When you consider that this is a thin shell or surface is very short and from the equations dr 5.4 are electrical voltage between phases of this dr. Is as follows:
Considering that this surface is a thin crust or a very short distance dr and 5.4 of the equation is the potential difference between dr term is the following.
When considering this surface as a thin shell or the Dr very short and 5.4 equation to the item different electric potential between Dr phase. As follows.