A-LevelPhysicsElectric fields9702/23 · May/June 2017 · Q312 Marks

An electron is accelerated from point A to point B by a uniform electric field, as illustrated in [Figure 3.1]. The distance between A and B is 12mm. The velocity of the electron at A is 2.5kms⁻¹ and at B is 18 Mms⁻¹.

(a)Define electric field strength.[1]
(b)(i)the acceleration of the electron,[2]
(b)(ii)the change in kinetic energy of the electron,[3]
(b)(iii)the electric field strength.[3]
(c)An α-particle moves from A to B in the electric field in (b). Describe and explain how the change in the kinetic energy of the α-particle compares with that of the electron. Numerical values are not required.[3]

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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.

3(a) force per unit (positive) charge
3(b)(i) a = (v² – u²)/2s
3(b)(i) = [(18 × 10⁶)² - (2.5 × 10³)²] / (2 × 12 × 10⁻³)
3(b)(i) = 1.3 (1.35) × 10¹⁶ m s⁻²
3(b)(ii) KE = ½ mv² or ½ m(v² – u²)
3(b)(ii) change in KE = 0.5 × 9.11 × 10⁻³¹ × [(18 × 10⁶)² – (2.5 × 10³)²]
3(b)(ii) = 1.5 (1.48) × 10⁻¹⁶ J
3(b)(iii) E = F/ e = ma / e or eV = ∆KE so E = ΔKE / (e x d)
3(b)(iii) E = (9.11 × 10⁻³¹ × 1.35 × 10¹⁶) / 1.60 × 10⁻¹⁹
3(b)(iii) or E = (1.48 × 10⁻¹⁶) / (12 × 10⁻³ × 1.60 × 10⁻¹⁹)
3(b)(iii) = 7.7 (7.69) × 10⁴ V m⁻¹
3(c) charge on α opposite to electron/charge on α is positive
3(c) ΔKE is negative/KE reduced
3(c) charge of α greater/twice that of electron causes larger/twice ΔKE (in magnitude)

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