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

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.

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gabbagabbahey
Homework Helper
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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.

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?

gabbagabbahey
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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?

it should point into the page

gabbagabbahey
Homework Helper
Gold Member
Good, so the fields oppose each other at that point....so what is the net field then?