Comment by jodrellblank

> "The work is done by the electric field, which is why we can have AC power where electrons don't have any net travel."

Still though, you asked how we know electricity uses electrons rather than protons and I'm sticking with "they're 2000x different in mass, there would be some measurable difference between them" even in an AC circuit. Oscillating a more massive conveyor belt back and forth 'in-place' is harder work than oscillating a lower mass belt. "Electrically charged" means "interacts with the electric field" and if energy in the electric field is moving heavier protons back and forth, wouldn't that be distinguishable from moving lighter electrons back and forth? Slower movement of protons, more heat generated, something like that?