It's a Thurs-Demo Twofer! (I thought I had blogged about this before, but I guess not.) I got the original Thurs-Demo post done early, so I figured that I could toss another one out there>
If there's one thing I like for demonstrations, it's Neodymium magnets. If there are two things I like, there is also liquid Nitrogen (LN2). If there are three things, then how about silver coins? Put them together and you've got something special.
The discovery of the connection between electricity and magnetism was pretty impressive bit of 19th century science. That a magnet, moving past an electrical conductor will induce an electric current is one of the founding concepts for our technological society. But there are some other connections between electricity and magnetism that some of you might not be aware of. Eddy currents, for example.
An eddy current occurs as a result of change in magnetic flux through an object. Basically, if you place a magnet next to a conductive material (like copper, aluminum, or silver) and keep it there, not much happens. But move the magnet - and weird things happen. If you are holding a very strong magnet and slide it across the surface of a large piece of copper or aluminum (or silver, if you're rich), something "pushes back" against the movement of the magnet. It feels like you're trying to move the magnet through syrup. What's happening is that by moving the magnet, you are changing the number of magnetic field lines going through the material - a change in magnetic flux. It just so happens that this changing magnetic flux causes little tiny electric currents in the conductive material. And these little tiny electric currents develop their own magnetic force - forces that directly oppose the movement of primary magnetic field. The original motion (movement is the result of a force acting on an object) is opposed by another force (the eddy current's magnetic field) thus it becomes harder to move the magnet across the surface of the conductor.
Or, if you have a bank of magnets, you can slow the movement of the conducting material past them. This is how some trains and roller coasters control their speed. The speeding axle, or car moves past a bank of strong magnets. The generated eddy currents create an opposing magnetic force and slow the train/coaster down. All without physically touching anything. Rather spiffy. And if you're wondering how the coin slot on the vending machine can detect whether or not you've given it a genuine American quarter - the bank of magnets in the coin slot slow the quarter down differently compared to a Canadian quarter (Canadian quarters, by the way, are magnetic).
One more thing - the better the conductor, the greater the effect. Cut little holes in the conductor - eddy currents are not as strong, because they don't get very large. Use a weaker magnet - smaller eddy currents. Thick, conductive metals and strong magnets - big eddy currents. Of course, you can use some liquid Nitrogen to super-cool the metal and make it even more conductive....
Roll the clip!