# Calculating 'Push' Force to Maintain Constant Speed

1. Jul 2, 2008

### Laurie01

1. The problem statement, all variables and given/known data
A conducting rail in contact with conducting wires, oriented perpendicular to both wires, is pushed with constant speed, causing an induced current of 0.69 A. B = 0.50 T and R (Resistor) = 2.0

Calculate the "push" force necessary to maintain the rail's constant speed.

2. Relevant equations

I assume I use F = q * v * b

3. The attempt at a solution

I don't think I am using the correct equation here because there is no charge on the conducting rail. I am thinking I need to use an equation that I am not yet familiar with. I went to the chapter in my book over induction and I can't find any equations dealing with a 'push' force.

2. Jul 2, 2008

### Staff: Mentor

You want the magnetic force on a current-carrying wire in a magnetic field, not the force on a single charge. Read this: http://hyperphysics.phy-astr.gsu.edu/Hbase/magnetic/forwir2.html" [Broken]

Last edited by a moderator: May 3, 2017
3. Jul 2, 2008

### Laurie01

Ohhh! Duhh. Lol. Thanks so much! That helped.

So, F = (0.69A)(0.34m)(.50T) = .117 N

And this would be considered the "push" force? Or would it be negative since the force is acting in the opposite direction?

4. Jul 2, 2008

### Staff: Mentor

I assume they just want the magnitude of the force.

The induced current leads to a magnetic force on the wire. If you don't push the wire with a force opposite to the magnetic force to cancel it out, the wire will slow down due to the magnetic force. (This is the point of Lenz's law: The induced current is not "free"--you must push on the wire to maintain it.)

5. Jul 2, 2008

### Laurie01

Thank you! That makes sense.