Nice demo of induction current

This time I have made a web page about a physical subject, the production of induction current around a closed loop when a changing magnetic field is going through the closed loop.

http://woelen.homescience.net/science/physics/exps/induction...

I think that this is a very nice experiment, also for teachers, because it requires no special equipment, just some tubes, copper wire, a diode, a LED and a suitable magnet.

Btw, my children like the crude flashing device I made very much. They keep on shaking it, producing bright flashes of white light

Indeed, that's what they do. Keeps metal-bits from poking holes in their stomachs. An NdFeB magnet would be a bit strong for that purpose, however.

Woelen: great! Now close the magnetic circuit with iron pole pieces to dramatically boost output, and add more windings* to generate power continuously during a cycle. Also, for 'duh' factor, put two antiparallel LEDs on there, so you don't waste one of the shakes.

*Winding = a bundle of copper. Windings are measured in turns, so your single winding has 1450 turns. The simplest transformer has two windings, etc.

A more powerful demonstration of induction can be seen here:
http://www.youtube.com/watch?v=ro6Dt9g7Ohw

Tim

Very nice, very simple!
Reminds me of asking my physics teacher a long time ago: Where/how is the work put into the system? Where does the energy come from? (assuming a frictionless system)

Cute but it reveals little about the nature of alternating current.
You can relay the magnets motion to another coil with a toy
compass along side as shown below. The needle will alternately
point to one end of the nearby coil or the opposite end exactly
in step with the motion of the magnet through the other coil.

Suspending the magnet from a rubber band and tugging down
and releasing it displays what is termed simple harmonic motion
the basis of the alternating current sine wave form.

Cut the loop of three rubber bands and tie both ends of one
band to one end of the other two to make a long band of
40 or 50 centimeters. Attach one end to the magnet and
the other to a fixed point overhead.
.

I tried the experiment with the campass, but it does not work. The fields are too fast and too much pulse like. If you are lucky you just see the needle shake a little. It does work though when a DC power supply is used for powering the coil (a few tens of mA of current already gives a good readout).

I made a new device, now using a coil with 3000 turns and a full wave rectifier, using Schottky diodes. The result really is impressive. Just moving a neodymium magnet through the hole by hand already leads to two flashes, quickly after each other. No vigorous shaking needed, no quick withdrawing needed.

A diagram of the system:



When the capacitor is omitted, then a really impressive behavior is obtained. When a 1 F goldcap is used, then you need to shake for a while in order to charge the capacitor and if the capacitor is charged well beyond 1.5 volts, then shaking leads to a more or less constant light output and when shaking stops, the light output slowly decreases and the capacitor remains charged at appr. 1.5 volts. So, you only need to shake for a long time the first time when a fresh capacitor is used. Goldcaps keep their charge for a very long time

I'll extend my webpage on this subject and add more pictures and videos.

Unfortunately it does not work as well with normal diodes, such as the 1N4001. These normal diodes have forward voltage drops well over 0.7 volts and when they carry a little more current, then this may be even 0.8 volts. A regular schottky rectifier diode has just 0.3 volts forward voltage drop in a circuit like this.

There also is another very funny effect. If the circuit with the LED is connected to the coil, or when the coil is simply shorted or a resistor is placed between the terminals, then you feel that pulling the magnet through the coil takes some effort. If the coil is open circuited, then you don't feel anything special. This is another beautiful demonstration. You feel that you have to provide power to the system. Mechanical energy is transformed into electrical energy.

[Edited on 28-10-09 by woelen]

Quote: Originally posted by woelen

I tried the experiment with the campass, but it does not work. The fields are too fast and too much pulse like. If you are lucky you just see the needle shake a little.

The classic demonstration

Quote: Originally posted by 12AX7

Indeed, that's what they do. Keeps metal-bits from poking holes in their stomachs. An NdFeB magnet would be a bit strong for that purpose, however. Also, for 'duh' factor, put two antiparallel LEDs on there, so you don't waste one of the shakes.

@ IrC

Some Simple Demonstration Experiments Involving Homopolar Motors

http://www.sbfisica.org.br/rbef/pdf/060904.pdf



Faraday Homopolar motor
i = current ( conventional flow , + => ( - )
ω = angular velocity

This video shows (+) polarity down into the magnet ( the reverse of the
drawing above ) so instead it rotates clockwise viewed from above.
http://www.youtube.com/watch?v=EkU_JmtH3PU

* Note : Corkscrewing the wire around the axis as shown in your video
greatly reduces the torque produced since the rotational force is now only
a partial vector of the principle force acting at right angle to the wire's length.

Rolling Homopolar motor
http://www.youtube.com/watch?v=q-mJl8IdmgI
Laplace Rail , is the same thing reacting to fixed rails powered externally.
http://www.youtube.com/watch?v=_-kQans2rww

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