Chapter-1: ELECTRIC CHARGES AND FIELDS
EXERCISES
1.1 What is the force between two small charged spheres having charges of 2 × 10–7C and 3 × 10–7C placed 30 cm apart in air?
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1.2 The electrostatic force on a small sphere of charge 0.4 uC due to another small sphere of charge –0.8 uC in air is 0.2N(a) What is the distance between the two spheres? (b) What is the force on the second sphere due to the first?
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1.3 Check that the ratio ke2/G mempis dimensionless. Look up a Table of Physical Constants and determine the value of this ratio. What does the ratio signify?
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1.4 (a) Explain the meaning of the statement ‘electric charge of a body is quantised’.
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(b) Why can one ignore quantisation of electric charge when dealing with macroscopic i.e., large scale charges?
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1.5 When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge.
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1.6 Four point charges qA = 2 mC, qB= –5 mC, qC= 2 mC, and qD= –5 mC are located at the corners of a square ABCD of side 10 cm. What is the force on a charge of 1 mC placed at the centre of the square?
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1.7 (a) An electrostatic field line is a continuous curve. That is, a field line cannot have sudden breaks. Why not?
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(b) Explain why two field lines never cross each other at any point?
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1.8 Two point charges qA= 3 mC and qB= –3 mC are located 20 cm apart in vacuum.
(a) What is the electric field at the midpoint O of the line AB joining the two charges?
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(b) If a negative test charge of magnitude 1.5 × 10–9 C is placed at this point, what is the force experienced by the test charge?
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1.9 A system has two charges qA= 2.5 × 10–7 C and qB= –2.5 × 10–7 C located at points A: (0, 0, –15 cm) and B: (0,0, +15 cm), respectively. What are the total charge and electric dipole moment of the system?
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1.10 An electric dipole with dipole moment 4 × 10–9 C m is aligned at 30° with the direction of a uniform electric field of magnitude 5 × 104 NC–1.Calculate the magnitude of the torque acting on the dipole.
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1.11 A polythene piece rubbed with wool is found to have a negative charge of 3 × 10–7 C.
(a) Estimate the number of electrons transferred (from which to which?)
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(b) Is there a transfer of mass from wool to polythene?
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1.12 (a) Two insulated charged copper spheres A and B have their centres separated by a distance of 50 cm. What is the mutual force of electrostatic repulsion if the charge on each is 6.5 × 10–7 C? The radii of A and B are negligible compared to the distance of separation.
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(b) What is the force of repulsion if each sphere is charged double the above amount, and the distance between them is halved?
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1.13 Figure 1.30 shows tracks of three charged particles in a uniform electrostatic field. Give the signs of the three charges. Which particle has the highest charge to mass ratio?
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