# Homework Help: Finding current through a bar

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1. Dec 19, 2016

### TheBigDig

1. A uniform bar has mass 0.0180 kg and is 30.0cm long. It pivots without friction about an axis perpendicular to the bar at point a (as seen in the diagram). The gravitational force on the bar acts in the −y-direction. The bar is in a uniform magnetic field that is directed into the page and has magnitude 0.350 T . What must be the current I in the bar for the bar to be in rotational equilibrium when it is at an angle 60.0o above the horizontal?

2. F = qv x B
dF = Idl x B

3. I've attempted to just plug in the values using the numbers given in the question but I'm unsure how to account for the angle using the above formulae.

2. Dec 19, 2016

### David Lewis

Draw a free-body diagram. Show all the forces acting on the bar.

3. Dec 20, 2016

### TheBigDig

Thanks. Found it by equating mg(L/2 cos theta) to IB L^2/2 and solving for I.

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