Solving the Puzzle: How Does the Magnetic Field Affect Balance?

[caljuice]

Well I have the answer already, just don't understand it.

A rectangular coil of wire is hung from one arm of a balance, as my beautiful diagram will show. With the magnetic turned off, a Mass M is added to the pan on the other arm to balance the weight of the coil. When a constant magnetic field is turn on and there is a currentr of 8.50 A in the coil, how much additional mass m must be added to regain the balance? The magnetic field is also going into the page. Forgot to draw the X's.
Image: http://img262.imageshack.us/img262/6318/balanaceyd8.th.jpg

Because of the magnetic field, the bottom length of the coil exerts a downward force but why is there a net force downwards, causing the torque? Shouldn't the upwards force from the magnetic field at the top of the rectangular coil cancel it out? Thanks in advance.

 

[m.e.t.a.]

I agree with you, the upward and downward forces would surely cancel out. Not to say that you diagram isn't excellent, but all I can think is that perhaps there is some small extra detail that we're missing here. Is the magnetic field distributed evenly throughout the entire diagram, for example, or is it localised?

 

[caljuice]

Ah yes. It was only a Mfield around the bottom part, since the question didnt mention that i assumed it was for the whole diagram and they just got lazy. But that makes much more sense, my bad.

 

[m.e.t.a.]

Nah, there was nothing wrong with your understanding, you were just faced with a poorly written question.