A-LevelPhysicsElectric fields9702/42 · May/June 2019 · Q69 Marks

State what is meant by electric potential at a point.

(a)State what is meant by electric potential at a point.[2]
(b)Two parallel metal plates A and B are held a distance d apart in a vacuum, as illustrated in Fig. 6.1. Plate A is earthed and plate B is at a potential of +V₀. Point P is situated in the centre region between the plates at a distance x from plate B. The potential at point P is V. On Fig. 6.2, show the variation with x of the potential V for values of x from x = 0 to x = d.[3]
(c)Two isolated solid metal spheres M and N, each of radius R, are situated in a vacuum. Their centres are a distance D apart, as illustrated in Fig. 6.3. Each sphere has charge +Q. Point P lies on the line joining the centres of the two spheres, and is a distance y from the centre of sphere M. On Fig. 6.4, show the variation with distance y of the electric potential at point P, for values of y from y = 0 to y = D.[4]

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Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

6(a) work done per unit charge B1 (work done) moving positive charge from infinity B1
6(b) straight line with non-zero gradient from x = 0 to x = d B1 line with gradient of constant sign and end-points between which △V = Vo and Ax = d B1 line passes through (d, 0) and (0, +Vo) with negative gradient throughout B1
6(c) V constant (and non-zero) from 0 → R and from (D – R) → D B1 equal (non-zero) values of (magnitude of) V at Rand (D – R). B1 curve (with a minimum) from R to (D – R) with V always positive B1 minimum at mid-point of curve B1

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