Bihar Board - Class 12 Physics - Chapter 2: Electrostatic Potential And Capacitance Short Answer Question
Q 1. Is the electric potential positive or negative near an isolated positive charge? Give reason too.
Answer:- 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?
Answer:- Work done is zero because the 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?
Answer:- 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?
Answer:- 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 center 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?
Answer:- When an uncharged conductor B is brought close to A, equal and opposite charges 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 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?
Answer:- 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?
Answer:- Work done is zero in moving a charge from one point to another on an equipotential surface.
Q 8:- Can two equipotential surfaces intersect each other? Give reason too.
Answer:- No two equipotential surfaces 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?
Answer:- 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.
Answer:- 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?
Answer:- 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?
Answer:- 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?
Answer:- 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?
Answer:- The electric field at a point just near the surface of a charged conductor,
E = 0 n
Where , n is a unit vector drawn normally outward at a point on the surface of a charged conductor.
15. Distinguish between polar and nonpolar dielectrics.
Answer:- 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?
Answer:- 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?
Answer:- Electric field strength at all points between the plates of a charged parallel plate capacitor is exactly the same. So, the 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?
Answer:- 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?
Answer:- 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?
Answer:- 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?
Answer:- 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?
Answer:- The capacitance of the given 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?
Answer:- Because the two plates of a capacitor are exactly parallel to each other, i.e., separation between them is the 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?
Answer:- For the 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 becomes K times of its original value.
Q 25:- Why must the electrostatic field be normal to the surface at every point of a charged conductor?
Answer:- 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.
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