# Magnetic Interaction: parallel rods, cross bar free to slide

Tags:
1. Mar 14, 2015

### MaryCate22

1. The problem statement, all variables and given/known data
Two horizontal parallel conducting rods are connected such that a conducting crossbar free to slide along them has a constant current I running through it (Figure 1) . The rods are separated by a distance ℓ and are in an external uniform magnetic field of magnitude B directed out of the screen. The crossbar has a length ℓ and mass m.

Part A: In which direction does the crossbar move?
Part B: If there is a coefficient of static friction μs between rods and crossbar, what is the minimum current I0 necessary for the crossbar to move? For the length ℓ of the bar use the notation L.
Express your answer in terms of some or all of the variables L, B, m, μs, and the acceleration due to gravity g.

2. Relevant equations
F=ILsin(theta)

3. The attempt at a solution
For Part A I think it moves to the right. Got this using the right hand rule.
I'm not sure how to go about Part B.

F=I0LB

I don't know how to incorporate the coeff of friction in this. What can I set it equal to?

2. Mar 14, 2015

### Staff: Mentor

The force on the rod due to interaction of the magnetic fields must > the force due to friction.