Originally published at: https://boingboing.net/2020/02/28/how-to-make-homopolar-motor-wi.html
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It’s really not that complicated. When there’s an electrical current in a magnetic field, the current experiences a force at right angles to both the field and the direction of the current.
In the examples here, the field is essentially vertical and the current is (at least in part) radial from the axis of the magnet. What’s left is tangential to the cylinder defined by the magnets – so it spins.
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“Uh-huh-mazing”, as the man says.
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Ah, the old ‘right hand rule’, where if you make a partial fist with your right hand and leave the thumb and index finger sticking out, the curled fingers show the direction of the magnetic field, the thumb sticking out shows the direction of the electrical current, and the index finger points in the direction of movement (in the cas of a moveable wire).
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Here’s a high speed version:
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Back in the late 70s I was taking both an electrical course and a physics course. This came up in both. Problem was the electrical course considered current flow from positive to negative. The physics course considered current flow from negative to positive (flow of electrons), which meant you had to switch hands. To this day I keep confusing the hands. Thank God for the internet.
I have had similar simple motor plans fail because the copper had a clear coating on it that I didn’t realize was there.
PSA
In metals there’s no practical difference. Electrons going one way or holes going the other, the force is the same. (you can work this out, pretty easily. F=q B x V . Try it.)
The potential difference, on the other hand, is not – which is why the Hall effect proves that there really are positive carriers.
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