Class 12 Physics Chapter 2 Important Questions Electrostatic Potential and Capacitance

Q 1:- Is the electric potential positive or negative near an isolated positive charge? Give reason too. Ans:- Electric potential near an isolated

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Electrostatic Potential and Capacitance. The best solution of the problem is to practice as many Physics Class 12 Chapter 2 Important Questions as possible to clear the doubts.

Q 1:- Is the electric potential positive or negative near an isolated positive charge? Give reason too.
Ans:- Electric potential near an isolated positive charge is positive. It is because a small positive test charge will experience a repulsive force due to isolated positive charge and so positive work is to be done against the repulsive force while bringing the test charge from infinity to given position.

Q 2:- A positive charge +Q is located at a point. What is the work done,if a unit positive test charge is carried once around this charge along a circle of radius r about this point?
Ans:- Work done is zero because electrostatic field is a conservation field and work done against a conservation force is zero for a close path in which initial and positions are the same.

Q 3 :- What is the work done by the electrostatic field of a nucleus in a complete circular orbit of an electron around it? What, if the orbit is elliptical?
Ans:- Work done is zero in both the case because of conservative nature of electrostatic field or force.

Q 4:- What is meant by spherical symmetry of electric potential due to a point charge?
Ans:- Spherical symmetry of electric potential due to a charge means that the electric potential at all points of a sphere of radius r with its centre located at the point charge q is exactly same at q/4πε0r.

Q 5:- A charge conductor 'A' is placed on an insulating stand. An uncharged conductor B is brought close to A, as shown in the following figure. How will the charge and potential of conductor A will change?
Ans:- When an uncharged conductor B is bought close to A, equal and opposite charge are induced on B as shown in the figure. The total charge of a conductor remains unchanged but due to presence of induced charges in its neighbourhood, electrostatic potential of a conductor 'A' decreases from its original value.

Q 6:- In which direction, protons tend to go in a region of varying potential? In which direction, electrons would tend to go?
Ans:- Protons, being positively charged particles, tend to move from higher potential to lower potential (or in the direction of electric field). However,electrons, being negatively charged particles, tend to move from lower potential to higher potential (or in a direction opposite to that of electric field.

Q 7:- How much work is done in moving a 500 μC charge between two points on an equipotential surface?
Ans:- Work done is zero in moving a charge from one point to another on an equipotential surface.

Q 8:- Can two equipotential surface intersect each other? Give reason too.
Ans:- No two equipotential surface can ever intersect with each other because if they do intersection, there will be two different potentials which is not possible.

Q 9:- Does electric potential increase or decrease along the direction of an electric field line?
Ans:- Electric potential always decrease along the direction of an electric field line.

Q 10:- Draw an equipotential surface for a system, consisting of two charges Q and -Q separated by a distance r,in air.
Ans:- The system behaves ad an electric dipole.A plane passing through mid-point O and parpendicular to dipole axis (i.e.,a plane in equatorial line) will be an equipotential surface (V=0) as shown in the following figure.

Q 11:- What is the value of electrostatic potential of the earth?
Ans:- Electrostatic potential of the Earth is taken to be always zero.

Q 12:- Does the electrostatic potential energy of a system of discrete point charges belong to entire system or to individual charges?
Ans:- The electrostatic potential energy of a system of discrete point charge belongs to the entire system.

Q 13:- A positive charge +q is moved in an electrostatic field from a point at high potential to a point at low potential. How do its potential energy and kinetic energy change?
Ans:- When a positive charge moves from a point at a high potential to another point at a low potential, its a moving down the potential gradient (or in direction of electric field) and consequently, its potential energy decrease. In turn, its kinetic energy increase. In fact, loss in potential energy reappears as the gain in kinetic energy of charge +q.

Q 14:- What is the value of electric field at a point just near the surface of a charged conductor having uniform surface density σ?
Ans:- The electric field at a point just near the surface of a charged conductor,

\vec{E}=\frac{σ}{ε_0}\hat{n}

Where \hat{n} is a unit vector drawn normally outward at a point on the surface of a charged conductor.

Q 15:- Distinguish between polar and nonpolar dielectrics.
Ans:- In the polar dielectric, each molecule has a finite (though small) value of electric dipole moment because centers of positive and negative charges do not coincide in a molecule. On the other hand, in a nonpolar dielectric, molecules of dielectric do not have any permanent dipole moment.

Q 16:- Can there be a potential difference between two conductors of same volume carrying equal positive charges?
Ans:- Yes, two conductros may have different shapes and consequently, different capacitances. Hence,in spite of having same charge (say, Q), their potentials may be different. So, there will be a potentials may be a potential difference between two conductors.

Q 17:- What is the ratio of electric field strength at any two points between the plates of a parallel plate capacitor?
Ans:- Electric field strength at all points between the plates of a charged parallel plate capacitor is exactly the same. So, ratio of field strengths at any two points will be one.

Q 18:- What meaning would you give to the capacitance of a single conductor?
Ans:- Capacitance of a single conductor means the capacitance of a capacitor whose one plate is the given conductor and the other plate is at infinity.

Q 19:- On what factors does the capacitance of a parallel plate capacitor depend?
Ans:- The capacitance of a parallel plate capacitor depends upon the area of its plates, separation between the plates and the dielectric constant of the dielectric medium present between the plates.

Q 20:- Can we give any amount of charge to a capacitor?
Ans:- No, we can not give charge beyond a limiting value to a capacitor. The limiting value is governed by the dielectric strength of the dielectric medium present between the plates of a capacitor.

Q 21:- Where does the energy of a capacitor reside?
Ans:- The stored energy in a charged capacitor resides in the electric field between the plates of a capacitor. Thus, energy is stored as electrostatic potential energy.

Q 22:- What happens to the capacitance of a capacitor if the potential difference between its plates is (a) doubled, and (b) tripled?
Ans:- The capacitance of the give capacitor will remain unchanged because capacitance of a capacitor is independent of the potential difference between its plates as well as charge.

Q 23:- Why is a parallel plates capacitor named so?
Ans:- Because the two plates of a capacitor are exactly parallel to each other, i.e., separation between them is same at all points.

Q 24:- For the same quantity of electric charge,how will the potential difference across the plates of the capacitor change on inserting a dielectric of dielectric constant K between the plates? What is its effect on the capacitance of the capacitor?
Ans:- For same quantity of electric charge, potential difference between the plates of a capacitor is reduced by K times on inserting a dielectric medium of dielectric constant K between the plates. Due to this lowering of potential difference, the capacitance of the capacitor become K times of its original value.

Q 25:- What is the main use of a Van de Graaff generator?
Ans:- A Van de Graaff's generator is used to accelerate positively charged particles, e.g., protons, deutrons, alpha particles and other positively charged ions to high energies needed for experiment to probe the small scale structure of matter.

Q 26:- Can a Van de Graaff's generator be used to accelerate neutrons? Why?
Ans:- Neutrons cannot be accelerated by a Van de Graaff's generator because neutrons have no charge.

Q 27:- What is the value of \oint \vec{E}.\vec{dl} in an electric field, and why?
Ans:- The value of line integral of an electric field over a closed path, i.e., \oint \vec{E}.\vec{dl} is zero, because electric is a conservation field.

Q 28:- What is the work done in moving a test charge q through a distance of 1cm along the Equatorial axis of an electric dipole?
Ans:- Work done is zero because potential at any point on the equatorial line of an electric dipole is zero. Hence, work done,

W = q(V2 - V1) = 0

Q 29:- Equipotential Surfaces are perpendicular to electric field lines. Why?
Ans:- We know that no work is being done in moving a test charge from one point to another on an equipotential surface. It means that there is no component of electric field along the equipotential surface and electric field is directed at right angle to equipotential surfaces. Alternatively, we can say that equipotential surfaces are perpendicular to electric field lines.

Q 30:- Calculate the electric potential st the centre point of a uniformly charged circular ring of radius R having a charge q. What is the effect on potential if charge q is unevenly distributed along the ring?
Ans:- For a uniformly charged ring, the distance of the centre point O from all points of a ring is exactly same equal to the radius of a ring. Hence, electric at the centre of a ring,

V=\frac{q}{4\pi\epsilon_0 R}

The value of potential remains unaltered even if the charge is distributed unevenly on the ring.

Q 31:- How can you protect a sensitive instrument from outside electrical influence?
Ans:- We can protect a sensitive instrument by making use of electrostatic shielding effect. We take a hollow metallic vessel and place the given instrument on an insulating base inside that hollow vassel. Now, whatever may be the value of external electric field, the instrument will remain unaffected because field inside hollow metallic vessel is always zero.

Q 32:- What is the physical significance of capacitance?
Ans:- The capacitance of a capacitor tell us about the potential difference produced between the two plates of a capacitor when a finite charge is given to it. If capacitance is high, one can give more charge to the capacitor for same potential difference between its plates.

Q 33:- What is the significance of dielectric strength of a dielectric medium?
Ans:- The dielectric strength of a dielectric medium is the maximum value of electric field which it can withstand without breaking. Therefore, we can design a capacitor having a maximum rating of potential difference between its plates so that there is no leakage of charge from the capacitor.

Q 34:- For a given potential difference, does a capacitor can store more charge with a dielectric or without a dielectric?
Ans:- For a given potential difference, a capacitor can store more charge with a dielectric present between its plates.

Q 35:- If the two plates of a charged capacitor be suddenly connected to each other by a thin metal wire, what will happen?
Ans:- The capacitor will be immediately discharged and electrostatic potential energy stored in the capacitor will be released producing heat and sometimes a spark too.

Q 36:- What is the basic use of a capacitor?
Ans:- A capacitor is used to store large amount of electric charge or energy event at a low potential.

Q 37:- Can we give as much charge to a capacitor as we wish?
Ans:- No, we cannot give as much charge to a capacitor as we wish. The maximum charge should be such so that the electric field between the plates of a capacitor is less that the dielectric strength of the dielectric present.

Q 38:- Is it possible to move a charge in an electric field without doing any work? If so, how and if not, why?
Ans:- Yes, it is possible to move a charge in an electric field without doing any net work done if electric potential of initial and final points is same. Thus, if

V_A=V_B, then

W_{A}^{B}=q(V_B-V_A)=0

Q 39:- What is the net charge on a charged capacitor, and why?
Ans:- Since in a charged capacitor, two plates have equal and opposite charges (Q and -Q), hence net charge on a charged capacitor is always zero.

Q 40:- We have two copper spheres, each of radius R, but one sphere is solid and the other is hollow. Which sphere has higher capacitance, and why?
Ans:- Capacitance of both the sphere is exactly equal having a value, C = 4πε0R.

Q 41:- On joining two or more capacitors in series, the equivalent capacitance has a smaller value even than that of capacitor of the least capacitance in the series grouping. Then, what is the use of series grouping of capacitance?
Ans:- In series grouping, the combination can withstand a higher voltage than individual capacitor. For example, we have four capacitors of 1 μF each which can withstand a potential of 250 volts. Their series combination has a capacitance of only 0.25 μF but can withstand a potential of 1000 volts.

Q 42:- A capacitor of capacitance C is charged to a potential V. What is the total electric flux of the electric field through a closed surface around the capacitor?
Ans:- We know that two plates of a capacitor have charges +Q and -Q respectively. Hence, net charge on a capacitor is zero. Consequently, electric flux of the electric field through a closed surface around the capacitor is always zero.

Q 43:- Is the electrostatic potential necessarily zero at a point, where the electric field strength is zero? Give an example to illustrate your answer.
Ans:- If the electric field strength is zero at a point, the electrostatic potential is not necessarily zero at that point. For example, at a point midway between two equal and similar charges, the electric field strength is zero, but the electrostatic potential is twice that due to a single charge.

Q 44:- What do you mean by potential energy of an electric dipole, when placed in electric field?
Ans:- An electric dipole always tends to orient itself along the direction of electric field. Work has to be done in rotating the dipole to some other orientation and this work in rotating the dipole gets stored in the dipole in the form of its potential energy.

Q 45:- A man inside an insulated metallic cage does not receive a shock, when the cage is highly charged. Explain, Why?
Ans:- It is because, the potential at each point inside the cage is same as that of the cage itself. Since there is no potential difference between the man and the highly charged cage, the man does not receive any shock.

Q 46:- Why must electrostatic field be normal to the surface at every point of a charged conductor?
Ans:- The electrostatic field at the surface of the conductor cannot have tangential component. Had it been so, the free charges on the surface would experience force and move i.e. will not remain static. It would lead to the flow of surface current. Therefore, the field lines must enter or leave the conductor at right angles to its surface.

Q 47:- Why 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 electric field due to polarization of the dielectric opposes it. Hence, the electric field gets reduced.

Q 48:- Why the Van de Graaff generator is enclosed inside an earth connected steel tank filled with air under pressure?
Ans:- It prevents the leakage of charge due to ionization. The reason is that in air under pressure, as soon as free ions are produced, they recombined to form neutral air molecules.