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The curl of a gradient, \( \nabla \times \nabla\phi \), is

  • A.always a non-zero vector for a non-constant field
  • B.equal to the Laplacian of the field
  • C.equal to the gradient of the divergence
  • D.always the zero vector

Answer: D

Writing out any component gives a difference of mixed second derivatives, such as \( \phi_{zy} - \phi_{yz} \), which vanishes whenever those derivatives are continuous. So

\[ \nabla \times \nabla\phi \;=\; \mathbf{0}. \]

This is why every gradient field is irrotational, and conversely why an irrotational field can be written as a gradient

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