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The divergence of a curl, \( \nabla \cdot (\nabla \times \mathbf{A}) \), is

  • A.the gradient of the divergence of the field
  • B.non-zero wherever the curl is non-zero
  • C.always zero
  • D.always equal to the Laplacian of the field

Answer: C

Expanding gives six mixed third derivatives that cancel in pairs, so

\[ \nabla \cdot (\nabla \times \mathbf{A}) \;=\; 0. \]

Every curl field is therefore solenoidal, which is why a solenoidal field can be represented by a vector potential and why magnetic flux density, having zero divergence, is written as the curl of one

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