... when the old electrons have been whisked away in a generator?

In a generator you have a magnetic field cutting through coils of wire, creating an electromotive force which is pushing the electrons down the wire. But after the electrons have been pushed away and used (in a light bulb for example), where do the new ones come from?

 

Conductors trade off electrons, in a closed loop circuit like you describe, the electrons in the loop are just going in a circle like a Nascar driver (through coil & load)..
In a lightbulb, they come from earth ground.. It is the Alpha and the Omega for AC current.. Well actually, remember electrons actually move really slowly linearly through a wire, so you probably won't be using the electrons that left the earth into the conductor when you turn an appliance on, before you turn it off..

 

... when the old electrons have been whisked away in a generator?

In a generator you have a magnetic field cutting through coils of wire, creating an electromotive force which is pushing the electrons down the wire. But after the electrons have been pushed away and used (in a light bulb for example), where do the new ones come from?

Electrons dont exist only in the generator conductors but also in every conductor in the circuit. The magnetic field just pushes them to move.

 

As electrons are shoved out the negative node of the generator, more electrons are sucked in the positive node of the generator. Round and round we go.

 

Electrons are like water in the sea, their everywhere, a fundimental component of matter that isn't bound too tightly to any one atom. You can even create a surplus or deficite on demand in an object, the link is so loose.