# Homework Help: Pushing a rod in a b field.

1. Sep 26, 2011

### serverxeon

imagine a metal rod floating in a region of magnetic field. You give the rod a slight nudge. Will the rod continue to go forever?

Considering conservation of energy i think it wouldn't. But i can't come to terms with any reasonable reasoning.

2. Sep 26, 2011

### Spinnor

Suppose the rod is perpendicular to the magnetic field and the "push" vector is perpendicular to both the rod and the field. The Lorentz force tells us that the rods movement will cause the rod to have a slight imbalance of charge along the rod. But after the push the rod should continue in motion at constant velocity? There are other orientations of the rod and the push that also have to be considered.

3. Sep 26, 2011

### serverxeon

Sorry to mention, i'm referring to perpendicular cases only.

Will the rod remain in motion? What about the potential difference across the two ends? Will it even out over time?

4. Sep 26, 2011

### danielakkerma

Hi,
Sorry to intrude, but is your rod a ferromagnet? Or does it carry charge?
Is the external field created unitary in space?
Can you provide some sort of sketch for clarity?
Thanks,
Daniel

5. Sep 26, 2011

### serverxeon

I don't have a computer with me these few days, so i can't get a drawing out. But here's the best i can do.

[PLAIN]http://img52.imageshack.us/img52/9687/magqj.png [Broken]

The system is similar to this, except ignore the black lines. Only the rod exist.

And, to one of the question, if the magnetic field is uniform, i guess being whichever type of magnetic material won't make a difference?

Last edited by a moderator: May 5, 2017
6. Sep 27, 2011

### danielakkerma

Hi, now that you've provided this illustration, I get your point at last;
You're it, its being ferromagnetic won't change anything as long as its perpendicular.
So we're to assume that it's travelling on this railing you've presented, and you're asking what is to happen to it once you apply a minor velocity to it.
Sadly, there are no pleasantry effects, here, as the induced field in the rod will cause it to slow down. Once its velocity reaches zero, the effective force becomes zero(Lorentz's force, q(vXB) and it stops, without giving you "much of a bang", this is unlike a spring, where you would osscilatory motion to the dependence of the force solely on the displacement; In this case, where the velocity is involved, it's a totaly different matter.
Daniel