CBSE Guess > Papers > Important Questions > Class XII > 2011 > Physics > Physics By Mr. Ajay Sharma

CBSE CLASS XII

Electrostatic Pot and Capacitance

(Q.1) What is the net charge on a charged capacitor? ( 1 mark ) (Q.2) What will happen to the capacity of a parallel plate capacitor if area of each plate is doubled and distance between the plates becomes halved? ( 1 mark )

(Q.3) What happens to the electric field inside a dielectric when it is placed in an external electric field? ( 1 mark )

(Q.4) What will be the effect on potential if a medium of dielectric constant K is introduced? (H) ( 1 mark )

(Q.5) What would be the work done if a point charge +q is taken from a point A to the point B on the circumference of the circle, which is having a +q point charge at the centre? (H) ( 1 mark )

(Q.6) a) What is the unit of electric flux?
b) Is it correct that the capacitance of a spherical conductor is equal to the radius of the conductor in CGS system? ( 1 mark )

(Q.7) Is electric potential necessarily zero at a point where electric field strength is zero. ( 1 mark )

(Q.8) Does electric potential increase along electric lines of forces. ( 1 mark )

(Q.9) A charge Q is distributed over two concentric hollow spheres of radii r and R (> r) such that the surface densities are equal. The potential at the common centre is (H) ( 1 mark )

(Q.10) Potential due to a single point charge is ( 1 mark )

(Q.11) A lightning conductor affords protection to the building since it ( 1 mark )

(a) Conducts the electric charge to the earth
(b) Repels away the charge clouds
(c) Diverts back the charge to the adjacent ground
(d) Transform back the charge to the clouds.

(Q.12) If we increase separation ‘d’ between the plates of a parallel plate condenser to ‘2d’ and fill wax to the whole empty space between its two plates, then capacitance increases from 1 pF to 2 pF. What is the dielectric constant of wax?(H) ( 1 mark )

(a) 1
(b) 2
(c) 3
(d) 4

(Q.13) In a charged capacitor, the energy is stored in ( 1 mark )

(a) The negative charges
(b) The positive charges
(c) The field between the plates
(d) Both ‘a’ and ‘b’

(Q.14) Two spherical conductors A and B of radii a and b (b > a) are placed concentrically in air. B is given a charge + Q and A is earthed. The equivalent capacitance of the system is(H)

( 1 mark )

(Q.15) We assume that earth is at zero potential because capacitance of the earth is ( 1mark)

(a) Infinite
(b) Zero
(c) 104 Farad
(d) 106 Farad

(Q.16) Fill in the blank with suitable option :Electric field inside a dielectric ———— when it is placed in an external electric field. ( 1 mark )

(a) Increases
(b) Decreases
(c) Remains same
(d) Becomes zero

(Q.17) Atomic Polarizability has dimension of(H) ( 1 mark )

(a) Area
(b) Volume
(c) Force
(d) Torque

(Q.18) A parallel plate capacitor is located horizontally such that one of the plates is submerged in a liquid while the other is above the liquid surface. When plates are charged the level of liquid (H)

( 1 mark )

(a) Rises
(b) Falls
(c) Remains unchanged
(d) May rise or fall depending on the amount of charge

(Q.19) The capacitors of capacities 3µF, 9µF and 18µF are connected first in series and then in parallel. The total capacity in the two cases in the ratio (Cs :Cp) of: ( 1 mark )

(a) 1 : 15
(b) 15 : 1
(c) 1 : 1
(d) 1 : 3

(Q.20) A potential difference of 250 volt is applied across the plates of a capacitor of 10 pF.The charge on the plates of the capacitor will be ( 1 mark )

(a) 2.5 10^-9 C.
(b) 4.5 10^-9 C.
(c) 2.5 10^-8 C.
(d) 3.5 10^-9 C

(Q.21) Increasing the charge on the plates of a capacitor means: ( 1 mark )

(a) Increasing the capacitance
(b) Increasing the potential difference between the plates.
(c) Decreasing the potential difference between the plates.
(d) No change in the field between the plates.

(Q.22) If the potential of a capacitor having capacity 6 mF is increased from 10 V to 20 V, then increase in its energy will be ( 1 mark )

(Q.23) A parallel plate capacitor is filled with dielectric as shown in Fig. Its capacitance has ratio with that of without dielectric as(H)

( 1 mark )

(Q.24) Consider a parallel plate capacitor of capacity 10 mF with air filled in the gap between the plates. Now one half of the space between the plates is filled with a dielectric of dielectric constant K = 4 as shown in figure. (H)

( 1 mark )

The capacity of the capacitor changes to

(a) 25 mF
(b) 20 mF
(c) 40 mF
(d) 5 mF

(Q.25) The order of capacitances used in power supplies is ( 1 mark )

(a) 1 microfarad to 10 microfarad
(b) 11 microfarad to 20 microfarad
(c) 15 microfarad to 20 microfarad
(d) 20 microfarad to30 microfarad

(Q.26) A condenser of capacity C1 is charged to a potential V0. The electrostatic energy stored in it is U0. It is connected to another uncharged condenser of capacity C2 in parallel. The energy dissipated in the process is (H) ( 1 mark )

(Q.27) A finite ladder is constructed by connecting several sections of 2 mF, 4 mF capacitor combinations as shown in the figure. It is terminated by a capacitor of capacitance C. What value should be chosen for C, such that the equivalent capacitance of the ladder between the point A and B becomes independent of the number of sections in between them? (H)

( 1 mark )

(a) 4 mF
(b) 2 mF
(c) 18 mF
(d) 6 mF

(Q.28) A solid conducting sphere having a charge Q is surrounded by an uncharged concentric conducting hollow spherical shell. Let the potential difference between the surface of the solid sphere and that of the outer surface of the hollow shell be V. If the shell is now given a charge of – 3Q, the new potential difference between the same two surfaces is(1mark )

(a) V
(b) 2 V
(c) 4 V
(d) - 2 V

(Q.29) An electric dipole has the magnitude of its charge as q and its dipole moment is p. It is placed in uniform electric field E. If its dipole moment is along the direction of the field, the force on it and its potential energy are respectively(H) ( 1 mark )

(a) q.E and max.
(b) 2 q.E and min.
(c) q.E and p.E
(d) Zero and min.

(Q.30) A parallel plate capacitor of capacitance C is connected to a battery and is charged to a potential difference V. Another capacitor of capacitance 2C is similarly charged to a potential difference 2V. The charging battery is now disconnected and the capacitors are connected in parallel to each other in such a way that the positive terminal of one is connected to the negative terminal of the other. The final energy of the configuration is (H)(1mark )

(Q.31) A charge +q is fixed at each of the points x = x₀, x = 3x₀, x = 5x₀, .... on X-axis and charge –q is fixed on each of the points x = 2x₀, x = 4x₀, x = 6x₀, .... . Here x₀ is a positive constant. Take the potential at a point due to a charge Q at a distance r from it to be . Then the potential at the origin due to above system of charges will be (H) ( 1 mark )

(Q.32) Identical charges – q each are placed at 8 corners of a cube of each side b. Electrostatic potential energy of a charge + q which is placed at the centre of cube will be(1mark )

(Q.33) A unit charge moves on an equipotential surface from a point A to point B. Then (1mark )

(a) V_A – V_B = + ve
(b) V_A – V_B = 0
(c) V_A – V_B = – ve
(d) It is stationary

(Q.34) A condenser having capacity 2 F is charged to 200V potential, its plates are joined to any resistance wires. What is the heat produced? ( 1 mark )

(Q.35) Two parallel plates are 2 cm apart and a potential difference of 50 V is applied across them. What is the electric filed intensity between the two plates ? ( 1 mark )

(a) -2500 V/m
(b) +2500 V/m
(c) -1500 V/m
(d) +1500 V/m

(Q.36) A potential difference of 250 volt is applied across the plates of a capacitor of 10 pF. What is the charge on the plates of the capacitor? ( 1 mark )

(a) 1.5x10^-9 C
(b) 2.5x10^-9 C
(c) 4.2x10^-4 C
(d) 1.25x10⁵ C

(Q.37) How much energy stored in a capacitor of 100mF when it is charged to a potential of 100 volt. ( 1 mark )

(a) 0.4 joule
(b) 0.125 joule
(c) 0.5 joule
(d) 1 joule

(Q.38) Four metallic plates each with surface area ‘A’ and interplate separation ‘d’ are placed as shown. Alternate plates are connected to points A and B. What is the equivalent capacitance of the system?(H)

( 2 Marks )

(Q.39) Four capacitors are connected as shown in the fig. What is the equivalent capacitances between A and B?

(Q.40) If a unit charge is taken from one part to another part over an equipotential surface, then what is the work done on the charge? ( 2 Marks )

(Q.41) Give the relation between the electric field and potential for closely spaced equipotential surfaces. ( 2 Marks )

(Q.42) What are equipotential surfaces? ( 2 Marks )

(Q.43) What principle do we use to find the potential at a point due to a system of charges?(2Marks )

(Q.44) What is a Vande Graaff Generator? Draw a Vande Graaff generator?(3 Marks)

(Q.45) What is electrostatic shielding? During thunderstorms it is safer to be inside a car or stand under the tree.Explain your answer. ( 3 Marks )

(Q.46) In a parallel plate capacitor with air between the plates has an area 6*10^-3m² and the distance between the plates is 6 mm.find the capacitance of the capacitor.if this capacitor is connected to a 200v supply, what is the charge on each plate of the capacitor? (H) ( 3 Marks )

(Q.47) The electric field at a point due to a point charge is 20Nc^-1 and the electric potential at that point is 40jc^-1 .Calculate the distance of the point from the charge and the magnitude of the charge. ( 3 Marks )

(Q.48) A parallel plate condenser with plate area A is filled with two dielectric K₁ and K₂ each occupying equal space lengthwise. If the separation between two plates is t for each dielectric then what is the capacity of the condenser. (H)

( 3 Marks )

(Q.49) A parallel plate capacitor having a plate separation 2mm possesses a capacitance of 18PF.The capacitor is connected to a 100v supply.Explain what would happen , if a 2 mm thick mica sheet of dielectric const k=6 were inserted between the plates(a) while the voltage supply remains connected.?(b) after the supply was disconnected. (H) ( 5 Marks )

(Q.50) What is the potential energy of a dipole placed in an external field? Also give an expression for Torque experienced by the dipole in a uniform electric field. What will be the torque experienced by the dipole if it is in the direction of the electric field? ( 5 Marks )

(Q.51) A 300PF capacitor is charged by a 200v supply.it is then disconnected from the supply and is connected to another uncharged 300PF capacitor.How much energy is lost in the process? ( 5 Marks

(Q.52) Derive the formula for effective capacitance when

(i) Capacitors C₁ and C₂ are connected in series.
(ii) Capacitors C₁ and C₂ are connected in parallel.
What happen to the charge on the capacitor if it is disconnected from the battery? ( 5 Marks )

(Q.53) Differentiate between electrostatic potential and electrostatic potential energy?
A metal sphere 0.30 m in radius is positively charged with 2 C. Find the potential at distance 1m from the sphere and at the centre of the sphere.

Submitted By Mr. Ajay Sharma
Email Id : [email protected]