The topic here is the motor effect. In Figure 2 a wire joined to a battery and a closed switch runs through the gap of a horseshoe magnet.
Figure 2: the switch is closed. Work out which way the wire in the gap is pushed.
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The field between the poles runs from north to south, so from left to right. Conventional current leaves the battery at its positive end, the longer line on the right, and so flows along the wire through the gap towards the viewer. Fleming's left hand rule has the first finger along the field (right), the second finger along the current (towards you), and the thumb then shows the force: upwards. That is the last option. Down would come from reversing the current, for example by starting from the wrong end of the battery. Left and right cannot be right, because the force is at right angles to both the field and the current, and the field already points left to right.
How the marks are earned
- Choose up: with the field running left to right and the current flowing towards you along the wire, Fleming's left hand rule gives a push upwards.
- One answer only: two answers earn nothing, and a circled or underlined right option counts if the box is blank.