how do generators produce electricity? i mean i understand an external force rotates the bar magnet which induces a current, but where do the charges come from? like for example the generator from the power plant gives us power in our homes which we pull for personal use, but where do the charges that we use get replenished? are the electrons drawn from the air or something? what happens at the atomic level when mechanical energy is converted into electrical energy? im not getting the concept.