Parasitic magnetic coupling "induces a certain voltage" across a trace?

[incf]

This came up in another thread.

Suppose a PCB has a 5V power supply rail. And it makes one "loop" near or around a trace carrying a 10A 100kHz signal (part of a SMPS).

One commenter said something like [1] "the alternating magnetic field will induce a current in the trace carrying the 5V rail" and another corrected that remark by saying something to the effect that "it will induce a fixed/predetermined voltage across the loop/trace and the current will be whatever circuit impedance limits it to"

• Question 1: is this true for parasitic/air core transformers "that they really are ideal transformers" in that they don't saturate or appear to have the transformer equivalent of ESR.
  
• ie. that the amount of energy being coupled/the induced current is limited purely by the impedance of loop.
• Question 2: Is it possible to estimate the (order of magnitude) amount of EMF induced by a 10A 100kHz trace into a 1 turn loop accidentally formed on a 1.6mm thick PCB? While I vaguely remember some equations I lack intuition.
• I imagine it's probably only microvolts.

Any trace that crosses at a right angle with a high current trace seems like it may be subject to some amount of magnetic coupling. Even on a 4 layer PCB where the middle two layers are ground and the high current trace is on the top, and the signal trace on the bottom.



edit: [1] original quotes:

There is always a loop, right?

Between any signal and the ground plane is a loop. (a small one)

Yes, any area in which the magnetic field can get into will induce a current, which in turn will induce a voltage depending on the impedance. So the shorter the trace the smaller this area.
In your case I think capacitive coupling will be worse. You can measure with a LCR meter the capacitance between your analog line and coil, then run a simulation to see what the result will be, 100kohm is usually too high for analog signals, enough to deserve a coaxial RG174 cable by itself.

AC magnetic flux induces an EMF (voltage) around a loop.  That voltage will produce a current through the loop equal to EMF/(total impedance), and that current will produce voltages across each impedance in the loop according to their individual values, adding up to the total EMF.