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The vector \( \nabla\phi \) at a point of the surface \( \phi = \) constant is

  • A.of magnitude equal to the value of the constant
  • B.normal to the surface there
  • C.tangent to the surface there
  • D.zero, the field being constant on the surface

Answer: B

Moving along the surface leaves \( \phi \) unchanged, so the directional derivative in every tangential direction vanishes and the gradient is perpendicular to all of them. That is what makes the gradient the natural way to find a normal, and hence a tangent plane, to a surface given implicitly

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