A-LevelPhysicsMagnetic fields9702/42 · May/June 2018 · Q89 Marks

Explain how a uniform magnetic field and a uniform electric field may be used as a velocity selector for charged particles.

(a)Explain how a uniform magnetic field and a uniform electric field may be used as a velocity selector for charged particles.[3]
(b)Particles having mass m and charge +1.6 × 10⁻¹⁹ C pass through a velocity selector. They then enter a region of uniform magnetic field of magnetic flux density 94 mT with speed 3.4 x 10⁴ m s⁻¹, as shown in Fig. 8.1. The direction of the uniform magnetic field is into the page and normal to the direction in which the particles are moving. The particles are moving in a vacuum in a circular arc of diameter 15.0 cm. Show that the mass of one of the particles is 20 u.[4]
(c)On Fig. 8.1, sketch the path in the uniform magnetic field of a particle of mass 22 u having the same charge and speed as the particle in (b).[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[B1] electric and magnetic fields at right-angles to one another (may be shown on a clearly labelled diagram)
[B1] particle enters fields (with velocity) normal to the (two) fields (may be shown on a clearly labelled diagram)
[B1] no deviation for particles with selected velocity
[C1] magnetic force equals/is the centripetal force
[C1] Bqv = mv²/r
[M1] M = Bqr/v
[A1] = (94 × 10⁻³ × 1.6 × 10⁻¹⁹ × 0.075) / (3.4 × 10⁴) division by 1.66 × 10⁻²⁷ shown, to give m = 20 u
[B1] sketch: semicircle clear (in same direction)
[B1] with larger radius

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