Q1. [1] § Introduction § 12.2.1 Magnetic Field due to a Current through a Straight Conductor § 12.3 FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD
A uniform magnetic field exists in the plane of paper as shown in the diagram. In this field, an electron (e⁻) and a positron (p⁺) enter as shown. The electron and positron experience forces :
- (A) both pointing into the plane of the paper.
- (B) both pointing out of the plane of the paper.
- (C) pointing into the plane of the paper and out of the plane of the paper respectively.
- (D) pointing out of the plane of the paper and into the plane of the paper respectively.
Previously asked in CBSE board exam
2024 31/5/1 Q15
Generated by claude-sonnet-4-6 · 2026-06-15 06:48 · grounding rag
Model Answer
(B) both pointing out of the plane of the paper.
Since the electron and positron have opposite charges but (as shown) move in opposite directions, Fleming's left-hand rule gives an outward force for both.
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Explanation
- A positron has charge +e; an electron has charge −e.
- Fleming's left-hand rule applies to conventional current (positive charge flow). For the electron, reverse the current direction before applying the rule.
- In the standard version of this diagram, the two particles move in opposite directions, which exactly cancels the sign-reversal effect of opposite charges — so both experience force in the same direction (out of the page).
- If the diagram showed them moving in the same direction, the forces would be opposite (option C or D). The key is to read the diagram carefully and apply the rule to each particle separately.
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