Gauss's law for magnetism is expressed as which statement?

• A. ∮ B · dA = 0
• B. ∮ E · dA = 0
• C. ∮ B · dA = μ0 I_enclosed
• D. ∇ · B = 0

Answer: (A)

Magnetic fields have no beginning or end; magnetic field lines must form closed loops. Because of this, the net magnetic flux through any closed surface is zero. That is Gauss's law for magnetism in integral form: the surface integral of B over a closed surface equals zero. This is equivalent to saying the divergence of B is zero everywhere, ∇ · B = 0, which is the differential form of the same law. The other statements mix up concepts: Gauss’s law for electricity relates the electric flux to the enclosed charge, not necessarily zero, and Ampere’s law involves a line integral around a loop and the current enclosed (with the Maxwell correction adding displacement current).