charged spheres in the torsion balance as point charges. The distance between the two charged spheres is much greater than the radii of the spheres. EXAMPLE . Consider two point charges q and q at rest as shown in the figure.
y x 1m q q They are separated by a distance of 1m. Calculate the force experienced by the two charges for the following cases: (a) q = + μC and q = + μC (b) q = + μC and q = – μC (c) q = + μC and q = – μC kept in water ( e r = ) 12th - 12th - - - - - Unit Electrostatics Therefore, W =− =− Note that the strength of the force between the two charges in water is reduced by times compared to the force between the same two charges in vacuum. When common salt ( NaCl ) is taken in water, the electrostatic force between Na and Cl ions is reduced due to the high relative permittivity of water ( e r = ). This is the reason water acts as a good solvent.
Note EXAMPLE . Two small-sized identical equally charged spheres, each having mass g are hanging in equilibrium as shown in the figure. The length of each string is cm and the angle θ is ° with the vertical. Calculate the magnitude of the charge in each sphere.
(Take g = ms − ) L L a Solution If the two spheres are neutral, the angle between them will be o when According to Newton’s third law, the force experienced by the charge q due to q is =− . Therefore, =− − N. The directions of and are shown in the above figure in case (a) (b) q = + μC, q = – μC, and r =