Note 1 to entry: The flux through a surface S of the vector V is denoted by ${\displaystyle \underset{\text{S}}{\iint }V\cdot e_n\text{d}A}$. If the surface is closed, the flux is denoted by ${\displaystyle \underset{\text{S}}{∯}V\cdot e_n}\mathrm{d}A$.