Q1. [4] § Introduction § 12.1 MAGNETIC FIELD AND FIELD LINES § 12.2 MAGNETIC FIELD DUE TO A CURRENT-CARRYING CONDUCTOR
AB is a coil of copper wire having a large number of turns. The ends of the coil are connected with a galvanometer as shown. When the north pole of a strong bar magnet is moved towards the end B of the coil, a deflection is observed in the galvanometer.
Answer the sub-questions based on the case below. Part (a) and (b) are compulsory; an internal choice has been provided in Part (c).
- (a) State the reason for using galvanometer in the activity and why does its needle deflects momentarily when magnet is moved towards the coil.
- (b) What would be observed in the galvanometer in a situation when the coil and the bar magnet both move with the same speed in the same direction ? Justify your answer.
- (c) State the conclusion that can be drawn from this activity. Will there be any change in the momentary deflection in the galvanometer if number of turns in the coil is increased and a more stronger magnet is moved towards the coil ?
Previously asked in CBSE board exam
2022 31/4/1 Q14 (main)
Generated by claude-sonnet-4-6 · 2026-06-15 06:52 · grounding stimulus
Model Answer
(a) A galvanometer is used because it can detect very small currents. When the magnet moves towards the coil, the magnetic flux through the coil changes, inducing an EMF and a momentary current — causing the needle to deflect. The deflection is momentary because current is induced only while flux is changing.
(b) No deflection would be observed. When coil and magnet move with the same speed in the same direction, there is no relative motion between them, so magnetic flux does not change and no EMF is induced.
(c) Conclusion: A changing magnetic flux (relative motion between magnet and coil) induces an electric current — this is electromagnetic induction.
Yes, deflection increases — more turns increase the flux linkage and a stronger magnet produces greater flux change, both resulting in higher induced EMF and larger deflection.
Source: Chapter – Magnetic Effects of Electric Current (Electromagnetic Induction activity)
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Explanation
- Examiner expects the term relative motion in (b) — this is the key justification.
- For (c), two sub-points are expected: the conclusion (electromagnetic induction) and the effect of more turns + stronger magnet (more deflection). Don't miss either.
- "Momentary" deflection must be explained by flux changing only while motion occurs — steady flux = no current.
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