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

[sbielby413]

http://http://session.masteringphysics.com/problemAsset/1074673/3/knight_Figure_32_14.jpg
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.

 

[sbielby413]

http://session.masteringphysics.com/problemAsset/1074673/3/knight_Figure_32_14.jpg

sorry i forgot the image.

 

[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.

 

[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?

 

[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?

 

[sbielby413]

it should point into the page

 

[gabbagabbahey]

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