1. Dec 6, 2012

letsgo

So here are two wires. With currents in the specified direction.

What I want to know, is how do you determine the direction of the magnetic moment of the upper wire, which would ultimately lead to me determining what direction its torque is in, therefore letting me determine what direction it rotates in (the answer is CCW, I do not know how to get it though).

I know that the B-field on it from the second wire, points out of the page.
And I know that the Force acting on it (due to the lower wire), is to the right.

Cross-Product Rules:
τ = μ x B
τ = F x r

I know when the current is in a loop, to find μ direction you just wrap your hand around, and your thumb points in the direction. But this wire is just a wire... I have no clue how to use a right-hand rule on the current alone.

2. Dec 6, 2012

aralbrec

That's right. What's the magnitude of the force above P compared to the magnitude of the force below P? Which way will I2 rotate (I assume P is a pivot).

What's the force per unit length at each point on I2? How would you compute the total torque given this force per unit length?