Conservation of Charge and Charging by Friction

Charge conservation says the total charge in an isolated system never changes. You can move it around, pile it up on one object and strip it from another, but you cannot make it or destroy it. Rubbing two different materials together is the classic way to see this. Some materials hold their outermost electrons more loosely than others. When you shuffle socks on carpet or rub a glass rod with silk, surface contact lets electrons hop from the material that holds them weakly onto the material that holds them more tightly.

At the microscopic level nothing exotic happens: no protons move, nuclei stay put, only electrons migrate. Whichever object gives up electrons is left with a shortage of negative charge, so it reads positive. Whichever object receives those electrons now has extra negative charge, so it reads negative. The two magnitudes match exactly, because every electron that leaves one surface lands on the other. That is conservation of charge in action, not an abstract law but a direct bookkeeping statement: charge lost by one object equals charge gained by the other.

This idea sets up everything else in the unit. Once charge sits on an object, it creates a field around it, and Gauss's law later lets you relate the total charge enclosed by a surface to that field. Conservation of charge is also why induction and polarization work the way they do, and why net charge on an isolated system is always a fixed, countable multiple of e. If you rub two identical materials together, there is no asymmetry in how tightly each holds electrons, so no consistent transfer direction exists and both stay essentially neutral.