Magnetic field lines

As with the electric field, we visualise $\vec{B}$ with field lines: curves tangent to the vector $\vec{B}$ at every point, traced in the direction of $\vec{B}$. Where the lines bunch together the field is stronger; where they spread out it is weaker.

Principle — Properties of magnetic field lines

• They leave the North pole and enter the South pole outside the magnet; inside they go from South towards North.
• They are always closed: they never start or end (no monopole for them to start from or finish at).
• The magnitude of $\vec{B}$ is proportional to the density of lines: $|\vec{B}| = \mu_0\,N_\text{lines}/S_\perp$.

The most characteristic property, and the one that clearly distinguishes $\vec{B}$ from $\vec{E}$, is that magnetic lines close back on themselves. Electric lines start on positive charges and end on negative ones; magnetic lines, in the absence of monopoles, have neither beginning nor end and always form closed loops that pass right through the magnet.

Field lines of a magnet: they leave the North pole, re-enter the South pole outside, and close by passing from South to North inside.

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Topics: Magnetismo Concepts: Campo magnetico

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