2. 3 The principle of superposition If more than one charge are dis ibuted each generates a field e The resultant E is simply the vector sum of individual (r)=∑E2 Q r-r 2士丌enr-r where (r-ri)l is a unit vector pointing trom ri to. This is called the principle of superposition Explanation: (1) experimentally set up (2)theoretically, the Maxwell theory(equations )is a linear theory; so electromagnetism is easy to study, in comparison with other fundamental forces