Are magnetic field lines constant? Here is a photo of metal filings around a bar magnet.  If I was to try this a lot of times around the same magnet, would the lines of filings be identical each time? Or would the field lines change from instance to instance?

 A: The metal filings do not sit 'on' field lines. What does happen is that long thin filings orient themselves along the direction of the magnetic field. If you have enough of them then they join end-to-end because each has a tiny induced field with north and south ends, and N ends attract to S ends.
If you repeated the experiment then the lines of filings would look similar (i.e. follow curves of more or less the same shape), but the actual lines would be in different places.
And if you repeated it with small spherical filings, they may not even line up - more likely would just form random blobs.
In reality, although a magnetic field is often described by the use of 'field lines', in normal situations there are actually no 'lines' - the magnetic field is continuous throughout space, and the lines we draw just mark routes parallel to the local field vector. They are actually no more real than lines of longitude, or contour lines on a map. I say 'in normal circumstances' because in some extreme conditions (for example within the body of a high-temperature superconductor) magnetic flux can concentrate in 'flux lines' - but that is not something you will ever see with iron filings and a bar magnet.
A: Remember that you can calculate with equations the magnetic field in any point of the space and in this case you won't find restrictions in the equations, that means that the magnetic field don`t have some kind of parallel lines, that's just an effect as you can read in the Penguino answer, maybe this website can give you a better mental image.


The website from the previous examples.
