I have an electromagnet (turned off) with an iron core in front of a permanent magnet. At this stage they attract each other. Will any current do if I want them to repel each other, or is there a formula for calculating how much current is required to set the polarity of the electromagnet?


closed as unclear what you're asking by Bence Kaulics, PeterJ, rdtsc, uint128_t, Dmitry Grigoryev Jun 20 '16 at 9:19

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  • \$\begingroup\$ Of course "any current" won't do, you need to overcome the magnetic field of the permanent magnet inside the core. \$\endgroup\$ – Dmitry Grigoryev Jun 20 '16 at 9:19

Not just any current will do to make them repel. The current must be larger than a certain minimum.

As you have correctly noticed, the core and the permanent magnet (PM) are attracted when the current is zero. This means the field from the PM is creating a field in the iron core.

As you increase the current slightly above zero, in opposition to the PM, there will still be sufficient field strength from the PM to magnetise the iron core, just weaker than before. The core and PM will still be attracted.

As you increase the current further, at some point the magnetic field from the coils will exactly oppose that of the PM, and drive the core's field down to zero.

Further increases in the coil current will now magnetise the core in opposition to the PM, which will be repelled.

While there is a formula that is able to calculate how much current is needed to oppose the field of the PM, it not only involves the strength of the PM, but also the spacing to the core, and the geometry of the air space between the core and the PM. Calculating from first principles would be so intractable that the simplest route would be to hang the PM from a string, and measure its deflection as you change the current.

  • \$\begingroup\$ I'm not saying you have this wrong, just wondering why in a now deleted answer (which also wasn't wrong) you say "This does not answer the question. Add the calculations to show how the amper turns of the electromagent ..." and then later say "I am well enough qualified to know that I do not want to attempt an answer without being able to be helpful by putting some numbers on the hand-waving, ... " and then you do just that whilst causing an answer that isn't really all that substantively lacking from yours to be removed. \$\endgroup\$ – placeholder Jun 18 '16 at 20:37
  • \$\begingroup\$ @placeholder Thanks for your interest. My answer is not ideal in my terms, as I don't provide a formula. It is however better than the other one, as it points out that the field in the iron core is a function of both the field of the PM and the field from the coils. I explicitly point out that the system will still attract even with a small current flowing, whereas the other answer implies that the iron polarity is dependent on the current flowing alone, which would theoretically repel for any current. The OP was specifically asking about 'any current' which I have answered. \$\endgroup\$ – Neil_UK Jun 19 '16 at 6:22
  • \$\begingroup\$ Thank you, exactly what I was looking for, really appreciate your effort to answer my clumsy question \$\endgroup\$ – cxn Jun 20 '16 at 15:56

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