An aluminium rod suspended horizontally between the poles of a horseshoe magnet experiences a sideways force when current is passed through it. A student claims that this force arises because the rod is in a magnetic field and carries current, and that the same principle must apply to every part of a circular coil placed in an external magnetic field. (i) Name and state the rule that gives the direction of force on the straight rod. (ii) Using the same principle, explain why a rectangular coil carrying current in a uniform magnetic field tends to rotate rather than translate sideways.
Generated by claude-sonnet-4-6 · 2026-06-26 01:13 · grounding rag
Model Answer
(i) Fleming's Left-Hand Rule:
Stretch the thumb, forefinger, and middle finger of the left hand so they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of current, then the thumb points in the direction of the force on the conductor.
(ii) In a rectangular coil placed in a uniform magnetic field, the two sides carrying current in opposite directions experience forces in opposite directions (by Fleming's left-hand rule). These two equal and opposite forces act at different positions on the coil, forming a couple. This couple produces a turning effect (torque), causing the coil to rotate rather than move sideways as a whole.
Source: Chapter 12, Section 12.3 — Force on a Current-Carrying Conductor in a Magnetic Field
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Explanation
- (i) Always name the rule first, then state it clearly with all three — field, current, and force/motion. Examiners expect all three fingers to be mentioned.
- (ii) The key idea is that opposite sides carry current in opposite directions → forces are opposite → they form a couple → rotation, not translation. Use the word couple or torque for full marks. Don't just say "force acts" — explain why it rotates.
- Keep the rectangular coil explanation tied to the same principle (Fleming's left-hand rule) as the question links both parts together.