To obtain the electrical force on the differential surface element, we consider that the surface element is enclosed in a pillbox, as shown in figure 3. The resulting force is then
Figure 3: Pill box surrounding differential surface element.
If we shrink the pillbox (along the 'h' dimension, normal to the surface) until it only encloses the interface we obtain the force per unit area due to electrical forces
The force is given by the dot product of the surface normal and the jump in the MST.
