# Magnetism - Quick solenoid question

## 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 = $$\mu$$0ni

## 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$$\pi$$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.

Last edited:

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Doc Al
Mentor
Show exactly what you plugged in for each factor.

Doc Al
Mentor
A 200-turn solenoid having a length of 25cm and a diameter of 10cm carries a current of 0.29A.
...
B = 4$$\pi$$x10-7 Tm/A x (200/0.25) x 7.5A = 7.28x10^-5.
Looks to me like you are using the wrong current.

Oops. I apologise for wasting time. I used a current from a different question by accident :).