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The field corresponding to a potential \( V = 3x^2 \) is

  • A.\( -6x\,\mathbf{a}_x \)
  • B.\( 6x\,\mathbf{a}_x \)
  • C.\( -3x^2\mathbf{a}_x \)
  • D.\( -6\,\mathbf{a}_x \)

Answer: A

Correct answer: \( -6x\,\mathbf{a}_x \)

\[ \mathbf{E} = -\nabla V = -\frac{dV}{dx}\mathbf{a}_x = -6x\,\mathbf{a}_x. \] The minus sign is not decoration: the field points from high potential to low, that is downhill, while the gradient points uphill.

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