Question

A solenoid has a length 1 m and inner diameter 4 cm and it carries a current of 5 A. It consists of five close packed layers, each with 800 turns along its length. The magnitude of the magnetic field at the centre is nearly equal to

• A. zero
• B. 25 mT
• C. 5 mT
• D. 1 Mt

Answer 1

The correct option is B

25 mT

Number of turns per unit length, $\frac{5\times 800}{1}=4000$ turns per meter.
Length, L ≫ d
\Rightarrow The solenoid is long and can be treated as an ideal solenoid.
The magnetic field at the centre is given by
$B=\mu 0nl=(4\pi \times {10}^{-7})\times (5\times 4000)=25mT.$