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Q & A: mercury motor

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Most recent answer: 11/13/2011
Q:
With a small pool of mercury a neodymium magnet in the center and applying electric current (directly to the mercury) and a copper wire I observed that the mercury rotated around the copper wire. I want to understand this phenomenon. The only thing that I found is something called Magnetohydrodynamics, the Navier-Stokes equations that describe the motion of a fluid and a short explanation on the internet about Faraday's motor. The thing that I would like is a cualitative explanation of the phenomenon and maybe later combine your explanation with the equations. I only know differential calculus right now so I'm trying to figure out what to do. I always wanted to do the Faraday motor, I thought it was really interesting, but the only thing I got is the motion of the mercury itself maybe I'll try it later with salt and water. Maybe the answer lies on the Navier-Stokes equations. I'll check that. Hope you can illustrate why that happens (mercury's circular motion around the wire)Thank you for reading this.
- Pi kid (age 21)
California
A:
It would be nice to have a picture, but it sounds like the current is flowing from some outer contact in toward a wire in the middle. The magnet probably has its field pointing mostly along the up-down direction. So there's a current at right angles to the field. The magnetic force on the current is along the vector cross product of the current direction and the field direction. That would be tangential. So there's a tangential force, i.e. one pointing the right direction to make the mercury rotate.

As for the fluid dynamics part, I think you only need that to figure out how much friction is made by the viscosity of the mercury. That friction keeps the mercury from spinning faster and faster, but isn't needed to see basically how the thing works.

Mike W.

(published on 11/13/2011)

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