Q.1. What is Coulomb’s law relating force between two charges placed near each other?
Ans. Mathematically force F between two point charges of strength Q1 and Q2, placed distance d apart is given by
Q.2. On what factors does the force acting between two point charges placed near each other depend?
Ans. The force acting between two point charges placed near each other depends upon (i) the strength of two charge (ii) distance between the charge and (iii) nature of medium surrounding the charge.
Q.3. Is electricity field intensity a vector quantity?
Ans. Yes, electric field intensity is vector quantity.
Q.4. What is polarization vector?
Ans. The sum of dipole moments per unit volume in a dielectric material is called the polarization vector or dielectric polarization, P.
Q.5. How is electric displacement in a dielectric material related to polarization?
Ans. The electric displacement or electric induction is defined as that property of the field which is evidenced by its ability to induce charge on conductors placed in it and is expressed as
Electric displacement D = E + P
Q.6. What is relation between electric flux density and electric field intensity?
Ans. Electric flux density, .
Q.7. What is electric susceptibility of a dielectric?
Ans. Electric susceptibility of a dielectric medium is defined as the ratio of bound charge density to free charge density and is represented by the symbol
Q.8. How is the electric susceptibility related to a relative permittivity?
Ans. Electric susceptibility
Q.9. What is Gauss’s Theorem?
Ans. According to Gauss’s theorem the total electric flux from a closed surface is equal to the total charge enclosed by the surface.
Q.10. What is Coulomb’s theorem?
Ans. According to coulomb’s theorem, the electric intensity at a surface of a conducting body, having a charge density of coulombs per unit of surface area is where , the absolute permittivity of the medium surrounding the conductor.
- 11. What is relationship between electric field intensity and potential gradient?
Ans. Electric field intensity at any point is equal to the negative potential gradient at that point i.e. E = -g.
Q.12. What is a meant by dielectric strength of a medium?
Ans. The dielectric strength is the property of a medium which enables it to withstand electric stress without damage. It is usually expressed in terms of the minimum electric stress (potential difference per unit distance) that will cause failure or breakdown of the medium under certain condition of service.
Q,13. What is permittivity of free space?
Ans. Permittivity od free space, F/m.
Q.14. What is equipotential surface?
Ans. The surface in an electric field at right angle to the lines of force or flux everywhere is equipotential surface.
Q.15. A negatively charged ebonite rod attracts a suspended ball of straw. Can we infer that the ball is positively charged?
Ans. No, because the negatively charged rod can attract an uncharged ball and also a positively charged ball.
Q.16. Two free protons and two free electrons are separated by the same distance. Compare coulomb’s forces of repulsion between the pair of protons and pair of electrons.
Ans. The forces are the same because each proton carries charge of same magnitude as each electron and distance between them is the same.
Q.17. What is the origin of frictional forces?
Ans. The frictional forces are electric in origin.
Q.18. Why does a charged glass rod attract a piece of paper?
Ans. Paper is a dielectric. When a positively charged glass, rod is brought near it, atoms of paper get polarized, with centre of negative charge of atoms coming closer to glass rod. Therefore, force of attraction (Fa) between glass rod and piece of paper becomes greater than the force of repulsion (Fr) between the glass rod and the piece of paper. This results in attraction of the piece of paper towards the glass rod.
Q.19. Can two similarly charged balls attract each other?
Ans. Yes, when charge on one, say A is much larger than the charge on the other, say B. On account of induction, the ball B carrying smaller charge shall acquire some net charge of opposite sign lying closer to A, Hence B will experience some net force of attraction.
Q.20. Two identical metallic spheres of exactly equal masses are taken, one is given a positive charge and the other an equal negative charge. Their masses after charging are different, comment.
Ans. To give positive charge, electrons have to be removed from the sphere. Therefore, mass of the sphere will decrease. Similarly, for giving negative charge, electrons have to be transferred to the sphere Therefore, mass of the sphere will increase. In actual practice, mass of electron is too small (= 9.1*10-31 kg), therefore change in mass on charging is negligibly small.
Q.21. Give some points of dissimilarity and similarity between electrostatic and gravitational forces.
- Electrostatic forces are between two charges and gravitational forces are between two masses.
- Electrostatic forces may be attractive or repulsive. But gravitational forces are always attractive.
- Electrostatic force between two protons is 1036 times stronger than gravitational force between them.
- Electrostatic forces operate over distances which are not large. Gravitational forces operate over very large distances.
- Both the forces obey inverse square law,
- Both are central forces.
- Both are conservative forces.
Q.22. A comb run through one’s hair attracts small bits of paper. Why? What happens if the hair is wet or if it is a rainy day?
Ans. This is because the comb gets charged by friction. If the hair is wet, or if it is a rainy day, friction between the hair and the comb reduces. The comb does not get charged, and it will not attract small bits of paper.
Q.23. Ordinary rubber is an insulator. But the special rubber tyres of aircrafts are made slightly conducting. Why is this necessary?
Ans. The special rubber tyres of aircrafts are made slightly conducting so that electricity generated on account of friction between the tyres and the runway goes to earth.
Q.24. Vehicles carrying inflammable materials usually have metallic ropes touching the ground during motion. Why?
Ans. When a vehicle moves, its body gets charged on account of friction due to air. The tyres also accumulate the charge on account of friction between the tyres and the road. The metallic ropes from the vehicle touching the ground enable the accumulated charges to flow to earth. This would otherwise be hazardous to the inflammable materials.
Q.25. A bird perches on a bare high power line, and nothing happens to the bird. A man standing on the ground touches the same line and gets a fatal shock. Why?
Ans. When a bird perches on single bare high power line, nothing happens to him as no current flows through his body because the circuit is not complete. In case of a person standing on ground, the circuit is completed through ground. Current flows through his body resulting in a fatal shock.
Q.26. What is an ideal dipole?
Ans. For a given dipole moment || = q (2a), when the charge q gets larger and larger, while the separation (2 a) gets smaller and smaller, we obtain an ideal dipole. Thus, an ideal dipole has almost no size.
Q.27. What is the direction of field intensity at a point on equatorial line of dipole?
Ans. The electric field intensity is along a line parallel to axis of dipole directed opposite to the dipole moment .
Q.28. When does, a charged circular loop behave as a point charge?
Ans. A charged circular loop behaves as a point charge, when the observation point on its axis is at a distance, which is very large compared to the radius of the circular loop.
Q.29. What is the nature of symmetry of the dipole field?
Ans. The dipole field has a cylindrical symmetry. The axis of the cylinder passes through the dipole axis.
Q.30. Why no two electric lines of force can intersect each other?
Ans. This is because at the point of intersection, we can draw two tangents to the two lines of force. This would mean two directions of electric field intensity at the point of intersection, which is not possible.
Q.31. Why do we obtain a neutral point in the space between two like charges?
Ans. This is because net electric field intensity at this point is zero, the intensities due to two charges being equal and opposite.
Q.32. What does (q1 + q2) = 0 signify?
Ans. q1 + q2 = 0, q1 = -q2, such a system of point charges is called an electric dipole. It is a pair of equal and opposite point charges separated by a small distance.
Q.33. An electric dipole is placed at rest in a uniform electric field, and released. How will it move?
Ans. A torque will develop and align the electric dipole in the direction of the electric field, if the dipole is not aligned already. The dipole shall not move as net force on the dipole is zero.
Q.34. Can two equivalent surface interest each other? Give reasons.
Ans. No, An equipotential surface is normal to the electric field intensity. If two equipotential surfaces intersect, then at the point of intersection, there will be two directions of electric field intensity, which is not possible.
Q.35. Electric potential of earth is taken to be zero. Why?
Ans. Earth is a good conductor of very large size. When some small charge is given to earth, its potential does not change. Hence potential of earth is assumed to be zero. It is like sea level being taken as zero level for measuring all heights.
Q.36. Can we produce high value on the human body without getting a sock?
Ans. Yes, we can. The person must stand on a highly insulating platform. Therefore, with high voltage on the body, no charge will flow to the ground through the body and the person will not get any shock.