A long solenoid carrying a current produces a magnetic field $B$ along its axis. If the current is doubled and the number of turns per cm is halved, the new value of the magnetic field is
AIPMT 2003,JEE MAIN 2022, Easy
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(a) $B = {\mu _0}ni \Rightarrow \frac{B}{{B'}} = \frac{n}{{n'}} \times \frac{i}{{i'}} = \frac{1}{{(1/2)}} \times \frac{1}{2} = 1 \Rightarrow B' = B$
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