Question Pool Electrostatic Potential and Capacitance

# Electrostatic Potential and Capacitance - SAMAGRA Question Pool & Answers | Class 12

Kerala Syllabus SAMAGRA SCERT SAMAGRA Question Pool for Class 12 Physics Electrostatic Potential and Capacitance

Qn 1.
If the distance between the plates of a capacitor is halved then what happens to its capacitance?

The value of capacitance become double

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Qn 2.

Why is it safer to be inside a car than standing under a tree during lightning?

Electrostatic shielding

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Qn 3.

A parallel plate capacitor has plates of area 200 cm2 and separation between the plates 1 mm. Calculate (i) the potential difference between the plates if 1n C charge is given to the capacitor (ii) with the same charge (1n C) if the plate separation is increased to 2 mm, what is the new potential difference and (iii) electric field between the plates.

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Qn 4.

Define electric potential difference between two points ?

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Qn 5.

“Electric potential at a point is 5 volt” .what do you mean by the statements ?

The work done to bring 1 C of charge from infinity to that point is

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Qn 6.

Give the unit and dimensions of electric potential

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Qn 7.

If " W  " is the work done to bring a charge " q " from infinity to a point then what is the electric potential at that point ?

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Qn 8.

What do you meant by electric potential at a point ?

It is defined as the work done to bring a unit positive test charge from infinity to that

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Qn 9.

What is meant by equipotential surface?

It is an imaginary  surface in which all points are of same potential at every

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Qn 10.

Inside a hollow charged spherical conductor, the potential

A) Is constant

B) Varies directly as the distance from the centre

C) Varies inversely as the distance from the centre

D) Varies inversely as the square of the distance from the centre

A ) Is a constant

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Qn 11.

Two electrons are held 3μm apart. When released from rest, what is the velocity of each electron when they are 8μm apart?

Let Up1 be the potential electric energy at rest (distance r = 3μm) and Up2 be the potential electric energy when they are 8μm apart and moving. The total

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Qn 12.

The distance AB between charges Q1 and Q2 shown below is 5.0 m. How much work must be done to move charge Q2 to a new location at point C so that the distance BC = 2.5 m?

If W is the work to be done to move Q2 from a position where its potential energy is Up1 and kinetic energy 0 (from rest) to another position where its potential

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Qn 13.

Draw the equipotential surface in a uniform electric field?

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Qn 14.

Draw the equipotential surface for two like charges separated by a small distance ?

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Qn 15.

Draw the equipotential surface for an electric dipole?

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Qn 16.

Draw the equipotential surface for a point charge?

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Qn 17.

Write the expression for the potential at a point on the surface of

i) the inner sphere and

ii) the outer sphere.

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Qn 18.

a) What is the use of van de Graff generator?

b) States the principle of van de Graff generator?

c) Explain its working with a neat diagram ?

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Qn 19.

State the principle/s of van de Graff generator?

1) Discharging action of sharp points: Electric discharge takes place in air or gases readily at pointed conductors

2) When a charged conductor is brought

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Qn 20.

a) Find the work done to assemble two charges q1 and q2 at the points of potentials V(r1) and V(r2) respectively?

b) If there is no external electric field what is the work done to separate them far apart.

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Qn 21.

An electric dipole is placed in a uniform electric field at an angle θ with the field E.

a) What happens to the dipole

b) Obtain the expression for energy of the above system.

c) Give the conditions for stable and unstable equilibrium of the dipole system.

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Qn 22.

N identical and equally charged (q) liquid drops coleases to form a single big drop .What is the charge and capacitance of the big drop. ?

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Qn 23.

a)Define electric polarization and give its unit ?

b)Writes the relation between polarization and electric field ?

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Qn 24.

Distinguish between polar and non polar dielectrics with examples?

Polar dielectrics

They posses permanent dipole moment.

The centres of positive charges and negative charges don't coincide each

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Qn 25.

The work done to move half a revolution of electron in an Orbit is zero. Why?

Orbit is an equipotential surface. So work done is

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Qn 26.

Prove that electric field is always perpendicular to an equipotential surface.

Work done to move a charge q from one point to another point on an equipotential is zero.

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Qn 27.

a)What do you meant by energy of a system of three charges ?

b)obtain the expression for the potential energy of the above system of charges?

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Qn 28.

a)What do you meant by energy of a system of two charges ?

b)obtain the expression for the potential energy of a system of two charges?

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Qn 29.

what are the different methods to increase the capacitance of a parallel plate capacitor.

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Qn 30.

a)What is the use of a capacitor ?

b)Obtain the expression for the capacitance of a parallel plate capacitor ?

c)What are the methods to increase the capacitance of a parallel plate capacitor ?

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Qn 31.

Where does the energy stored in a capacitor ?

The energy is stored in the electric field between the plates of a

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Qn 32.

When a dielectric slab is introduced between the plates of a capacitor what happens to its capacitance ?

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Qn 33.

When a dielectric is introduced between the plates of a charged but source less parallel plates capacitor then what happens to the electric field between the plates ?

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Qn 34.

The capacitance of a conductor is said to be one Farad if one coulomb of charge increases its potential by one volt.

1F = 1C /

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Qn 35.

Give the unit and dimensions of capacitance ?

[C] = [ M-1L-2T-4A2

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Qn 36.

Find the resultant capacitance when three capacitors of capacitance C1 ,C2 and C3 are connected in series ?

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Qn 37.

You are provided with three capacitors of capacitance C1, C2 and C3. How will you connect them to get maximum capacitance. Obtain the expression for total capacitance of the combination.

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Qn 38.

a) obtain the expression for the potential due to a electric dipole at any point.

b) what is the potential at a point on the axial and equatorial lines of the dipole.

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Qn 39.

a) what do you mean by energy of a capacitor

b) obtain an expression for energy stored in a capacitor.

c) a capacitor is charged and disconnected from the cell and a dielectric slab is introduced between the plates of the capacitor what happens to the energy of the capacitor and why?

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Qn 40.

What is the work done by the field of a nucleus in a complete circular orbit of the electron? What if the orbit is elliptical?Justify your answer.

Whenever the electron completes an orbit, either circular or elliptical, the work done by the field of a nucleus is zero.

W = F.s= Fs cosØ , where

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Qn 41.

In a Van de Graaff type generator a spherical metal shell is to be a 15 × 106 V . The dielectric strength of the gas surrounding the electrode is 5 × 107 Vm−1. What is the minimum radius of the spherical shell required?

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Qn 42.

Describe schematically the equipotential surfaces corresponding to

(a) a single positive charge at the origin, and

(b) a uniform grid consisting of long equally spaced parallel charged wires in a plane.

(a) Concentric spheres centered at the origin are equipotential surfaces.

(b) A periodically varying shape near the given grid is the equipotential surface.

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Qn 43.

Describe schematically the equipotential surfaces corresponding to

(a) a constant electric field in the z-direction,

(b) a field that uniformly increases in magnitude but remains in a constant (say, z) direction,

a) Equidistant planes parallel to the x-y plane are the equipotential surfaces.

(b) Planes parallel to the x-y plane are the

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Qn 44.

A bird perches on a bare high power line, and nothing happensto the bird. A man standing on the ground touches the same line and gets a fatal shock. Why?

Current passes only when there is difference in

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Qn 45.

Ordinary rubber is an insulator. But special rubber tyres of aircraft are made slightly conducting. Why ?

To enable them to conduct charge (produced by friction) to the ground. As too much of static electricity accumulated may result in spark and it result in

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Qn 46.

A comb run through one’s dry hair attracts small bits of paper.Why? What happens if the hair is wet or if it is a rainy day?

This is because the comb gets charged by friction. The moleculesin the paper gets polarised by the charged comb, resulting in anet force of attraction.

If

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Qn 47.

Prove that electric field at the surface of a charged conductor,

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Qn 48.

Vehicles carrying inflammable materials usually have metallic ropes touching the ground during motion. Why?

To enable them to conduct the accumulated charge (produced by friction ) to the ground; As too much of static electricity accumulated may result in spark and result

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Qn 49.

Does the kinetic energy of a small negative charge increase or decrease in going from Q to P ?

Attraction icreases from Q to P.Hence potential energy of negative charge decreases in going from Q toP.Hence Kinetic energy

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Qn 50.

Does the kinetic energy of a small negative charge increase or decrease in going from B to A?

Due to force of repulsion on the negative charge, velocity decreases and hence the kinetic energy decreases in going from B to A.(Potential energy

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Qn 51.

Give the sign of the work done by the external agency in moving a small negative charge from B to A ?

Work done = potential energy difference of a small negative charge between the points A and B = UA - UB = -q (VA

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Qn 52.

Give the sign of the work done by the field in moving a small positive charge from Q to P.

In moving a small positive charge from Q to P, work has to be done by an external agency against the electric field. Therefore,work done by the field is

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Qn 53.

Give the sign of the potential energy difference of a small negative charge between the points A and B ?

potential energy difference of a small negative charge between the points A and B = UA - UB = -q (VA – VB

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Qn 54.

Chose the correct one from the following ?Give reason.

A                         B                           C

a) VA<VB<VC                      b) VA >VB >VC

c) VA =VB=VC                      c) VA<VB>VC

Ans: b

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Qn 55.

Give the sign of the potential energy difference of a small negative charge between the points Q and P ?

As V α 1/r , VP > VQ. Thus, (VP – VQ) is positive.

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Qn 56.

A molecule of a substance has a permanent electric dipole moment of magnitude 1029 C m. A mole of this substance is polarised (at low temperature) by applying a strong electrostatic field of magnitude 106 V m1. The direction of the field is suddenly changed by an angle of 60º. Estimate the heat released by the substance in aligning its dipoles along the new direction of the field. For simplicity, assume 100% polarisation of the sample.?

Dipole moment of each molecules = 10–29 C m

As 1 mole of the substance contains 6 × 1023 molecules,

total dipole moment of