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Q & A: spinning water with a magnet and current

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Most recent answer: 12/19/2014
Q:
I have 54 volts of D/C current running into a brine solution (electrolysis). As a twist, though, I decided to put my strong, N52 Neodymium magnet, attached to the negative terminal, into the water. The water began to spin with surprising rapidity around the magnet. I have a general idea about what's happening, but am honestly for the most part pretty stumped. Could I get an explanation of this phenomenon from someone else? Preferably someone who's taken more than 2 physics courses? Thanks!
- Evan (age 23)
Indiana, USA
A:

That 54V is a voltage, not a current. Thanks to the salt in the brine, you do have a significant current, although to find out how much you'd want to put an ammeter in the circuit.

So now you have a current flowing in a magnetic field. There's a force, called the , on moving charges in magnetic fields. The force is at right angles to the current flow and to the field. Say that the field came out from the magnet along the direction of the current flow. The field lines then bend out and loop around back to the rear of the magnet. As they head away from the middle of the strong current region, they're not parallel to the current. So there's a force on the moving charged particles. The forces on the positive and negative ions in some region point the same way, because they are traveling opposite directions and the force depends on the product of the velocity and the charge. The direction of the force varies with position because the direction of the field does. The force lines make loops around the current, driving that spinning motion that you see.

So what you see is exactly what is expected. If you flip the magnet around you should get spinning in the opposite direction. Likewise if you switch the direction of the current the direction of the spin should reverse.

Mike W.


(published on 12/19/2014)

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