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.

  • \$\begingroup\$ "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." But does it really? What do you think an antenna is? Think about how you can charge up a conductor with static, then think about what AC is doing to your metal block at the close end and the far end whenever the amplitude or polarity changes. \$\endgroup\$
    – DKNguyen
    Jun 10 '20 at 0:45
  • 1
    \$\begingroup\$ Tea can flow in your teacup with a circular motion without a pressure difference driving it. \$\endgroup\$
    – Chu
    Jun 10 '20 at 0:51
  • \$\begingroup\$ I can understand the tea in a teacup, as there's a path - the inner surface of the cup and the center kind of is ignored. But a conductor metal block, it has the same homogeneous conductivity everywhere. \$\endgroup\$ Jun 10 '20 at 0:56
  • \$\begingroup\$ Watch magnetic braking caused by eddy currents: youtube.com/watch?v=M856bqqbZcM \$\endgroup\$ Jun 10 '20 at 0:59
  • \$\begingroup\$ There are an infinite number of possible circular current paths within a block of conductor. \$\endgroup\$
    – Chu
    Jun 10 '20 at 1:23

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.

  • \$\begingroup\$ This was, by far, the best answer that I received so far. Thanks, I think I understand it now. This clears some of my doubts on "skin effect" as well, where the path of current flow is on the periphery of the conductor and no current flows in the center despite the conductor being a solid block of material. \$\endgroup\$ Jun 10 '20 at 17:32
  • \$\begingroup\$ Too add, once this conducting wire loop is fused to the core, an infinite number of new conductor loops suddenly appear and currents flow in all those loops simultaneously, right? \$\endgroup\$ Jun 10 '20 at 17:43

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.

  • \$\begingroup\$ Thanks for the answer. I know I'm too naive (or perhaps haven't studied enough). I normally visualize current as something that flows in a path provided by a conductor. In a block of metal, since the whole metal is solid block where's the path, or how does the current choose its path of flow. And, weirdly, how do we measure such currents? Where do we connect our ammeter? \$\endgroup\$ Jun 10 '20 at 0:53
  • \$\begingroup\$ The current is flowing in every possible path at once, basically. \$\endgroup\$
    – Hearth
    Jun 10 '20 at 1:05
  • \$\begingroup\$ @Hearth, that what's confusing me from the beginning, on what basis is that current path chosen, since all of the material is conductive. \$\endgroup\$ Jun 10 '20 at 1:23
  • \$\begingroup\$ @BhanuNepal No current path is chosen--all paths are taken. \$\endgroup\$
    – Hearth
    Jun 10 '20 at 1:36
  • \$\begingroup\$ Which is also true when the current is driven by an electric potential difference. If you connect two wires together to form a branching circuit, current can and will flow in both wires, it doesn't just pick one. And microscopically, it flows more or less evenly through the whole cross-section of each wire, not just in a thin line down the center of the wire. \$\endgroup\$
    – The Photon
    Jun 10 '20 at 1:48

[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.]

  • 2
    \$\begingroup\$ This answer can't possibly help someone with OP's level of understanding. \$\endgroup\$
    – The Photon
    Jun 10 '20 at 1:46
  • \$\begingroup\$ @ThePhoton Well, the propagation delay stuff helped me. Heh. \$\endgroup\$
    – DKNguyen
    Jun 10 '20 at 2:42
  • \$\begingroup\$ Well, I didn’t realize that this was an interesting question. I was actually ashamed to ask it here, but it drove me nuts when I tried to imagine how current would flow in a solid block of metal which is held at a constant potential; my notion of current flow was that it required ends to flow from and to, and a block of metal simply has no terminals. \$\endgroup\$ Jun 10 '20 at 3:09
  • \$\begingroup\$ @analogsystemsrf you mentioned “return to source”, and that confuses me further. The source of electrons is the metal block itself, and it doesn’t need to return anywhere at all. \$\endgroup\$ Jun 10 '20 at 3:14
  • \$\begingroup\$ @BhanuNepal Stop thinking of the block as one perfectly uniform, homogenous piece of material \$\endgroup\$
    – DKNguyen
    Jun 10 '20 at 3:23

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