# Eddy current confusion

Every source tells that eddy currents are produced by change in magnetic flux in conductor, but according Gauss' law for magnetism net flux over a closed surface is 0, then how magnetic flux will change to produce eddy current?

• maybe you quote one of the many sources, since I do not understand you question – trula Sep 19 '20 at 13:40
• The strength or intensity of the field can change while maintaining Gauss' law. For example, suppose the field intensity changes uniformly everywhere. – garyp Sep 20 '20 at 15:05

Assume you have a coil winded along z axis and a large metal plate put in xy plane. Then applying AC voltage across the terminals of coil eddy current induced on the metal but in this case charges are moving and using Gauss' Law considering the change in the magnetic apparently flux changes. With another setup, for instance a metal disk is again in xy plane rotates under uniform magnetic field toward $$\hat{k}$$ direction and magnetic field covers small portion of the disk. In this case to visualize the eddy current, consider atoms as hard spheres and when the disk rotates each atom with their charges rotates as well but under the magnetic field atom's position barely changes conversely to the regime which goes under the magnetic field changes for small time difference. Basically, for the induced current on disk depends on the scanned area change by magnetic field. So, applying Gauss' Law may be hard but it works (even for moving charges).This link may help you to understand better with it's pictorial explanations.

• Notice that except for two very short sentences at the end, your answer is one long sentence. That makes it hard to follow. You might break it up into several sentences where each one expresses one thought or idea. That would be easier to read. – garyp Sep 20 '20 at 15:00
• @garyp thanks for the warning I hope it is better now. – asd.123 Sep 20 '20 at 15:11

Gauss' law for magnetism net flux over a closed surface is 0.

The polarity of static fluxes balances out to 0.

Static magnetic fields do not create eddy current losses because of the lack of motion of charges. You either need a moving magnetic material or a dynamic/alternating current or both.

The external flux is reduced by the amount of eddy currents and a drag force and heat loss occurs. The net flux is still zero even though the external flux on either side is weaker from the circulating eddy currents on the edges.

In a water stream the eddy currents on both edges of a stream reduce the overall flow rate, however the net water stored =0 as output = input.

Eddy current losses increase with the square of charge velocity which can be excited by magnet strength or AC coil voltage or frequency or any combination of these and other factors such as laminate thickness.