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When the direction of current is reversed, the polarity of an electromagnet should be reversed. I am trying to understand why this did not happen in an experiment I conducted today.

An electromagnet was connected to a 12v battery and I verified that it was working. I then touched the electromagnet to a strong regular magnet (rated to hold over 50 kg). There was a strong force between the magnets. I then disconnected the electromagnet from power, reversed the wires, and turned it back on. I expected to feel the electromagnet repel the other magnet, but instead they two magnets got stuck together again. It seems like reversing the wires had no affect on the polarity of the electromagnet.

Here is a picture of the electromagnet I used for reference: enter image description here

And here is the strong regular magnet.

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    $\begingroup$ Some of the commercial picker-upper magnets don't have a single pole at each end, like the nicely-labeled bar and horseshoe magnets you find in science kits, but have a more complicated polarization geometry. This helps them pick stuff up better even if the stuff is partially magnetized. Have you checked with a well-behaved permanent magnet that the poles are where you think they are? $\endgroup$ – rob May 21 '14 at 4:49
  • $\begingroup$ @rob Thank you for the suggestion. I will try that tomorrow. $\endgroup$ – Thorn May 21 '14 at 4:52
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The electromagnet appears to contain a ferromagnetic core. This core is magnetized by the electromagnetic coil. It retains a magnetic field after the coil is turned off, and its field may take some time to reverse after the coil polarity is reversed.

Moreover, if the test magnet is more powerful than the coil (which sounds to be the case), it will cancel the field of the coil completely. You are essentially attaching the ferromagnet to the permanent magnet.

Source: Wikipedia.

You might try again with only a coreless coil. Perhaps try an inductor rather than a dedicated solenoid.

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When one magnet is much stronger than another one, the stronger magnet will overcome the original magnetization (M) of the weaker magnet and attract it even when they were originally opposed.

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  • $\begingroup$ This is not the case for all magnets. $\endgroup$ – Blackbody Blacklight May 22 '14 at 7:17
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The permanent magnet appears to have a ferrous metal sheathing which is attracted to the permanent magnet, and thus cancels out much if not all of the permanent magnets ability to repel when the polarity is reversed. You can try eliminating the sheathing, but that would greatly reduce the power of the permanent magnet along with its ability to repel. Per my discussion with Magnetech a while back.

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