A-LevelPhysicsElectric fields9702/22 · Feb/Mar 2017 · Q57 Marks

An electron is travelling in a straight line through a vacuum with a constant speed of 1.5 × 10⁷ m s⁻¹. The electron enters a uniform electric field at point A, as shown in [Figure 5.1]. The electron continues to move in the same direction until it is brought to rest by the electric field at point B. Distance AB is 2.0 cm.

(a)State the direction of the electric field.[1]
(b)Calculate the magnitude of the deceleration of the electron in the field.[2]
(c)Calculate the electric field strength.[3]
(d)The electron is at point A at time t = 0. On [Figure 5.2], sketch the variation with time t of the velocity v of the electron until it reaches point B. Numerical values of v and t do not need to be shown.[1]

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Mark scheme

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

[B1] (a) to the right/from the left/from A to B/in the same direction as electron velocity
[C1] (b) v² = u² + 2as
[A1] (b) a = (1.5 × 10⁷)²/(2 × 2.0 × 10⁻²) = 5.6 × 10¹⁵ m s⁻²
[C1] (c) E = F/Q
[C1] (c) = (9.1 × 10⁻³¹ × 5.6 × 10¹⁵)/(1.6 × 10⁻¹⁹)
[A1] (c) = 3.2 × 10⁴ V m⁻¹
[B1] (d) straight line with negative gradient starting at an intercept on the v-axis and ending at an intercept on the t-axis.

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