A-LevelPhysicsElectric fields9702/42 · Oct/Nov 2018 · Q68 Marks

State what is meant by the electric potential at a point,

(a)(i)State what is meant by the electric potential at a point,[2]
(a)(ii)State the relationship between electric potential at a point and electric field strength at the point.[2]
(b)Two similar solid metal spheres A and B, each of radius R, are situated in a vacuum such that the separation of their centres is D, as shown in Fig. 6.1. [Figure 6.1] The charge +Q on sphere A is larger than the charge +q on sphere B. A movable point P is located on the line joining the centres of the two spheres. The point P is a distance x from the centre of sphere A. On Fig. 6.2, sketch a graph to show the variation with x of the electric potential V between the centres of the two spheres. [Figure 6.2][4]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

[B1] work done per unit charge
[B1] work done moving positive charge from infinity (to the point)
[M1] field strength = potential gradient
[A1] negative sign included or directions discussed
[B1] horizontal straight lines, at non-zero potential, within the spheres
[B1] magnitude of potential greater at surface of sphere A than at surface of sphere B
[B1] concave curve between A and B, with a minimum nearer to B
[B1] lines show V positive all the way from 0 to D

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