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The divergence of \( \mathbf{A} = \rho \, \mathbf{a}_\rho \) in cylindrical coordinates is

  • A.1
  • B.\( \rho \)
  • C.0
  • D.2

Answer: D

Correct answer: 2

\[ \nabla\cdot\mathbf{A} = \frac{1}{\rho}\frac{\partial}{\partial\rho}(\rho A_\rho) = \frac{1}{\rho}\frac{\partial}{\partial\rho}(\rho^2) = \frac{2\rho}{\rho} = 2. \] The \( \rho \) must go inside the derivative; differentiating \( A_\rho \) alone would give 1, which is the offered trap.

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