magnetomotive force (mmf) = 0.4 * pi * number of turns * amps

H field = mmf/ magnetic path length (MPL)

H = (0.4 * pi * number of turns * amps) / MPL

so here we see H field (magnetizing force field) is all about transformer dimension, number of turns and amps

B = permeability * H

and here we see B field (magnetic field) is based on core characteristics (perm) and H intensity..

permeability is the measurement of how much H is needed to make x amount of B

higher perms means the B field intensity increases more with less H field input

EMI filters used in all modern day electronics is a filter used right at the input of raw AC mains power for the electronic device. typically following a emi filter is transient suppression then on to rectification to create around 180v DC

The EMI filter is 2 identical coils on one core.. that has opposite currents.. it is used to filter out imbalanced currents that typically come from radio interference.. the imbalanced current is typically HF and it encounters inductance since the imbalance trys to make a magnetic field the inductance shunts the current (filters the noise).. the balanced current does not experience inductance and only feels the resistance of the copper.. a EMI filter is a perfect example of what happens with opposite currents and what happens with imbalanced currents since its job revolves around that..