Suppose I'm using 10 amps average, or 100 amps peak, in an audio system. The 100 amps peak comes from the need to recharge the amplifier energy storage capacitors ---- perhaps 100,000 uF --- at the peaks of the 60Hz sine waves.
The electrician who installed the 10 amp wiring inside the wall of the house was running short of "RETURN" wiring, so he simply ran the "HOT" wire thru the wall behind the audio system, to the power-outlet on the wall, and then used a short piece of RETURN wire back to the circuit-breaker box.
Thus the magnetic field of the HOT wire is not cancelled by the magnetic field of the RETURN wire.
If your vinyl record RIAA preamplifier has a 4" by 4" loop in its STAR ground, and that STAR ground loop is located 8" from the HOT wire in the wall, how much non-hum evil-singing (because of the harmonic-content) is induced into the preamplifier's ground?
Use Vinduce == [MU0 * MUr * Area / (2 * PI * Distance)] / dI/dT
This simplified to 2e-7 * Area/Distance * dI/dT.
Let dI/dT = 100 amps/10 microseconds. What is Vinduce?
Vinduce = 2e-7 * 0.1meter * 0.1 meter/ 0.2 meter * 1e+7 amp/sec
Vinduce = 2e-7 * 0.05 * 1e+7 = 0.1 volt.
All because the fluxes did not cancel.