I am a student of intermediate and now a days we are studying Electricity. I get confused when I sit to learn any topic of physics specially mathematical derivations.I cannot understand what formula is used and when will be used in this derivation.For Example:In "Capacitance of A Parallel plate Capacitor" topic first we used (E=sigma/epsilon not) then we used E=V/d. now I confuse why we used these formulas.And mostly I cannot remember that which formula is used in this and that derivation? and somtimes I forget to apply them because there are many topics in which these formulas are used again and again so that these topics are mixed in my mind and I get confuse. Please help me I will be thankful to you.It is a great problem of my study. thanks. If any one have any site in which I can find all derivations with details of everystep and each step is defined whether what is reason to use this and that formula ,please tell me. I wil be grateful to you.