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

Solution

## The correct option is C 25 mT Number of turns per unit length, 5×8001=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=μ0nl=(4π×10−7)×(5×4000)=25mT.

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