Neodymium Magnets
Eddy Currents

According to Wikipedia.org,  the free encyclopedia, an eddy current (also known as Foucault current) is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. It is caused when a moving (or changing) magnetic field intersects a conductor, or vice-versa. The relative motion causes a circulating flow of electrons, or current, within the conductor. These circulating eddies of current create electromagnets with magnetic fields that oppose the effect of the applied magnetic field (see Lenz's law). The stronger the applied magnetic field, or greater the electrical conductivity of the conductor, or greater the relative velocity of motion, the greater the currents developed and the greater the opposing field. http://en.wikipedia.org/wiki/Eddy_current

Copper, Brass and Aluminum are the most common materials that generate Eddy Currents when reacting to the presence of a moving strong magnetic field.

Eddy Currents Using My Bracelet

Added June 25-08

I'm using diametrically magnetized 1/4" dia by 1/2" cylinders and copper and aluminum tubes to generate Eddy Currents. With diametrically magnetized cylinders the North and South Poles are on the side of the cylinder as opposed to the flat ends.

Eddy Currents Stop a Rolling Sphere

Added June 25-08

I have been granted scavenging privileges at my local scrap yard. When I came across a 1" thick, 32 lb slab of Aluminum I bought it knowing I could dream up some form of Eddy Current experiments using it. Well, here is the first one.

Eddy Currents Generated by Really Powerful NIBs

Added May 25-08

In the Metal Recycling Industry, formerly known as junk yards, Eddy Currents are used via conveyor belt to separate copper, brass and aluminum from plastics and paper. I setup a platform topped with 1/4" plate glass. From below the glass I am able to move some sizable pieces of copper and aluminum above the glass quite easily. The Neodymium Magnet is a 2" by 2" by 1" N42 Block with a pull force of over 320 lbs.

The Slow Drop

The simplest Eddy Current experiment is preformed by dropping a Neodymium Magnet down a vertical copper or aluminum pipe. In stores like Home Depot and Lowe's Home Improvement you can get 1/2" or 3/4" copper pipe in lengths as short as 2 feet. Both have interior diameters slightly larger then their stated size so a 1/2" or 3/4" ball or cylinder will fall cleanly. Check to see that the ends weren't pinched in when it was cut. If it was file or sand it correctly.

A ball will fall faster than a cylinder. When I demonstrate this experiment I typically use a 1/2" pipe and drop a 1/2" steel ball first. Of coarse it drops like a rock. Then the Neodymium Sphere followed by the 1/2" NIB Cylinder.

The steel ball drops really fast. The 1/2" NIB Sphere takes about 4 seconds to fall a foot and the cylinder takes over 6 seconds. Another factor that influence the timing of the drop is the proximity of the magnet to the tube wall and the thickness of the tube wall. A NIB will fall slower through a thick wall tube than a thin one. Although hard to find, a schedule 40 or schedule 80 (thick walled copper pipes) work much better than your typical Type "L" or Type "M" plumbing pipes. Of coarse, they are also more expensive.

Eddy Current Freefall 2

People are totally amazed when I show them this one. This  demonstration uses a 1" dia by 1/2" thick Neodymium Disk, a couple of 2" wide by 1/8" thick, 18" long Aluminum bars, aluminum Binding Screw Posts and nylon tubing for a separator. I bought the aluminum parts at one of my local Home Depots. The fact that the viewer can closely watch this rather heavy magnet actually floating leaves them awestruck. Check out the video. 

Eddy Current Braking

This is a simple experiment designed to show how Eddy Currents can drastically slow-down a falling 1" dia by 1/8" thick Neodymium Magnet. The Aluminum Bar is 1 inch thick.

Eddy Current Braking Pendulum

Here I mounted a 1" dia by 1/2" thick NIB in one end of the pendulum and a skateboard ball bearing on the other. The Aluminum plate is 3/8" thick.

Eddy Current Gear

For this device I first fashioned an aluminum disk 4" in diameter and 3/8" thick. I drilled five 1/2" holes, 1 in the center and 4 equally spaced 0.35" from the edge. The center hole is for a Mandrel 1/4 Inch Shank Wheel Arbor. The other holes are for 1/2" dia by 1/2" thick N50 Neodymium disks. I super glued these in place with poles alternating N- S- N- S. Then I made a 7 1/2" dia by 1/10" thick  brass disk spinning top. I set it up on my drill press and fired it up.

Neither the Aluminum nor the Neodymium ever touches the brass. I mounted a mirror behind the rig to show the clearance of the parts.

Eddy Current Freefall 3

My latest Eddy Current Free Fall Drop Device is a 6 footer that I designed to work with my Big Boys. In this video I am dropping a 17.34 ounce 2" by 2" by 1" thick Neodymium Magnet Block with a pull force of around 325 pounds. The aluminum extrusions are separated by 1 1/8". 

 Earlier I dropped a 2" dia by 1" thick disk super magnet with a pull force of over 230 pounds. Dropping the 13.6 oz. (386 g) NIB between the vertical aluminum surfaces took about 8.5 seconds. As usual, my weird mind wondered what it would be like if it just fell for 8 and a half seconds in freefall.

Let's do some math. If I dropped it off a cliff for 8.5 seconds......

Seconds ... rate of fall ..... Distance traveled

1 ..........     32.15 ft/s ..........     32.15 ft
2 ..........     64.30 ft/s ..........     96.45 ft
3 ..........     96.45 ft/s ..........     192.90 ft
4 .........     128.60 ft/s ..........    321.50 ft
5 .........     160.75 ft/s ..........    482.25 ft
6 .........     192.90 ft/s ..........    675.15 ft
7 .........     225.05 ft/s ..........    900.20 ft
8 .........     257.20 ft/s ........     1157.40 ft
8.5 .......    273.27 ft/s ........     1430.67 ft         83.294 m/s     436.08 meters  
9 .........     289.35 ft/s .........    1446.75 ft
10 ........    321.50 ft/s .........    1768.25 ft
11 ........    353.65 ft/s .........    2121.90 ft
12 ........    385.80 ft/s .........    2507.70 ft

The 2" NIB fell at a rate of 8 1/2 Inches per second. If it was dropped off a cliff it would have fallen over 1/4 mile and be falling at 186 mph after 8 1/2 seconds.

Let's do a little more math. If the 0.386 kilogram magnet falling off a 436.08 meter high cliff would impact the ground (rock) below with a force of 1,650 Joules. This converts to 1,218 foot-pounds of energy. Instead it impacts the floor at a steady  0.2159 meters per second generating 0.0066 foot-pounds of force. 

By comparison, lets use a 44 magnum (Dirty Harry's powerful handgun) bullet of 210 grains exiting a muzzle with a velocity of 1300 fps. The energy of this bullet at the muzzle is 788 foot-pounds or 65% of the energy of the falling magnet. The magnet's impact energy is 1.55 times the bullet's. Enough of being a nerd...