# Homework Help: Conducting rod is free to move over a loop under influence of B field?

1. Apr 26, 2014

### GodPlaysDice

1. The problem statement, all variables and given/known data
The conducting rod ab shown in the figure (Figure 1) makes contact with metal rails ca and db. The apparatus is in a uniform magnetic field 0.800 T, perpendicular to the plane of the figure.

If the resistance of the circuit abdc is 1.50 Ω (assumed to be constant), find the magnitude of the force required to keep the rod moving to the right with a constant speed of 7.50 m/s . You can ignore friction.

I've already found that (if the rod moves with constant velocity 7.5 m/s) the EMF is 3 volts.

2. Relevant equations
F = qv x B
V = IR
q = It

3. The attempt at a solution
I thought I could somehow use ohms law to find the charge q i needed in the equation for the Lorenz Force by q = I t, but I don't have a t. I don't really know where to start solving this one.

2. Apr 26, 2014

### rude man

You could use energy conservation. What's the energy dissipation per second around the loop? Relate that to force and rate of change of distance.

You could also use the equation relating force to wire length, wire current and B.

Do it both ways as a check!