# Direction of r-hat?

1. Nov 11, 2012

### Smooth23

Okay, the biggest problem I'm still having 12 weeks into physics 2 is that I cannot for the life of me see how the direction of r-hat is determined. Can anyone explain it to me? Specifically in regards to Biot-Savart law, etc.

2. Nov 11, 2012

### Staff: Mentor

Please tell us which textbook you're using. Different textbooks unfortunately use different notations and conventions for things like this. If you can show us the equations that you're looking at, someone who knows the subject well can probably figure out what's what.

3. Nov 11, 2012

### Smooth23

Okay, I'm studying for a test, looking at former solutions here: http://campus.mst.edu/physics/courses/24/Old_exams/e3_spring2012_solved.pdf [Broken]
number 7 has me confused for line segments 1 and 2 why is r-hat in the given direction?

Last edited by a moderator: May 6, 2017
4. Nov 11, 2012

### Staff: Mentor

Okay, $\hat r$ is always supposed to point from the current (wire) segment ds towards the point P where you want to evaluate $\vec B$. It's drawn backwards on segment 1. It should point towards P. It doesn't make any difference in the final answer because the contribution from segment 1 comes out zero either way.

5. Nov 11, 2012

### Smooth23

Thanks, I realize it didn't make a difference in that particular problem. Another old test had a similar problem where it did make a difference, but was apparently labeled correctly that was really confusing the crap out of me.