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For \( \mathbf{F} = x\mathbf{i} + y\mathbf{j} + z\mathbf{k} \), the flux out of any closed surface enclosing volume \( V \) is

  • A.\( 3V \)
  • B.\( V \)
  • C.\( V/3 \)
  • D.zero for every such surface

Answer: A

The divergence is constant:

\[ \nabla \cdot \mathbf{F} \;=\; 1 + 1 + 1 \;=\; 3, \]

so the divergence theorem gives \( \iiint 3\,dV = 3V \) whatever the shape of the surface. For a sphere of radius \( a \) this is \( 3 \times \tfrac{4}{3}\pi a^{3} = 4\pi a^{3} \), matching the direct calculation

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