Physics
Physics
Problem
A switch S is connected in series with a battery and a long straight wire placed directly below a small compass needle. The switch is kept open for some time, then closed for a few seconds, and finally opened again. The needle shows a sharp deflection at the instants of closing and opening but remains steady in between. Explain the reason for the needle’s behaviour at all three stages.Intuitive explanation
The compass needle responds to changes in magnetic field, not steady fields themselves—this is the key insight. When the switch opens or closes, current through the wire suddenly starts or stops, creating a changing magnetic field that induces a deflection. Once current flows steadily (switch closed), the magnetic field becomes constant, so the needle settles back to its original position despite the field's presence. The same happens when you open the switch again: the sudden change causes deflection, but the return to zero field is what matters, not the field itself.
Intuitive explanation
The compass needle responds to changes in magnetic field, not steady fields themselves—this is the key insight. When the switch opens or closes, current through the wire suddenly starts or stops, creating a changing magnetic field that induces a deflection. Once current flows steadily (switch closed), the magnetic field becomes constant, so the needle settles back to its original position despite the field's presence. The same happens when you open the switch again: the sudden change causes deflection, but the return to zero field is what matters, not the field itself.