I can't understand the polarity of an induced emf of an inductor from a bar magnet. I understand inductor voltage polarities in normal closed circuits in that an inductor produces a voltage drop when current increases since di/dt is positive. Then when the circuit opens, di/dt becomes negative which flips the polarity of the voltage. But I can't understand the polarity in an open loop. Consider the following scenario. enter image description here

So clearly the current will run up the coil to create a field which opposes the incoming north pole (Obviously no current actually flows). From what I've read online we must use the induced EMF formula since considering the rate of change of current doesn't really work since it's an open loop. Nevertheless, I took the bottom of the coil to be ground. Then by the EMF formula it is clear that the EMF will be negative, let's say it's -9V. So this means the voltage at the top of the coil is 9V below ground thus meaning that the current is flowing from + to -. But from what I've seen online the current should flow from - to + or in other words from low potential to high potential. So what's wrong with my interpretation?

  • \$\begingroup\$ Why do you keep mentioning current; there is none in your scenario; induction is the key word and what direction you wind your coil affects polarity. \$\endgroup\$
    – Andy aka
    Nov 8, 2022 at 10:13
  • \$\begingroup\$ Sorry for the confusion. I just want to know how to determine which side of the coil as a higher potential with respect to the other side and why this is the case. In my diagram the top side of the coil is at a lower potential than the bottom of the coil but we would expect it to be the other way around wouldn't we? \$\endgroup\$ Nov 8, 2022 at 11:09
  • \$\begingroup\$ It depends on whether you wind clockwise or anti-clockwise from the top to the bottom. \$\endgroup\$
    – Andy aka
    Nov 8, 2022 at 11:16
  • \$\begingroup\$ Ok. Let's say it's wound anti-clockwise from top to bottom, what would the polarity be? \$\endgroup\$ Nov 8, 2022 at 11:27
  • \$\begingroup\$ Is that your actual question in a nutshell? \$\endgroup\$
    – Andy aka
    Nov 8, 2022 at 11:41

1 Answer 1


Try this image from here: -

enter image description here

The image is explaining two things; polarity of induced emf and the effect of Lenz's law. The yellow/orange N and S are to do with Lenz's law in the case that a current was flowing due to the induced emf.

The important thing to note is the induced voltage polarity when the north of the magnet is moving towards the coil. In the case above it is positive but, if you wound the coil in the opposite direction it would be negative.


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