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Under the Lorentz gauge condition, the electric potential V, is related to magnetic vector potential \(\vec{A}\) as :

  • A.\(\vec{\nabla} \cdot \vec{A}\) \(+ (1/\mu_{0}\varepsilon_{0}) \frac{\partial V}{\partial t} = 0\)
  • B.\(\vec{\nabla} \cdot \vec{A}\) \(+ \mu_{0}\varepsilon_{0} \frac{\partial V}{\partial t} = 0\)
  • C.\(-\vec{\nabla} \cdot \vec{A}\) \(- (1/\mu_{0}\varepsilon_{0}) \frac{\partial^{2} V}{\partial t^{2}} = 0\)
  • D.\(\vec{\nabla} \cdot \vec{A}\) \(- \mu_{0}\varepsilon_{0} \frac{\partial V}{\partial t} = 0\)

Answer: B

The Lorentz gauge condition is \(\vec{\nabla}\cdot\vec{A}\) \(+ \mu_{0}\varepsilon_{0}\,\partial V/\partial t = 0\).

The Lorentz (Lorenz) gauge chooses the potentials so that the divergence of \(\vec{A}\) cancels the time derivative of V divided by \(c^{2}\). With \(c^{2} = 1/\mu_{0}\varepsilon_{0}\):

\[ \begin{aligned} &\vec{\nabla}\cdot\vec{A} \\ &+ \frac{1}{c^{2}}\frac{\partial V}{\partial t} = 0 \end{aligned} \]

In this gauge both V and \(\vec{A}\) satisfy inhomogeneous wave equations, and the condition is Lorentz invariant. Writing \(1/\mu_{0}\varepsilon_{0}\) instead multiplies by \(c^{2}\) where it should divide, the minus sign gives the wrong relative sign, and the second time derivative belongs to the wave equation, not the gauge condition.

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