A-LevelPhysicsElectric fields9702/42 · May/June 2017 · Q69 Marks

State Coulomb's law.

(a)State Coulomb's law.[2]
(b)(i)Use Fig. 6.2 to explain whether the two spheres have charges of the same, or opposite, sign.[2]
(b)(ii)A proton is at point P where x = 5.0 cm. Use data from Fig. 6.2 to determine the acceleration of the proton. acceleration = ........................................................... ms⁻² [3][3]
(c)Use data from Fig. 6.2 to state the value of x at which the rate of change of electric potential is maximum. Give the reason for the value you have chosen.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[M1] force proportional to product of charges and inversely proportional to the square of the separation
[A1] reference to point charges
[M1] (near to each sphere,) fields are in opposite directions or point (between spheres) where fields are equal and opposite or point (between spheres) where field strength is zero
[A1] so same (sign of charge)
[C1] (at x = 5.0 cm,) E = 3.0 × 10³ V m⁻¹ and a = qE / m
[C1] E = (1.60 × 10⁻¹⁹ × 3.0 × 10³) / (1.67 × 10⁻²⁷)
[A1] = 2.9 x 10¹¹ ms⁻²
[M1] field strength or E is potential gradient or field strength is rate of change of (electric) potential
[A1] (field strength) maximum at x = 6 cm

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