This is similar to the integral in Eq. (2.17), but in terms of its evaluation, it is evaluated over each component independently. The only difference between this and the scalar integral in Eq. (2.17) is that the unit vectors may not be constant and, therefore, they may have to be resolved into Cartesian coordinates in which the unit vectors are constant and therefore may be taken outside the integral sign (see Sections 1.5.2, 1.5.3, and Example 2.7). In Cartesian coordinates, we may write