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?

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    $\begingroup$ 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... $\endgroup$ – Fabian Jan 2 '17 at 17:25
  • $\begingroup$ That's how magnetic fields are observed and detected experimentally. Its their property. There's no why in that. $\endgroup$ – UKH Jan 2 '17 at 18:03

Since monopoles don't exist. What it means is that a north pole can never be extracted independent of a south pole. So independent sources and sinks of lines of force don't exist unlike electrostatics, where a positive charge can be isolated from a negative charge (but a north pole can not be isolated from a south pole). Hence because sources and sinks don't exist, there is no concept of beginning/termination of lines of force (unlike electrostatics wherein lines of force begin in the +ve charge and terminate in the negative charge). The only way a curve can have no beginning or ending is if the curve loops into itself. Hence magnetic lines of forces create closed loops. Hope this helps.

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