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## Homework Statement

A horizontal wire 0.20 m long and 80 grams in mass is hung in a uniform B-field by two massless strings. The magnitude of the magnetic field is 0.070 T. When a 42 amp current passes through the wire, the wire swings upward forming an angle *.

a. What direction is the current going through the wire?

b. What does * equal?

c. What is the tension in each of the two strings?

Diagram: http://img.photobucket.com/albums/v696/talimtails/pp22A.jpg

* = theta

## Homework Equations

F = BILsin*

B = kI/r (possibly)

k (constant)= 2 x 10^-7 T-m/A (T = tesla, A = amps, m = meters)

+Application of the right hand rule, with thumb being current, other fingers being magnetic field, and force coming out of the palm.

## The Attempt at a Solution

For a, I said the current was going out of the page. I figured the magnetic field would be going down (N to S on the diagram) because magnetic fields usually are directed from + to -. So using my right hand, if the magnetic field (my four fingers excluding thumb) is down, the current (my thumb) seemed to be going out of the page towards me.

For b, I thought there must be some kind of acceleration that is making the wire move. If so, F = ma, but F also equals BILsin*. Therefore, I set the two equal to each other and solved for *.

sin* = ma/BIL = (.08 kg)(9.8 m/s^2)/(.07 T)(42 A)(.2m) ... * = 4/3.

However, I quickly realized that 4/3 is not a possible solution for * as it can never go over 1 for sin. This is where I get stuck. I'm not sure, to be truthful, that acceleration is a factor since it seems the magnetic field would be forcing the wire to the N pole rather than acceleration... but I could not find any other applicable equation. Is there some obvious factor I'm missing? Or.. if you'd rather not give that, how should I go about finding the angle (without giving me a direct answer). I'd be helpful if I could understand why and then do the calculations myself rather than being giving an answer.

For c, recalling early chapters, I would draw a free body diagram of the components I have and solve for T using that. In that case, would I consider the normal force (F_n) and the weight (mg)? I've never done this with magnetic fields before, so it leads me to believe that I should also consider the magnetic field.. as that seems to be the dominating force that determines the angle and therefore tension.