Extra Questions on Electric Charges and Fields
Q.1. Give two points of distinction between charge and mass.
Ans. Charge can be positive, negative or zero. But mass is a positive quantity. Further, electric charge is always conserved. But mass is not conserved as it can be changed into energy and vice-versa.
Q.2. Does Coulomb’s law of electric force obey Newton’s third law of motion ?
Ans. Yes, it obeys. Forces exerted by two charges on eachother are always equal and opposite.
Q.3. Is the electric force between two electrons greater than the gravitational force between them ? If so,
by what factor ?
Ans. Yes, electric force between two electrons, is greater than gravitational force between them by a factor of
Q.4. What is the importance of Coulomb’s law of electric force in vector form?
Ans. Coulomb’s law in vector form establishes that forces between any two point charges are equal and opposite.
These forces act along the line joining the charges. Therefore, electrostatic forces are central forces.
Q.5. What is the cause of charging ?
Ans. The cause of charging is actual transfer of electrons from one body to the other.
Q.6. An isolated conducting sphere is given a positive charge. Does its mass increase, decrease or remain
the same ?
Ans. Its mass decreases slightly as it loses some electrons.
Q.7. An ebonite rod is rubbed with fur or wool. What type of charges do they acquire ?
Ans. The ebonite rod acquires negative charge and fur/wool acquires an equal positive charge.
Q.8. What is meant by quantization of charge ?
Ans. Charge on any body or particle can be integral multiple of charge on an electron (-e), i.e., q= ne, where
n= 1, 2, 3,
Q.9. Consider three charged bodies P, Q and R. If P and Q repel each other and P attracts R, what is the
nature of force between Q and R?
Ans. Attractive, because Q and R will carry opposite charges.
Q.10. Why does an ebonite rod get negatively charged on rubbing with fur ?
Ans. This is because electrons in fur are less tightly bound than electrons in ebonite rod.
Q.11. A glass rod rubbed with silk acquires a charge + 1.6 x 10-12 C. What is the charge on the silk ?
Ans. Charge on silk is equal and opposite to charge on glass rod, i.e., q = -16 x 10-12 C.
Q.12. Does motion of a body affect its charge ?
Ans. No, charge on a body does not change with motion of the body.
Q.13. Name any two basic properties of electric charge.
Ans. (i) Quantization of charge
(ii) Conservation of charge.
Q.14. What is the value of charge on an electron ? Is a charge less than this value possible ?
Ans. e = -1.6 10-19 C. No, charge less than this value does not exist.
Q.15. What is the basic cause of quantization of charge ?
Ans. Only integral number of electrons can be transferred from one body to the other.
Q.16. Electrostatic forces are much stronger than gravitational forces. Give one example.
Ans. A charged glass rod can lift a piece of paper against the gravitational pull of earth on this piece.
Q.17. What is the dimensional formula for εo ?
Ans. M-1L-3 T4 A2.
Q.18. What is the relevance of large value of K (= 81) for water ?
Ans. It makes water a great solvent. This is because binding force of attraction between oppositely charged ions of the substance in water becomes 1/81 of the force between these ions in air.
Q.19. Does the coulomb force that one charge exerts on another change if other charges are brought nearby?
Ans. No, the coulomb force due to one charge on another charge is not changed.
Q.20. Force of attraction between two point charges placed at a distance d is F. What distance apart
should they be kept in the same medium so that force between them is F/3 ?
Ans. As F α1/r2 :. New distance = √3r.
Q.21. Can ever photons have a charge ? If not why?
Ans. No, photons can never have a charge. This is because charge cannot exist without rest mass.
Q.22. Which of the following quantities depends on state of rest or motion of a body : (a) mass (b) length
(c) time (d) charge (e) charge density and (f) phase ?
Ans. Mass, length, time and charge density depend on state of rest or motion of a body. However, charge and phase are invariant.
Q.23. In coulomb’s law, on what factors the value of electrostatic force constant K depends ?
Ans. The value of K depends on nature of medium separating the charges and on the system of units.
Q.24. Define electric dipole moment. Write its SI unit.
Ans. Electric dipole moment is equal to product of magnitude of either charge and distance between them. Its SI unit is C-m.
Q.25. In which orientation, a dipole placed in a uniform field is in (i) stable (i) unstable equilibrium ?
Ans. If is angle between P and É , then 0 = 0° for stable equilibrium, 0 = 180° for unstable equilibrium.
Q.26. Name any four vector fields.
Ans. Electric field, magnetic field, gravitational field and flow field (of a liquid) are vector fields.
Q.27. How does a free electron at rest move in an electric field ?
Ans. A free electron at rest will move in a direction opposite to electric field.
Q.28. Four charges of same magnitude and same sign are placed at the corners of a square, of each side
0-1 m. What is electric field intensity at the centre of the square ?
Ans. Electric field intensity at the centre of the square will be zero.
Q.29. Force experienced by an electron in an electric field Ē is F newton. What will be the force experienced by a proton in the same field ? Take mass of proton 1836 times the mass of an electron.
Ans. The proton will experience the same force, F newton, but in the opposite direction.
Q.30. What is the use of the concept of electric field intensity ?
Ans. It enables us to find force on any charge in the field, i.e., F = qE .
Q.31. Name the physical quantity whose SI unit is NC-1?
Ans. Electric field intensity.
Q.32. Why is electric field intensity inside a charged conductor zero ?
Ans. This is because there are no electric lines of force inside the body of a conductor.
Q.33. Draw electric lines of force due to an electric dipole.
Ans. The electric lines of force in respect of an electric dipole are as shown in Fig. 1(b).9.
II. What is the net force on a dipole in a uniform electric field ?
Q.34. How does a torque affect the dipole in an electric field ?
Ans. Torque tries to align the dipole along the field.
Q.35. Two point charges of +3 μC each are 100 cm apart. At what point on the line joining the charges will
the electric intensity be zero ?
Ans. At the centre.
Q.36. Which rule gives you the direction of torque ?
Ans. The direction of torque is given by right handed screw rule.
Q.37. What happens when an electric dipole is held in a non uniform electric field ?
Ans. It experiences some net force and some net torque.
Q.38. What is nature of symmetry of field due to a point charge ?
Ans. The field due to point charge has spherical symmetry.
Q.39. When is an electric line of force straight ?
Ans. Electric line of force is straight in the field of a single charge.
Q.40. When is an electric line of force curved ?
Ans. Electric line of force is curved in the field of more than one point charges.
Q.41. At what points, dipole field intensity is parallel to the line joining the charges ?
Ans. At any point on axial line or equatorial line of dipole.
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