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Class induction demonstration toy.

Class induction demonstration toy.

Class induction demonstration toy.

So I was watching the famous Walter Lewin do a lecture about induction. I'm getting ready to do the same for an aircraft mechanic class.

The idea I had was... I have a big (Ø40mmx20mm axially magnitized 52N) magnet I ordered from amazon. Yeah I could get my 87V fluke out and make a loop and see microamps... that would honestly probably be good enough.

However... What would I need to do to say drop the magnet down a tube... have an induction coil that it passes through... then have a transformer that could produce a reasonably sized spark across an air gap. The magnetos on aircraft engines do a pretty good job with a much weaker magnet.

I've got a 3d printer, full machine shop, solenoid winding machine... I just need some #of turns, and wire gages.


RE: Class induction demonstration toy.

There are so many scrap transformers around it seems silly to wind one for a demo. Get an old ignition coil if you want a spark. But read-up on how they work. Voltage generated depends on the rate of change in magnetic field passing through the coil. A dropping magnet is not fast enough to generate a spark. Sparks a made by interrupting current through the coil, not by closing a switch and starting current flow through the coil. A capacitor across the switch contacts is a very important component for non-obvious reasons.


Several suitable YouTube videos would probably be more educational than a clunky demo, or at least it would provide ideas for a better demo.

RE: Class induction demonstration toy.

Drop the magnet down a PVC pipe and time it. Then drop the magnet down a copper pipe and time it.
Good demonstration of eddy currents and Lenz's Law.

RE: Class induction demonstration toy.

Great idea Comcokid. The currents will be induced as circulating currents in the copper and lost as heat (a little).
You probably won't detect the rise in temperature but with a strong magnet you should see a noticeable difference in time.

"Why not the best?"
Jimmy Carter

RE: Class induction demonstration toy.

I do like this video: https://www.youtube.com/watch?v=gZpa7FYR_pw

The problem is the small amount of power available.

Instead of a spark I'd try a small incandescent bulb or other device that maximizes the extracted current and produces the maximum back EMF that resists the fall of the magnet.

(Check my math - there are some assumptions and it's late. No one will die, so my motive to be exactly right is really, really small.)

Figuring the magnet is about 7 grams per cc. If the diameter is 4 cm and the thickness is 2 cm, then the mass is about .2 kg with a weight at 1 G of about 2N. If the weight does not accelerate while it moves down the tube, the resisting force will also be 2N. If the drop is 1m, then there is 2 Joules or 2J/100cm available. So if it goes 1cm/second that's 2J/100seconds or .02 Watts. Obviously if it goes faster than some noticeable portion of the available energy will be taken in kinetic energy, so cranking it to 1m/second means that less energy is available for powering external devices, but more power.

I would probably set it up by tightly winding fine copper wire over a thin wall acrylic tube with narrow gaps to allow the magnet to be visible at various intervals.

I would use different electrical loads and possibly an Arduino to operate and electromagnet that releases the permanent magnet. The Arduino would also be able to select various loads using some transistor driven reed switches and monitor an IR sensor to capture the time and send that data back to a laptop or display on an attached, umm, display. Use additional reed switches to multiplex the loads to the analog inputs and the voltage across the load can be measured to see what the current is.

Momentarily, that's all I have.

RE: Class induction demonstration toy.

Lol. I'm an idiot mechanic(al) engineer. I can wind wire around circles, melt metal, and hit things with hammers and make things... I haven't figured out arduino/raspberry pi stuff... would like to...

But I LOVE THAT VIDEO. yes. I need that.

All that said... the magnet in a magneto is no where near as strong as this permanent thing...it's actually kinda scary...and it will shock the stuffing outta ya even by hand turning it past egap.

RE: Class induction demonstration toy.

Yes, but magnetos always move the magnet VERY quickly and most have a mechanism that snaps the magnet past the coil if the magneto is being driven slowly. It also has the very same contacts
Compositepro was talking about that are timed to open the contacts at precisely the moment the magnetic field is peaking in the coil. They are not just a magnet going past a coil.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Class induction demonstration toy.

The difference between your magnet and a magneto magnet is not so much the magnet as the air gap.
Not all magnetos have a snap action.
I was teaching my son about small motors. We had a one cylinder motor stripped down. I told my son to hold the spark plug wire and slowly turn the flywheel. No snap action on this one. He wasn't turning the flywheel very fast when he started to feel the output of the coil.
There is such a very great difference between the permeability of transformer steel and air that it is often feasible to ignore the iron and just calculate the air gap in a magnetic circuit.
In practice an error of 1/10,000 of an inch in measuring or constructing the air gap in some arrangements may result in a greater difference than will ignoring the entire iron part of the magnetic circuit.
Your air gap is from one end of the magnet to the other end.
When a bar magnet is bent into a horseshoe the air gap is much reduced and the same magnet has much more attracting force.
Horseshoe magnets are not just for luck!

Another good illustration of induction is a small bell or buzzer powered by one or two 1.5 Volt dry cells.
When the buzzer is powered, the clapper moves until the points open and then it moves back until the points close again. Repeat.
The current sets up a magnetic field. When the points open the field collapses and induces a high voltage in the coil.
This is often high enough to light a small neon lamp connected across the points.
It takes something around 70 Volts to light a neon lamp.
This is the same principle as a magneto. Back EMF generated by interupting the current through an inductor.
The faster the current is interrupted, the higher the voltage. A capacitor across the points will cause an even higher voltage.

"Why not the best?"
Jimmy Carter

RE: Class induction demonstration toy.

I do the magnet dropped down a copper tube demo and people are mystified by it. A rare earth magnet can take a very long time to fall...

RE: Class induction demonstration toy.

I did a demo years ago using strips of material, plastic, Cu, Al, and SS.
They were all fastened to a board that was on a steep incline (~70deg).
We had a set of magnets at the top held by a strip or wood. When a lever was pushed the magnets were all released at once.
I even had samples of Cu alloys (brass, CuNi and such) that were different enough to see the effects.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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