Solving Direction of Current Homework Equation

[jaredmt]

Homework Statement

the mass is hanging from the bar. the magnetic field points into the page. The magnetic force is the only thing supporting the bar (negligible mass) and the weight. What side of the battery, a or b, has the positive terminal of the battery?

Homework Equations

F = Il X B

The Attempt at a Solution

I tried this 2 different ways and keep getting a different answer than what my professor has.
since the magnetic field goes into the page i have:
B = -j
the force goes up, so i have that as
F = k
i considered left to right to be (+i)
so then i have the equation:
F = Il X B
or
k = I X (-j)
where I = (-i) direction, meaning I goes from right to left on the bar. this would mean that the positive terminal is on the b side of the battery. Am I doing this wrong? or does the professor have the wrong answer?edit: i must be doing something wrong. I am goin through all my homework and i seem to be getting the wrong direction on every question. can somebody walk me through how to find the answer to this?

edit2: ok well i was about to panick but now that i can do the right hand rule (which was a lot easier than i thought it was) I am getting all my directions right. but I am still curious to know what I am doing wrong with my cross products. I am not sure why they're all wrong.

 

[jdstokes]

I can't see your diagram, but assuming that the arrangement is like this

B
x x x x

x x x x
a ----------b
I

then the current must be flowing to the right (just use the right-hand rule) which implies that a is positive.