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## Homework Statement

A 200-turn solenoid having a length of 25cm and a diameter of 10cm carries a current of 0.29A. Calculate the magnitude of the magnetic field B inside the solenoid.

## Homework Equations

B = [tex]\mu[/tex]

_{0}ni

## The Attempt at a Solution

Well, I plugged the numbers into the equation and got 7.28x10^-5 T as the answer. However, this is wrong. The correct answer is meant to be 0.3 mT. I can't see where I've gone wrong because the magnetic field inside a solenoid is meant to be independent of its dimensions, right?

EDIT: I found the number of turns per unit length, then put that into the equation and still get the wrong answer. I'm confused.

B = 4[tex]\pi[/tex]x10-7 Tm/A x (200/0.25) x 7.5A = 7.28x10^-5.

I've no idea why I get that. Maybe the diameter has some significance that I haven't managed to see yet.

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