A current-carrying aluminium rod is suspended horizontally between the poles of a horseshoe magnet with the magnetic field directed vertically upward. When current flows through the rod from end B to end A, the rod is displaced to the left. What will happen to the direction of displacement if both the direction of the current AND the direction of the magnetic field are simultaneously reversed? Justify your answer.
Generated by claude-sonnet-4-6 · 2026-06-26 01:12 · grounding rag
Model Answer
When both the direction of current and the direction of the magnetic field are simultaneously reversed, the rod will still be displaced to the left — the direction of displacement remains unchanged.
Justification: According to Fleming's Left-Hand Rule, the direction of force on a current-carrying conductor depends on both the direction of current and the direction of the magnetic field. When only the current is reversed, the force reverses (rod moves right). When only the field is reversed, the force also reverses (rod moves right). But when both are reversed simultaneously, the two reversals cancel each other out, and the net force acts in the original direction — to the left.
Source: Chapter 12, Section 12.3 – Force on a Current-Carrying Conductor in a Magnetic Field
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Explanation
- The key concept here is that force direction depends on both current and field. Reversing either one alone reverses the force. Reversing both together is equivalent to no net change — the effects cancel.
- Examiners expect you to explicitly state the final direction (left) and give a clear reason using Fleming's Left-Hand Rule logic or the principle that double reversal restores the original direction.
- A common mistake is saying the displacement reverses — avoid this by thinking of the two reversals as multiplying two negatives: (−1) × (−1) = +1.