Heat generator science project?

--Hmmm sounds like a good place to post might be over at sci.physics.electromag but be aware that the kook ratio there is almost as bad as it is here, heh.

Yes, that will heat up the copper.

Another cool trick, is to spin a disk. Put a magnet up to it's face, and the drag will slow down the disk, (as long as the driving motor isn't TOO big). As long as the axis of the magnet is offset to the disk, it will slow it down.

Now, take a donut shaped magnet, and hold to one side of the spining disk, it will slow it down. Now, take that donut shaped magnet, and hold on the centerline of the shaft, and right up to the disk. No drag !!! Tha magnet can go right up to the disk, and not induce any drag, asd long as it is centered on the axis of rotation.

Now, get a voltmeter, and measuer the voltage from the center to the edge of the disk.

Now, to really blow your mind, glue the magnet to the disk, so it is rotating along with the disk, and you will still get a voltage at the rim of the disk, to the centerline.

Yup. It's an eddy current brake. Jerry Martes once made an eddy current brake dyno with a metal disc and electromagnets from alternators. It could dissipate several horsepower.

Very nice. I will try to do it tonight.

you can also spin up a stationary disk if you rotate the magnet.

"Ignoramus14150" wrote in message news:BMydnf9b_v-H1oPbnZ2dnUVZ snipped-for-privacy@giganews.com...

I work with a lot of magnets and such at work, and have played with this a lot. I'll tell you about a nifty trick when this explanation is over. When a field moves in the presence of a conductor, current is induced in that conductor. That's how transformers work. That's why the iron core is usually laminated using strips instead of a solid core, which prevents those currents from going in all the wrong directions, which would overheat the transformer. There's always going to be eddy currents (what currents going around and around inside a conductor are called) when a magnetic field moves in the presence of a conductor. By the reverse token, any electrical current creates a magnetic field. So you have a magnetic field creating electrical current which creates another magnetic field, which opposes the first field. If I were to take a disc and place in it a few magnets in a circular layout and place it near another disc, I can make that disc spin by spinning the magnetic disc. If the discs were non-ferrous and good conductors (like copper or aluminum this works really well. In fact, that's what makes eddy current clutches and brakes work so well. No contact or friction, but it will get warm from the electrical resistance in the conductor creating heat. I don't understand this as well as I should (I'm no engineer, but I do work with them...) but I'm learning. For the trick... We have a big magnet in the lab that we use to characterize superconducting magnet crystals that we made. We put the crystal over the magnet in a little tray, fill the tray with liquid nitrogen, and when the crystal is cold enough, we carefully and with great force lift away the newly magnetized crystal and move it to a machine where we scan the magnetic fields, looking for signs of crystal defects. I was playing with a copper ring the other day and dropped it flat over the magnet. It fell slowly because the ring created an opposing magnetic field which slowed its descent. Knowing that copper's resistance actually decreases when it's chilled to lower temperatures (as all conductors do to one degree or another) I chilled the ring with liquid nitrogen and repeated the process. It would fall slower when I dropped it as expected, but the ring actually fell off to one side, a victim of it generating its own opposing magnetic field, a little stronger than before. The ring (a vacuum gasket, actually) was about two or thee inches in diameter and the magnet is about the same, four or so inches high and about 500 milligauss in strength right on the magnet top surface, for the enquiring minds who want to know.

A little bit of useful web information:

I've got a Thelma eddy current truck brake. It's about 24" diameter, weighs about 800 lbs, and can absorb about 250hp continuous, several times that for short periods, and about 2000 ftlbs. torque. It has about 10 or so 24 volt field coils each the size of a coffee can between two massive iron rotors with cooling vanes. Used originaly for logging truck braking in the mountains. I plan to hook it up to a set of chassis rolls that are rusting in my back yard one of these days to make a chassis dyno. Main work will be making the concrete pit to set it in...

Sounds exciting. I just made a eddy current brake prototype, not really to be used for braking, only for demonstration. Epoxy is setting right now.

We use something very similar at work, as a dyno, for testing bus engines. It generates serious heat!

One nice thing about eddy current dyno's is that they are pretty easy to control. The field current is very small in relation to the power being controlled, and absorbtion torque increases with speed/rpm so you can hold a steady speed with a fairly basic controller.

Hi oldjag

It is possible to set a set of rollers above ground, and even trailer mounted for HP levels around the 200 HP level. But, you probably knew that. If you decide to build that pit, be sure to account for the effect the sound from any engines will have on your neighbors, if any.

Jerry

Check out his thing science guru Bill Beaty built:

Chris