Practice portal › Magnetism and Matter › Bar Magnet and Dipole in a Field
Asked in GSEB Board July 2021 · Field of a bar magnet and earth's field
Idea: magnetic field lines have no beginning and no end, because there are no magnetic monopoles. Every line that enters a closed surface must leave it again somewhere.
Adding the contributions over the surface, the inward and outward fluxes cancel exactly: ∮⃗B· d⃗A=0.
Compare electricity, where lines start and stop on charges and ∮⃗E· d⃗A=q/(ε₀). There is no magnetic analogue of an isolated charge -- cut a magnet and you get two smaller magnets, never a lone pole.
So the net magnetic flux through any closed surface is zero.
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