Magnetic Field Lines
Magnetic field lines are imaginary lines used to represent the direction and strength of the magnetic field; they emerge from the north pole and enter the south pole outside a magnet.
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Properties of field lines
Magnetic field lines are drawn so that the tangent at any point gives the direction of the field at that point. Outside a bar magnet, they emerge from the north pole and enter the south pole; inside the magnet, they go from south to north, forming closed loops. Lines never intersect and field is stronger where they are closer together.
Interaction of two magnets
When two magnets are placed near each other, their field lines distort and combine. Like poles facing each other produce a region of weakened field and neutral points; unlike poles produce stronger field lines connecting N to S between them.
Why do we say magnetic field lines are 'imaginary', yet very useful?