A long solenoid produces magnetic field B along its axis. If its current is doubled and the number of turns per centimetre is halved, the new magnetic field is:
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Question

A long solenoid produces magnetic field B along its axis. If its current is doubled and the number of turns per centimetre is halved, the new magnetic field is:

G1 Physics · Accepted Answer

Step 1: Internal magnetic field of an ideal long solenoid is B=μ₀nI. Step 2: Doubling current sends I→2I, while halving turn density sends n→n/2. Step 3: B′=μ₀(n/2)(2I)=μ₀nI=B. Field remains unchanged, option B. Final answer: (B) B.