Question 7

Two charged spheres, R and S, are both stationary on a smooth, insulated surface
inclined at an angle of 25° to the horizontal. Sphere S, of mass 0,01 kg and carrying a
charge of –6 x 10-9 C, is connected to a 0,03 m long, light inextensible string attached
to point P at the top of the incline.
Sphere R, carrying a charge of +5 x 10-9 C, is held such that the distance between the
centres of the spheres is r, as shown in the diagram below.
Ignore the effects of friction.

Sphere R exerts an electrostatic force of magnitude 1,2 x 10-3 N on sphere S.
7.1 State Coulomb’s law in words. (2)

7.2 Calculate the distance r between the spheres. (3)

7.3 Draw a labelled free-body diagram for sphere S. (4)

7.4 Calculate the:
7.4.1 Tension in the string (4)
7.4.2 Net electric field at point P (5)

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