A-LevelPhysicsMagnetic fields9702/42 · Oct/Nov 2020 · Q107 Marks

A long straight vertical wire A carries a current in an upward direction. The wire passes through the centre of a horizontal card, as illustrated in [Figure 10.1]. The card is viewed from above. The card is shown from above in [Figure 10.2]. Two wires A and B are now placed through a card. The two wires are parallel and carrying currents in the same direction, as illustrated in [Figure 10.3].

(a)On [Figure 10.2], draw four lines to represent the magnetic field produced by the current-carrying wire.[3]
(b)(i)Explain why a magnetic force is exerted on each wire.[2]
(b)(ii)State the directions of the forces.[1]
(c)The currents in the two wires are not equal. Explain whether the magnetic forces on the two wires are equal in magnitude.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[B1] concentric circles centred on the wire
[B1] separation of lines increasing with distance from wire
[B1] arrows show anti-clockwise direction
[B1] current in (each) wire creates a magnetic field (at the other wire)
[B1] current (in wire) at 90° to field causes force
[B1] force on each wire towards other wire/attractive
[B1] Newton's third law pair of forces so yes (forces are equal) OR force proportional to product of both currents so yes (forces are equal)

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.