1. The problem statement, all variables and given/known data I am reading about electrodynamics, and in different formula, there is a SI form and a Gaussian form. 2. Relevant equations What is the relationship in the different unit system for c or B or E and other relative variables? 3. The attempt at a solution sometimes there's a difference of 4(pie)/c, sometimes there's only 1/c, but i don't see when is which and which.