Question

A closely wound long solenoid of length 40 cm has magnetic field of $8\pi \times {10}^{-4}$ Tesla in central part, current in solenoid is 2 A. Find number of turns in solenoid (assume solenoid as ideal) ?

• A. 100
• B. 1000
• C. 4000
• D. 400

Answer 1

The correct option is D 400

$B={\mu }_{0}ni8\pi \times {10}^{-4}=4\pi \times {10}^{-7}n\left(2\right)$
$n=\frac{8\pi \times {10}^{-4}}{8\pi \times {10}^{-7}}=1000=\frac{N}{l}$
$N=1000\times 40\times {10}^{-2}$
= 400