Experiment 4: Eddy Currents

As many of you know I used to be an aeronautic engineer. I helped design holographic inspections for the rockets for NASA and I also worked with Eddy Currents, used as a non-destructive testing tool. You see, there are often microscopic scratches on the surfaces of jet engine parts.the vibrational forces exerted on an engine in use, and over time, can cause these scratches to propagate into actual cracks. In order to prevent catastrophe, engine makers use ultrasound and eddy currents to detect these microscopic cracks, so they can be sanded away. Eddy currents are very cool. Anyone with a strong magnet can see them in action. So I thought I'd include an experiment that demonstrates this phenomena - which is great for kids of all ages!

What you'll need:

•   A strong magnet
• A section of copper pipe
• A section of some other material pipe (or even a toilet paper/paper towel roll)

Process:

1. Stand thepipes up on end, or your student can hold them in their hand, vertical.
2. hold the magnet over the opening on one end of the non-copper tubing
3. let the magnet drop, it will fall right down at the forces of gravity
4. now hold the magnet over the opening at one end of the copper tubing
5.  let the magnet go, watch what happens!

Understanding the experiment:
The magnet will float in the copper tube. This is the result of a specific form of Electro-Magnetic force called the Eddy current. As you know, copper is a conductor of electricity. When the magnetic field interacts with the surface of the tube, the metal begins to generate its own magnetic field/current. This was discovered by a scientist by the name of Michael Faraday. The magnetic field created in the metal opposes the one surrounding the magnet. What happens when the same polarized ends of 2 magnets interact? They push eachother away. The same is true here. The magnetic field generated in the pipe is calle the Eddy Current. This all falls within the Physics law called Lenz's Law. Older kids can research Michael Faraday and the Russian physicist Heinrich Lenz. For another simple experiment demonstrating Lenz's Law, check out the Swinging Magnet experiment http://www.ndt-ed.org/TeachingResources/NDT_Tips/LenzLaw.htm