Maybe this is a simple question for physicists. I was watching this video demonstrating the magnetic field around conducting wires:
https://www.youtube.com/watch?v=V-M07N4a6-Y
And find it difficult to understand why the iron powders do move towards the conducting wire when they align their body to the magnetic field? I was thinking the magnetic force should be in the tangential direction of the Magnetic field line, then where is the origin of this centripetal force that drives the powers move towards the center?
Dear Sheng,
to my opinion the iron particles first arrange themselves in a ring according to the circluar magnetic field which is concentric to the wire (or bundle of wires). This can be clearly seen in the video. But with incresing numbers of iron particles the tangential magnetic forces between them increase due to the inherent µ of the iron. These forces try to shorten the circumference of that ring with the consequence that this ring is forced to shrink in diameter and thus the particles in that ring move gradually towards the center. It is like a stretched circular rubber band whose thickness gradually grows (by what ever means).
Dear Gerhard,
Thank you! That is an excellent analogy you are using! I immediately understand!
Additional remark:
the shrinking of the ring diameter ends when all iron particles in that ring join together without any gap, so that there is no chance for further movement (shortening the circumference) for them in tangential direction (i.e. parallel to magnetic field lines).
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