I know, there's an overwhelming amount of information on the Internet about what eddy currents are, but on the fundamental front, I simply fail to grasp how currents could circulate/or even exist in a piece of metal. I always thought that in order for current to flow, conductors must be in different potentials. A solid piece of metal (if connected to a power source, say the positive terminal of an ac) would stay at the same potential, with every spot on the metal at an equal potential. I can't imagine how current would flow within a block of conductor metal when all of the metal is a block.
I can't imagine how current would flow within a block of conductor metal when all of the metal is a block.
Imagine a loop of wire, surrounding a region of changing magnetic field. This loop of wire will have a voltage induced around it, and a current will flow through it.
If that region is air, then fill it in with metal. If that region is an iron core, then fuse the conductive loop to the core. Neither changes the conditions in the loop, you will still have current flow in response to the changing field across the inner region.
I always thought that in order for current to flow, conductors must be in different potentials.
This rule depends on a couple of simplifying assumptions that taken together are called the "lumped circuit approximation".
One of these assumptions is that there are no significant changing magnetic fields passing through the circuit. Without this assumption, we can't even actually define a potential at each point in a system.
But there are lots of situations where this assumption isn't valid. For example, in a magnetic generator, the internal working of the generator depends on the assumption being violated, even if we can still analyze the rest of the circuit outside the generator using lumped circuit analysis.
Similarly, eddy currents occur when changing magnetic fields excite currents in the metal. They don't depend on differences in potential because they come from the changing magnetic field, rather than from fields produced by electric charge. And those changing magnetic fields even mean the potential isn't well-defined in the regions where the eddy currents are found.
[This is a great question. Here is my first attempt at an explanation.]
Its all about electric charges. [IMHO the "magnetic fluxes" hides behavior. Perhaps the magnetic_potential would better explain issues. But I'm still learning about that.]
Or changes in charges. Which means changing voltages.
The sea-of-electrons, that is a metal, will move under the electric field of external charges.
As a sinusoidal voltage varies up and down in a wire near the surface of a metal plate, the local density of electrons at surface of the metal will --- slightly --- vary because of the externally applied electric field.
Does this matter? a thin piece of copper foil, your standard PCB foil thickness of 35 microns (1.4 mils) has a propagation delay of 150 nanoseconds, thru the foil from one side of the sheet to the other side.
This delay, 150 nanoSeconds, is about ONE MILLION times slower than light; thus ONE MILLION times slower than charges move along the surface of the metal
These propagation delays become very pronounced for steel and iron, appearing at much lower frequencies (much longer delay times).
If you view the metal as filled with many DELAY regions, then nothing is happening instantaneously, anywhere.
All these delays of charge movement still need some balancing, so there is some requirement for transient-displaced charges to ---- eventually ---- return to their source within the sea-of-electrons.
I'm trying to understand the "eddy current" behavior myself.
I think the need for balancing, despite all the delays within the metal, cause the eddy current behavior.
[thank you for letting me try to answer this question.]