Question

01. A long solenoid consist of 20 turns per cm. What current is necessary to produce a magnetic field
of 20mT inside the solenoid ? ans - 8A]
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Answer 1

magnetic field inside a long solenoid is given by , $B=\mu_0ni$

where B is magnetic field , $\mu_0$ is permeability of the space. i.e., $\mu_0$ = 4π × 10^-7 Tm/A

n is number of turns per unit length and I is current through solenoid.

here, n = 20 turns per cm = 20 × 100 = 2000 turns per m

B = 20mT = 2 × 10^-2 T

now, 2 × 10^-2 = 4π × 10^-7 × 2000 × i

⇒i = 100/4π = 7.95 ≈ 8

hence, current through solenoid is 8A.