# Homework Help: Strength of magnetic field from a current filled wire at a point

1. Nov 12, 2008

### sbielby413

http://http://session.masteringphysics.com/problemAsset/1074673/3/knight_Figure_32_14.jpg [Broken]
that is the diagram that accompanies the question. The 3 parts of the question ask what is the magnetic field strength at points 1, 2 and 3.

I know that B=B_1+B_2+.... for each point. In order to find the strength of the field at a point you must take into consideration all forces. I also know that u_0/4pi*(I s cross r/r^2) gives the strength of the magnetic field at a point, as does (u_0*I)/(2*pi*d)

I have attempted the problem several times, and keep using the second equation from above for the points on the outsides of the 2 wires (points 1 and 3). However, I keep getting the answer incorrect. I haven't started point number 2, but I think once I figure out the principle behind points 1 and 3 it will come easier.

Last edited by a moderator: May 3, 2017
2. Nov 12, 2008

### sbielby413

3. Nov 12, 2008

### gabbagabbahey

You need to consider the direction of the individual fields too, not just there magnitudes...if they point in the same direction at a given point, then the net field will be the sum of the two individual fields $B_1+B_2$, but if they are in opposite directions, you will end up with the difference of the two fields $|B_1-B_2|$....use the right hand rule to determine the direction of the individual fields at each of the points in question.

4. Nov 12, 2008

### sbielby413

using the right hand rule, should B for the top wire should point out of the page and the bottom one into the page? my right hand rule is a little rusty.

also, am i using the right equations?

5. Nov 12, 2008

### gabbagabbahey

Yes, the field at point 1 due to just the top wire is out of the page....how about the field at point 1 due to just the bottom wire?

6. Nov 12, 2008

### sbielby413

it should point into the page

7. Nov 12, 2008

### gabbagabbahey

Good, so the fields oppose each other at that point....so what is the net field then?