A-LevelPhysicsElectric fields9702/42 · Feb/Mar 2019 · Q58 Marks

State what is meant by an electric field.

(a)State what is meant by an electric field.[1]
(b)An isolated solid metal sphere has radius R. The charge on the sphere is +Q and the electric field strength at its surface is E. On Fig. 5.1, draw a line to show the variation of the electric field strength with distance x from the centre of the solid sphere for values of x from x = 0 to x = 3R. [Figure 5.1][4]
(c)The sphere in (b) has radius R = 0.26 m. Electrical breakdown (a spark) occurs when the electric field strength at the surface of the sphere exceeds 2.0 x 10⁶ Vm⁻¹. Determine the maximum charge that can be stored on the sphere before electrical breakdown occurs.[3]

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Mark scheme

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

[B1] region where charge experiences an (electric) force
[B1] graph: field strength zero from x = 0 to x = R
[B1] curve with negative gradient, decreasing from x = R to x = 3R
[B1] line passes through field strength E at x = R,
[B1] line passes through field strength 0.25E at x = 2R and field strength 0.11E at x = 3R
[C1] field strength = q/4πε₀x²
[C1] 2.0 × 10⁶ = q / (4 × π × 8.85 × 10⁻¹² × 0.26²)
[A1] q = 1.5 × 10⁻⁵ C

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