A long solenoid carrying a current produces a magnetic field

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A long solenoid carrying a current produces a magnetic field B along its axis. If the current is double and the number of turns per cm is halved, the new value of the magnetic field is

1. 4B
2. B/2
3. B
4. 2B

Correct Option: C

B = μ₀ni

B₁ = (μ₀)		n		(2i) = (μ₀ni) = B
		2

⇒ B₁ = B