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True or false: the flux of a field through a closed surface must vanish if the field is solenoidal.

  • A.False, the flux depending on the shape of the surface
  • B.False, solenoidal fields having no flux defined
  • C.True, but only for surfaces enclosing the origin
  • D.True, the divergence theorem turning the flux into an integral of zero

Answer: D

Solenoidal means \( \nabla \cdot \mathbf{F} = 0 \), so the divergence theorem gives

\[ \oiint_{S} \mathbf{F} \cdot \mathbf{n}\,dS \;=\; \iiint_{V} 0\,dV \;=\; 0 \]

for every closed surface in the region, whatever its shape or position. This is what \"source-free\" means in integral terms

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