Science — CBSE Class 10 board question

(i) When key K is plugged in, the current in Coil 1 changes from zero to a steady value. This changing current produces a changing magnetic flux through Coil 2, which induces an EMF (and hence deflection). Once current becomes steady, the flux no longer changes, so no EMF is induced and the galvanometer shows no deflection.

(ii) When the key is plugged out, the current in Coil 1 decreases from steady value to zero. This again causes a change in magnetic flux through Coil 2, so the galvanometer shows a momentary deflection, but in the opposite direction to that observed when the key was plugged in.

(iii) Conclusion: A changing magnetic field (or changing current) in Coil 1 induces an electric current in the neighbouring Coil 2. A steady current does not induce any current in the adjacent coil.

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• The key concept is electromagnetic induction: current is induced only when there is a change in magnetic flux.
• Examiners expect the words "changing flux" or "changing current" in (i); without this, full marks are unlikely.
• In (ii), mentioning opposite direction earns the mark — the reversal indicates the flux is now decreasing instead of increasing.
• In (iii), the conclusion must clearly state that a changing (not steady) current/field causes induction — this is the core principle of Faraday's law at this level.