# Magnetic field, current, and a slide wire

#### Heart

Hi, I really need help with this question.

Here's the question:

"The figure shows a U-shaped conducting rail that is oriented vertically in a horizontal magnetic field. The rail has no electric resistance and does not move. A slide wire with mass m and resistance R can slide up and down without friction while maintaining electrical contact with the rail. The slide wire is released from rest."

"Determine the value of a terminal velocity if l = 25.0 cm, m = 10.0 g, R = 0.100 ohms, and B = 0.550 T."

I know that V_emf = vlB
I = V/R = vlB/R
If I can find I, then I could determine v. The problem is they don't give V_emf, so I cannot find I. and I have no idea why would they give the mass for, when generally for this type of problem they don't. Any hints would be greatly appreciated.

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#### Heart

F = IlB
P = Fv = (v^2)(l^2)(B^2)/R

I just don't see how could the mass be relevant to this problem. If it was a particle then may be I could use the mass to do something with the circular motion but this just doesn't make sense. Please help.

#### Heart

I think I might get it now, I just didn't read the question carefully and miss the "vertical part".

#### Heart

OK, I got it, still cannot believe I could miss the keyword "vertical". Thanks and sorry for this. Please don't get mad at me. I'm going to sleep now at almost 8 A.M. :zzz:

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