- #1
Michael King
- 10
- 0
Homework Statement
Show that it is always possible to pick a gauge so V' = 0.
Homework Equations
We weren't given any, but I've been working with:
(a)[tex]\vec{E} + \frac{\partial\vec{A}}{\partial{t}} = -\nabla{V}[/tex]
(b)[tex]\vec{A'} = \vec{A} + \nabla{\psi}[/tex] (where [tex]\psi[/tex] is a scalar function)
(c)[tex]V' = V - \frac{\partial{\psi}}{\partial{t}}[/tex]
The Attempt at a Solution
The way I've attempted it is not to choose the Lorentz or Coulomb gauge, but to make
[tex]\nabla\cdot{\vec{A}} + \mu_{0}\epsilon_{0}\frac{\partial{V}}{\partial{t}} = x[/tex] (where [tex]x[/tex] is a scalar function)
Furthermore, from Griffths solutions I found:
[tex]\nabla\cdot{\vec{A'}} = - \mu_{0}\epsilon_{0}\frac{\partial{V'}}{\partial{t}}[/tex]
In our case, the above equals 0 as V' = 0.
I took the divergence of (b):
[tex]\nabla\cdot\{vec{A'}} = \nabla\cdot\{\vec{A}} + \nabla^{2}{\psi}[/tex]
[tex]0 = \nabla\cdot\{\vec{A}} + \nabla^{2}{\psi}[/tex]
[tex]- \nabla\cdot\{\vec{A}} = \nabla^{2}{\psi}[/tex]
[tex]- (x - \mu_{0}\epsilon_{0}\frac{\partial{V}}{\partial{t}}) = \nabla^{2}{\psi}[/tex]
[tex]-x + \mu_{0}\epsilon_{0}\frac{\partial{V}}{\partial{t}} = \nabla^{2}{\psi}[/tex]
[tex]\mu_{0}\epsilon_{0}\frac{\partial{V}}{\partial{t}} = \nabla^{2}{\psi} + x[/tex]
[tex]\mu_{0}\epsilon_{0}\frac{\partial^{2}{\psi}}{\partial{t^{2}}} = \nabla^{2}{\psi} + x[/tex]
... which gives me an inequality (i.e. an extra x, and also an extra factor of mu_0 and epsilon_0).
I've also been trying to use the solution to Griffths problem 10.7 as reference but I am really stuck on the steps to take.