# Where are the electrons of a cathode ray tube coming from when using AC?

I see in this picture a beam of electrons leaving the cathode. But I read before, that the electrons can flow out of a copper wire only, if they are replaced by other electrons. I guess, that in the battery we can replace it with another one, but in the AC current we can't power companies do not create or destroy electrons.

• They come from the power supply and return to ground eventually. Mar 4 '17 at 22:42
• I can't really understand this: "we can't power companies do not create or destroy electrons.". It means, that we can't give energy to some business firms, who doesn't create of destroy electrons. It is probably not what you wanted to explain. I fixed the other parts of your question, but I suggest next time do the same without me. Mar 5 '17 at 0:57
• I have no doubt that the A/C current must be rectified before being used in a CRT. Apr 24 '19 at 15:59
• @DavidWhite you can run a cathode ray tube with an old fashioned induction coil with an interrupter. It produces an alternating current but one with much higher voltage peak in one direction than the other. en.wikipedia.org/wiki/Induction_coil#The_interrupter Jun 21 '20 at 1:46

Let us first examine what happens to the electrons in a conductor of length $$l$$ that is connected to an AC voltage source. The electrons oscillate to and fro with the oscillating voltage. Conservation of charge says that in a length $$dl$$ of the conductor in a time interval $$dt$$, whatever number of electrons flow out of $$dl$$ must be gained by the remaining part of the system, which in our case is the rest of the conductor. Remember that the conservation must hold globally.