# Why are magnetic field lines closed? [duplicate]

If there were two charges placed at a large distance, won't their (say) magnetic fields interact? What if that large distance is something like close to infinity? Even if they experience a feeble force from each other, doesn't that imply that the magnetic field lines are not closed, as the particle could have been placed anywhere around the source charge at any distance?

Would not this generally say that every electrically charged particle in the universe is part of an infinitely large magnetic field?

• Are you aware that there are no magnetic monopoles? So there are no magnetic charges and field lines can only be produced by magnetic dipoles... Jan 2, 2017 at 17:25
• That's how magnetic fields are observed and detected experimentally. Its their property. There's no why in that.
– UKH
Jan 2, 2017 at 18:03

• @VincentThacker: Absolutely, but what needs to happen is that, it needs to curl such that for any path, $\int_C \vec{F} \cdot \mathrm{d}\vec{r} \neq 0$, else $\vec{F} = \nabla \, f$ and the magnetic vector potential $\vec{F} : \vec{B} = \nabla \times \vec{F}$ will be a conservative field with an embedded scalar potential. Any helical shape of arrow quivers which would revolve its life over to infinity with $\nabla \times \vec{F} \neq \vec{0}$ should work. This closed lines pedagogy is a simplistic one where we can avoid talking about curls and divergences. Oct 10, 2021 at 13:33