Symmetry properties of the scalar potential and the vector potential How do the scalar potential $\phi(x,t)$ and vector potential $A(x,t)$ behave under the parity and time-reversal transformations?
 A: Recall the relations $\textbf{E}=-\boldsymbol{\nabla} V(\textbf{r},t)-\frac{\partial}{\partial t}\textbf{A}(\textbf{r},t)$ and $\textbf{B}=\boldsymbol{\nabla}\times \textbf{A}(\textbf{r},t)$. Now, $\textbf{E}$ is an polar vector i.e., under parity $\textbf{E}\rightarrow -\textbf{E}$, and $\textbf{B}$ is an axial vector i.e., under parity $\textbf{B}\rightarrow\textbf{B}$. Since, $\boldsymbol{\nabla}$ is odd under parity, $\textbf{A}$ must also be odd under parity. Similarly, $V(\textbf{r},t)$ is also odd under parity for $\textbf{E}$ to be a polar vector. 
To understand that the behaviour of $\textbf{E}$ under parity one can use the formula $$\textbf{E}(\textbf{r})=\frac{1}{4\pi\epsilon_0}\int\frac{\rho(\textbf{r}^\prime)d^3\textbf{r}^\prime}{|\textbf{r}-\textbf{r}^\prime|}$$ from Electrostatics. Note that, under parity $d^3\textbf{r}^\prime\rightarrow -d^3\textbf{r}^\prime$ and the integrand is invariant under parity. Therefore, under parity $\textbf{E}\rightarrow -\textbf{E}$. To derive the behaviour of $\textbf{B}$ under parity use the fact that the current density $\textbf{J}\rightarrow -\textbf{J}$ under parity.
A: The scalar potential V does not change sign under parity . 
I think it must have even parity.  It can be shown from the expression of electric field. 
 $\textbf{E}=-\boldsymbol{\nabla} V -\frac{\partial}{\partial t}\textbf{A}$ 
I think this equation should  remain invariant under parity.
For that V should not change sign under parity.  
Under parity, 
$\textbf{E}\rightarrow - \textbf{E}$
$\textbf{B}\rightarrow \textbf{B}$
$\boldsymbol{\nabla}\rightarrow - \boldsymbol{\nabla} $ 
$\textbf{A}\rightarrow -\textbf{A}$
$\textbf{V}\rightarrow \textbf{V}$
Under time reversal operation,
$\textbf{E}\rightarrow \textbf{E}$
$\textbf{B}\rightarrow -\textbf{B}$
$\boldsymbol{\nabla}\rightarrow \boldsymbol{\nabla} $ 
$\textbf{A}\rightarrow -\textbf{A}$
$\textbf{V}\rightarrow \textbf{V}$
