Maybe a better way of asking the question would be what is the equation for E - the electric field, between point electrical contacts on a sheet. I mean a large sheet - up to two feet by two feet. This might be different from the sheets that I've begun to associate with resistors - small and thin. Is there one equation for different large sheets? What types of equations are there describing this situation? If there are different equations for different sheets, are there equations that relate E to properties of sheets - that are more descriptive than the name of the type of material composing the sheet?