Eddy currents in metal objects © Siemens Stiftung 2016 CC BY-SA 4.0 international

Eddy currents in metal objects

Image

Graphic:
The principle of eddy current brakes.

English, Spanish, German
Image (41.6 kByte)
2018-10-26
If a metallic disc moves through a static magnetic field whose field lines are perpendicular to the direction of movement, voltages are induced in the disc and eddy currents form. According to Lenz's law, the induced current generates a magnetic field to oppose the change in magnetic field that produced the current.
The braking effect is examined at the edges of the static magnetic field. At the position where the disc enters the external magnetic field, the current tries to maintain the state “without the magnetic field”. Therefore, the current is directed such that its induced magnetic field offsets the external magnetic field. The fields oppose each other (in the graphic, the induced field is directed “outward”), resulting in magnetic repulsion. The resulting force counteracts the direction of movement, slowing down the disc.
The situation is reversed at the position where the disc exits the magnetic field. The induced current tries to maintain the state “with the magnetic field”. Therefore, it is directed such that its induced magnetic field strengthens the external magnetic field (in the graphic, the induced field is directed “inward”). This results in magnetic attraction. The resulting force once more counteracts the direction of movement, slowing down the disc again.

Information and ideas:
This principle is also applied to eddy current sorting of residual waste containing aluminum.
Illustration
Physics; Technology
Grade 7 to 9; Grade 10 to 13
Middle/high school
Electricity; Magnetism
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© Siemens Stiftung 2016
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