State one similarity and one difference between the fields of force produced by an isolated point charge and by an isolated point mass.

1. (a)State one similarity and one difference between the fields of force produced by an isolated point charge and by an isolated point mass.[2]
2. (b)An isolated solid metal sphere A of radius R has charge +Q, as illustrated in Fig. 5.1. [Figure 5.1] A point P is distance 2R from the surface of the sphere. Determine an expression that includes the terms R and Q for the electric field strength E at point P.[2]
3. (c)A second identical solid metal sphere B is now placed near sphere A. The centres of the spheres are separated by a distance 6R, as shown in Fig. 5.2. [Figure 5.2] Point P lies midway between spheres A and B. Sphere B has charge –Q. Explain why: (i) the magnitude of the electric field strength at P is given by the sum of the magnitudes of the field strengths due to each sphere[1]
4. (c)(ii)the electric field strength at point P due to the charged metal spheres is not, in practice, equal to 2E, where E is the electric field strength determined in (b).[2]

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