Gravity is Optional
Eddy currents - Explanation

Any time a magnet moves near a conductor, it causes a current to flow in the conductor.

Any time a current flows in a conductor, it creates a magnetic field.

This is how all motors (and generators) work - moving a magnet near a wire produces current, putting current through a wire near a magnet causes either the wire or the magnet to move (depending on the design). In either case, the "magnet" can be replaced with an electromagnet. It does not matter, it is the moving magnetic field that causes the effect.

In "normal" circuits, the current flows away from the generator, through some wires, and eventually back to the generator. This is how the power plant makes the electricity you use in your house.

However, for most of the experiments in this section, the current stays inside the sheet of aluminum, or copper pipe, where it was generated. These currents are referred to as eddy currents .. because they flow in small circles (eddies) inside the conductor rather than in an external circuit. (In the case of the pipe, it flows around the pipe. That is, in the pipe around the falling magnet.)

In many applications, great efforts are expended to reduce eddy currents. In particular, the iron cores of transformers and motors are made of many iron plates which are electrically isolated from each other to reduce these currents. In other cases, the cores are made of compressed iron particles. (This is common in ferrite tuning cores used in inductors, TV tuners, and the like.)

In most applications, eddy currents simply consume energy that could be used for something else. They also make things hotter.

However, there are a number of products that use eddy currents to perform useful work. The most common application is the AC synchronous motor (squirrel cage motor). All the old electric clocks contained these. A similar application is the old mechanical power meter found outside most houses. These contained a large aluminum disk that was driven by eddy currents. Some stoves use eddy currents to cook. These can be recognized by having a glass top and the fact that they remain cool unless there is a pot on the "burner".

Eddy currents are also used as zero friction brakes .. typically in laboratory scales.


Ok, you don't believe me. I really did create a force field. (Hey, I said so, so it must be true.)

Well, there are several ways to prove that eddy currents exist. The simplest is to stop them. In the videos, you can see that one of the copper tubes has a slit on the side. If a current was flowing around the pipe, then this slit would stop it. And, in fact, you can feel the magnet move faster when it passes the slit.

Many people suggest that there is a sudden change in air pressure as the magnet passes the slit. It is easy to prove that that is not what is happening .. simply drop another, non-magnetic, object thru the tube. (I use a battery.)

Well, a ring is simply a slice of a pipe. Cutting through one side of a ring is similar to putting a slot in a pipe. It is not possible to drop a ring over a magnet because of "a force field". However, when a ring is cut on one side, it will drop over the magnet .. proving that eddy currents exist.

When I connect a current meter (milliamp meter) across the gap and pull the magnet thru the center, the needle moves, proving that moving a magnet thru a conductor (ring / tube) produces a current.

Thus, slots, and amp meters, prove that eddy currents are produced in these experiments. The fact that it is possible to drop a ring with a gap over a magnet, but not possible for a ring without a gap, proves that the force is produced by those same eddy currents.

The real mystery is why some eddy currents appear to cause the aluminum to be pushed away, and others appear to attract it. In fact, when the magnet is moved toward a piece of aluminum, it will always push it away. Yet, when the same magnet is pulled away from the same piece of aluminum, the aluminum will move in the same direction as the magnet. One action appears to generate a force field and the other a tractor beam.

What is happening is that when the magnet is moving toward the aluminum, the eddy currents move in one direction. However, when the magnet moves away from the object, the eddy currents move in the opposite direction. In both cases, the eddy currents produce a magnetic field oriented such that it resists any change in distance between the magnet and the conductor (aluminum). Thus, the conductor will always move in the same direction as the magnet.

Since real current is flowing, there will be IR (heating) losses. This is what allows an inductive stove to heat a pan. This is also how the magnet stops things from moving. (It simply converts the mechanical energy into heat.)

When some metals are cooled enough, they become superconductors. In simple terms, this means that IR losses go to zero. As a result, when a magnet is placed over a superconductor, it simply floats. The eddy currents in the superconductor create a magnetic field that exactly balances the field from the magnet.

Author: Robert Clemenzi
URL: http:// / EddyCurrents / Explanation.html