One of the big pratika we do in the B Session trainings is the stripped-down motor, that is, a motor constructed with only a battery, magnet, and a coil of wire. I’ve been building these for decades, and am always amazed at how much subtle physics there is to find. In Tetun we sometimes call it the ‘Motór Arbiru’, that is, the Arbitrary Motor, because sometimes it is nearly impossible to determine why one person’s is sputtering unresponsive while their colleague’s spins merrily away.
To explain how it turns has always been a bit of an exercise in hand-waving. The coil becomes an electromagnet when the current flows through it; that much is clear. Then it is in a position to push and pull on the permanent magnet below. This can be proven: stop the current or remove the permanent magnet and the motor grinds to a halt.
But at what point is that push or pull actually happening, and how is it translated into a torque to result in spinning motion?
We’ve been using tiny magnet wire – 30 gauge – because we got a large shipment of it and so that’s what we distributed to all the schools. It is hard to tell if or how much of the enamel insulation you’ve got stripped off of the two ends, or at what point that stripped part is actually contacting the safety pin end loops that we use to support it. It’s a great activity precisely because there is a hidden complexity, and because of the inevitable diversity of results when everyone makes one.
Well, when we were collecting questions after this activity in Suai someone asked something I’d never thought of in 25 years of doing this: Is it necessary to make a coil? Would it work with a straight wire?
Of course it wouldn’t work. I’ve taken this activity to the level of Maxwell’s equations, right-hand rule, superposing angular force vectors of magnetism and gravity, all walking lockstep with Newton’s action and reaction law. Without a coil, all this falls apart and you’re left with a single wire resting glumly between the supports, incontrovertibly stationery.
Holding off my desire to pontificate on high-level physics, I jumped at the chance to place this question in the most delightful category, the one reserved for questions that we can test right here on the spot with the materials on the table.
Well, in the time it took me to point that out, one group had already tried it and called all attention to their single wire, which to my great astonishment, was whirring away, thumbing its nose at me, Uncle Newton, and all of electromagnetism. Here’s the video.
I first went to San Francisco’s Exploratorium in freshman year of high school, and while there saw a most elegant exhibit with an enormous wire carrying around 20 amps of current through the jaws of a large magnet. Like most Exploratorium exhibits, it was wide open to grab and play around with. When the current flowed, the wire jumped and I couldn’t push it back down into the field with all my strength.
The experience of that exhibit, etched more indelibly than anything I found at MIT, came rushing to my rescue here in Suai, and I said in Tetun, with no loss of dignity: “I stand corrected: every wire with a current experiences a force when it enters a magnetic field. Furthermore, that wire is not completely straight; it’s got curves like a jump rope.”
Indeed our Arbitrary Straight Wire Motor actually has a slightly bent wire and if it happens to get (arbitrarily) connected to the current at the right time each time it comes around, it gets a momentary torque and that’s all it takes to make it spin.
If you want to work it out in detail, you find the direction of the conventional current, positive to negative, and point your index finger down the wire in that direction. Get your magnet stuck on the battery with the north pole up. With your left hand, point your fingers out of the top of the magnet and curl them around toward the bottom of the magnet. They represent part of the magnetic field. Now, without moving the index finger from its direction with the current, point your middle finger in the direction of those left-hand fingers, that is, the magnetic field lines. Finally, put out your thumb at right angles to both of the others, and that will represent the direction of the force. A slight component in the direction of the jump rope’s rotation will make it happen.