2. 3 The principle of superposition If more than one charge Qi,(i= 1, 2 …)are tributed each generates a field e The resultant e is simply the vector sum of individual E Q2 E(r)=2Ei 24丌or-;/2(r-ri)1, where(r-rill is a unit vector pointing from ri to r This is called the principle of superposition Explanation 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