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Why magnetic lines comes from north to south out side of the magnet

is any magnetic lines comes from south to north if so in which direction

What is the reason of magnetic lineS

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4 Answers 4

Making magnetic field lines go from north to south is just a convention. An equally valid convention could have been magnetic field lines going from south to north. The magnetic field lines are a conceptual tool to visualize the magnetic field.

An interesting point about magnetic fields is that they are divergenceless (i.e. $\nabla\cdot\vec{B}=0$). What this means is that the elementary unit that gives a static magnetic field is a dipole (e.g. a bar magnet).

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Not quite (to your second sentence), because magnetic fields have preferred directions. –  Chris Gerig Jun 13 '12 at 19:42
What do you mean? –  PhysGrad Jun 13 '12 at 19:56
They are (pseudo)vectors; their direction points out of N and into S. So you can relabel if you want, but you cannot fix N,S and then choose a convention for the direction. –  Chris Gerig Jun 13 '12 at 20:09
I see. However the last sentence of my answer was referring to that the elementary unit of static magnetic fields, unlike static electric fields whose elementary unit is an electric monopole (i.e. an electric charge), is a magnetic dipole. –  PhysGrad Jun 13 '12 at 20:28

There aren't really magnetic lines - they are maps of the magnetic field. It's just like a contor line map (http://en.wikipedia.org/wiki/Contour_line).

The north end and south end are arbitrary names.

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A picture is worth a thousand words.

enter image description here

Iron filings display the "lines", like small dipoles as @PhysGrad has mentioned. The compasses are larger dipoles and the permanent magnet itself is the largest. One can imagine tiny dipoles following "lines", so in a sense they exist to the accuracy of the experiment.

The image displays the need for a convention , one could call it red and blue poles.

Actually the convention is a bit bizarre it was adopted by watching a compass pointing to earth's north, which makes the magnet in the north pole a south magnetic pole!

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  1. By convention.
  2. If the magnetic line refers to the visualization of B-field, it actually goes from south to north inside a magnet, and north to south outside the magnet. This is because B-field is divergenceless, and consequently magnetic lines are closed loops.
  3. Magnetic lines are just visualization of magnetic field. They are not "real" lines. In fact, magnetic field permeates space, not just where you draw the line.
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