We all know (or maybe know) that to move, we need to spend energy. If you want to drive a car, you gotta spend gasoline.
Also, we know that in energy transformation, in real-world almost some part of it is converted into heat produced because of the friction between motion bodies. (for example, part of the gasoline of the car in burned to overcome car's friction with air, and thus would be converted into heat, and won't serve any movement at all).
Now a question has obsessed my mind. How electrons circulate around nucleus for ever? Where does electron get its energy from?