 J
(b) 49.3 J
(c) 29.6 J
(d) – 0.7 J

B

Question. A charge Q is placed at the origin. The electric potential due to this charge at a point P in space is V. The work done by an external force in bringing another charge q from infinity upto the pointP is
(a) ( q + Q) V
(b) ( Q – q ) V
(c) QV
(d) qV