As shown in the diagram an aluminium rod 'AB' is suspended horizontally between the two poles of a strong horse shoe magnet in such a way that the axis of rod is horizontal and the direction of the magnetic field is vertically upward. The rod is connected in series with a battery and a key. State giving reason : (a) What is observed when a current is passed through the aluminum rod from end B to end A ? (b) What change is observed in a situation in which the axis of the rod 'AB' is moved and aligned parallel to the magnetic field and current is passed in the rod in the same direction ?
Generated by claude-sonnet-4-6 · 2026-06-15 06:51 · grounding rag
Model Answer
(a) When current flows from B to A with the magnetic field directed vertically upward, applying Fleming's Left-Hand Rule, the rod is displaced towards the left. This is because a force acts on the current-carrying conductor perpendicular to both the current and the magnetic field.
(b) When rod AB is aligned parallel to the magnetic field and current is passed, no displacement (no force) is observed. This is because the force on a current-carrying conductor is maximum only when current and magnetic field are perpendicular to each other; when they are parallel, the force becomes zero.
Source: Chapter 12, Section 12.3 — Force on a Current-Carrying Conductor in a Magnetic Field
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Explanation
- For (a), the key phrase is Fleming's Left-Hand Rule: field upward, current B→A (horizontal) → force toward the left. The textbook Activity 12.7 directly states the rod displaces left.
- For (b), examiners expect you to state no force/no displacement and give the reason: force is zero when current is parallel to the field (angle = 0°). The textbook states displacement is largest when current is at right angles to the field, implying zero when parallel.
- Always name Fleming's Left-Hand Rule for part (a) — it fetches marks.