Regarding Maxwell’s equations above, there are two scalar potentials that may be defined: the electric scalar potential and
the magnetic scalar potential. However, inspection of Maxwell’s equations shows that the equations are not, in general, curlfree.
Therefore, scalar potentials cannot be used to solve for general electric and magnetic fields. There are, however, two
situations in which scalar potentials may be used:
(1) If the time derivative of the magnetic flux density in Faraday’s law [Eq. (11.24)] is zero, the electric field intensity is
curl-free, and the electric potential may be used in lieu of the electric field intensity: