Current in the primary coil generates a magnetic flux around the coil (affecting the low reluctance magnetic core, and also the secondary coil).

Knowing that the emf (or current) is induced in the 2nd coil is in antiphase with the current in the primary coil, my question is why isnt the emf/current induced in the magnetic core in antiphase with the one in the primary coil?

If it is induced by a smaller than proportional amount than that in the secondary coil, that is the eddy current; why is it in phase with the current in the primary coil while that current induced in the secondary coil is in antiphase with the primary coil?

enter image description here

  • $\begingroup$ What CURRENT is in the core? $\endgroup$ May 6, 2017 at 13:49
  • $\begingroup$ What is "reluctance"? $\endgroup$
    – DanielSank
    May 6, 2017 at 14:02
  • $\begingroup$ "Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating electric energy it stores magnetic energy. In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance. It is a scalar, extensive quantity, akin to electrical resistance. The unit for magnetic reluctance is inverse henry, H−1." $\endgroup$
    – Shawn.T.
    May 6, 2017 at 14:24
  • $\begingroup$ (From wikipedia) $\endgroup$
    – Shawn.T.
    May 6, 2017 at 14:25

1 Answer 1


There are two things going on here:

  1. First, the phase vs anti-phase between the two coils is somewhat arbitrary, as it depends on which way the coils were wound and which terminal you call "+". In your figure, the primary is wound around the field lines left-handed while the secondary is wound right handed: That's a relative minus sign compared to if they'd been wound the same way. Similarly, you can call either the top or bottom of the windings the "plus" end. (Transformers are often marked with a dot to indicate which one to call plus to get standard phasing)
  2. But more importantly, you've made an assumption about current that's not true: The current in the primary does not determine the current in the secondary. (Consider a secondary that's an open circuit, has various resistors on it, or a short circuit: For the same primary current, those will all have different secondary currents) The (changing) primary current just induces a voltage in the secondary; what happens in response to that is given by the circuit connected to the secondary coil.

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