JEE Main · 2022 · 25 Jun 2022 · Shift 2 · Magnetic Effects of Current

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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 magnetic field will be equal to

(A)B✓ Correct
(B)2 B
(C)4 B
(D) $\frac{B}{2}$

Answer: (A)

Solution

Magnetic field inside a solenoid:

B = μ_{0} n I

, where n is turns per unit length and I is current. Initially

B_{1} = μ_{0} n I

. After changes:

B_{2} = μ_{0} (\frac{n}{2}) (2 I) = μ_{0} n I = B_{1}

. The new field equals the original field B.

JEE Main · 2022 · 25 Jun 2022 · Shift 2 · Single correct option

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