Not all vector fields have zero curl, but if the curl of a vector field happens to be zero, then the above form can be used
because ∇ C is zero. This type of field is called a curl-free field or an irrotational field. Thus, we say that an irrotational
field can always be written as the gradient of a scalar field. In the context of electromagnetics, we will use the second form in
Eq. (2.115) by convention.
To understand the meaning of an irrotational field, consider the Stokes’ theorem for the irrotational vector field C defined
in Eq. (2.115):