A-LevelPhysicsMagnetic fields9702/41 · Oct/Nov 2018 · Q89 Marks

A particle has mass m, charge +q and speed v. The particle enters a uniform magnetic field of flux density B such that, on entry, it is moving normal to the magnetic field, as shown in [Figure 8.1]. The direction of the magnetic field is perpendicular to, and into, the plane of the paper.

(a)Explain what is meant by a magnetic field.[2]
(b)(i)On [Figure 8.1], draw the path of the particle through, and beyond, the region of the magnetic field.[3]
(b)(ii)There is a force acting on the particle, causing it to accelerate. Explain why the speed of the particle on leaving the magnetic field is v.[1]
(c)The particle in (b) loses an electron so that its charge becomes +2q. Its change in mass is negligible. Determine, in terms of v, the initial speed of the particle such that its path through the magnetic field is unchanged. Explain your working. speed = ...........................................................[3]

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Mark scheme

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

[M1] region where there is a force
[A1] experienced by a current-carrying conductor/moving charge/(permanent) magnet
[B1] single path, deflection in 'upward' direction
[B1] acceptable circular arc in whole field
[B1] no 'kinks' at start or end of curvature, and straight outside region of field
[B1] force (on particle) is normal to velocity/direction of motion/direction of speed
[B1] magnetic force provides/is the centripetal force
[C1] Bqv = mv²/r or r = mv/Bq
[A1] (if q is doubled), new speed = 2v

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