Why is the magnetic flux through a closed surface always zero?

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The magnetic flux through any closed surface is always zero because there are no magnetic monopoles—isolated magnetic charges—that can act as sources or sinks for a magnetic field.12

In contrast to electric fields, where electric charges serve as starting or ending points for field lines, magnetic field lines are continuous and form closed loops. Because every magnetic field line that enters a closed surface must also exit it, the net magnetic flux is zero. This principle is formally expressed as Gauss’s law for magnetism, which is a fundamental component of Maxwell’s equations in electromagnetic theory.341