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source of electrons



How would you answer the question a student wrote to me today: "Where does
an electron gun in a TV get an unlimited supply of electrons?"

We're just starting electrostatics so I don't know if he is worried about
the current coming out being less than the current going in (i.e.
conservation of charge) or what.

Second question: is the "earth + atmosphere" system charged? Do we receive
more positive or negative particles as cosmic rays? Seems like I read
about this but I can't remember the answer.

Thanks,
Larry