# Homework Help: Magnetism of an electromagnet

1. Apr 14, 2014

### howdoIphysics

Hey I'm new here so I don't know if this is the right forum, but from what I could see this was the most relevant.

1. The problem statement, all variables and given/known data

A long solenoid with 50 turns per cm carries a current of 0.2 A. The solenoid is filled with an iron
alloy. The magnetic field B inside the iron alloy is measured to be 1.58 T acting parallel to the axis of
the solenoid.

Calculate the:
i) size of the applied field Bapp;
ii) size of the magnetization M in the iron alloy;

2. Relevant equations

This is my problem really as I don't know so far I've been using the magnetic field of a solenoid = (N/L)
*I*μ0, however I don't know if this correct or if it needs the permeability of the core to produce an accurate value.
I've also been using Bt= Bapp0*M rearranged to M= (Bt-Bapp)/μ0 to answer the second question.

3. The attempt at a solution

Well this is just plugging numbers into the equations so I'm confident that I haven't done anything wrong here.
Assuming the solenoid field is Bapp, Bapp=5000*0.2*1.3×10−6 = 0.0013.
Putting that into the rearranged equation. M = (1.58 - 0.0013)/1.3×10−6 = 1.2106.

My problem here is those values seem very small and ridiculously large respectively, have I done something wrong or am looking for errors where there are none?

Last edited: Apr 14, 2014
2. Apr 14, 2014

### BruceW

hey, welcome to physicsforums! uh, your answers look good to me. Although when you hand in your work, you should probably give units, and maybe be a bit more careful on the rounding-off. But it's essentially good. And yes, one is very large and the other is very small. But this is just because SI units are like that. Looking at your equation M= (Bt-Bapp)/μ0 And since μ0 is very small in SI units, the ratio between Bt-Bapp and M will always be an 'extreme' ratio, in SI units.

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