# Torque on current loop

1. Oct 16, 2009

### dlslhc

1. The problem statement, all variables and given/known data
A wood cylinder of mass m = 0.250 kg and length L = 0.100 m with N =10 turns of wire wrapped around it longitudinally, so that the plane of the wire coil contains the long axis of the cylinder. The cylinder is released on a plane inclined at an angle theta to the horizontal, with the plane of the coil parallel to the incline plane. If there is a vertical uniform magnetic field of magnitude 0.5T, what is the least current i through the coil that keeps the cylinder from rolling down the plane?

2. Relevant equations
force = NiL x B

3. The attempt at a solution
The answer is 2.45A, i try to find all the forces and solve when torque is zero, but i cant elimate the sin theta or cos theta thing. Please help, thankyou.

Last edited: Oct 16, 2009
2. Oct 16, 2009

### Redbelly98

Staff Emeritus
Can you show what you worked out so far? It might be possible for somebody to spot your error (if any) that way.

p.s. if it helps, you can copy-and-paste this theta symbol: θ

3. Oct 16, 2009

### dlslhc

2NiLBr sinθ = μmgr cosθ
mgsinθ = μmg cosθ

the answer is 2.45 if g = 9.8, i used 10 before lol, correct now?

Last edited: Oct 16, 2009
4. Oct 17, 2009

### Redbelly98

Staff Emeritus
Looks good (I presume you are able to eliminate the sinθ & cosθ now).