Iron filings can be used to visualize the magnetic field lines of a magnet.

For example, from Wikipedia:

The magnetic field of a bar magnet revealed by iron filings on paper. A sheet of paper is laid on top of a bar magnet and iron filings are sprinkled on it. The needle shaped filings align with their long axis parallel to the magnetic field. They clump together in long strings, showing the direction of the magnetic field lines at each point.

Question: When looking at magnetic field lines formed by iron filings, how can we determine which magnetic pole is North and which is South?

  • 3
    $\begingroup$ I do not understand what you mean by "magnetised field lines". $\endgroup$
    – Farcher
    Jan 4 '19 at 16:47
  • $\begingroup$ Suppose I posted the same picture as you have without the 'N' and 'S' labels, how could you tell if I had turned the picture around? (This is just a hint, I don't have time to write an answer now.) $\endgroup$
    – jacob1729
    Jan 4 '19 at 23:05

We can't. A magnet's North and South poles don't have any particular intrinsic property by themselves, only in how they interact with another North or South pole. If two pole attract, they are opposite, but we can't say which one is which. If two poles repel, they are either both North or both South, but again, we can't tell which it is. We need to compare to against the Earth's magnetic field as a reference. Only then can we identify a magnet's North pole, which is the one that points toward the Earth's magnetic south pole (which is, ironically, located near the Earth's geographic north pole)


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