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)****[18]**