How Do Eddy Currents Work?

by Paul Bright

Lenz's Law

Lenz's Law states that electrical currents near a magnetic field will go in an opposite direction of the field's currents. Inside a conductor, which is anything with moving electrical charges, the currents will swirl in a direction that is perpendicular to the magnetic field. These currents are referred to as Eddy currents.

Opposing Forces

Eddy currents are created when the conductor's magnetic field is exposed to some sort of change, usually when the conductor moves into another magnetic field or when a stationary object enters the conductor's magnetic field. The eddy current will be created in the opposite direction of the field's movement, strengthening or weakening dependent upon the changing distance between the conducting object and the stationary object. The creation also spawns heat and kinetic energy.

Using Eddy Current Resistance

One particular use of eddy currents is with trains. Trains typically travel with a lot of inertia, making them difficult to safely break or halt motion. Eddy currents can be used in the breaking system of trains to create a gradual, smooth stop. A train would expose its metal wheels to a magnetic field, creating eddy currents. The faster the wheels are spinning, the greater the eddy current resistance created; therefore, causing a gradual slow down of the train. This breaking system, also used in roller coasters, is similar to a friction stop.

Metal Detection

Since most metals have different levels of resistance, an eddy current can be used to differentiate between metals. This technology is used in vending machines. Coins that are entered into the coin slot would roll past an eddy current. The current would slow the coin's roll at a speed dependent upon the coin metal's conductivity. That speed is then translated into information that is sent to the vending machine computer to identify the numerical value. Thus, a penny would slow down at a different rate than a nickel, and so forth and so on. This use of eddy currents also can differentiate between real coins and counterfeit ones, since counterfeit coins are often not made of the same resistant material as regular coins.