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Question regarding cathodes and anodes

  1. Jun 1, 2014 #1
    Hello everyone!

    I just had a quick question regarding cathodes and anodes. In chemistry, I know that an anode is a site of oxidation and the cathode is the site of reduction. However, I noticed that with a cathode-ray tube, for instance, a cathode emits electrons to an anode?

    Are these two the same things or not? Can someone please differentiate?

    Thanks!
     
  2. jcsd
  3. Jun 1, 2014 #2

    adjacent

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    The important thing to remember is that cathode is negative and anode is positive.
    That's the common thing in both Cathode-ray-tubes and electrolysis things IMO.
     
  4. Jun 1, 2014 #3

    Borek

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    Cathode is always the one through which electrons flow into. Everything else should follow.
     
  5. Jun 1, 2014 #4

    davenn

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    so why in a vacuum tube is the cathode emitting electrons and the positively charged anode "collecting" them "

    I have always found it annoying/confusing when they say what you did for chemistry
    but the opposite for tubes and semiconductor diodes

    what is the reason for the difference ?

    Dave
     
  6. Jun 1, 2014 #5

    ZapperZ

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    I don't think this is correct. As davenn has pointed out, in electron tubes/guns, the cathode is where the electrons originate and "flow out", not into. The anode is where they flow into.

    The typical answer is that the cathode is the terminal with the lowest potential or the electron source while the anode is the highest potential or the electron sink.

    Zz.
     
  7. Jun 1, 2014 #6

    AlephZero

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    To be more pedantic:

    Cathode is always the one that electrons flow into from outside the device.

    Clearly the negative electric charge flowing into the cathode has to get to the anode somehow inside the device, either by negative charges moving one way, and/or by positive charges moving the other way.
     
  8. Jun 1, 2014 #7

    davenn

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    yup, that's a clearer definition :) did see something along those lines in wiki

    cheers
    Dave
     
  9. Jun 1, 2014 #8

    NascentOxygen

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    There can be a lot of confusion around this. I used to think I understood it, but no longer. The terms are best avoided in general conversation if you eschew ambiguity, IMO.

    Do the terminals of a rechargeable cell take turns in being the anode? Do you connect the red charger lead to the cell's anode or its cathode?

    Does the anode of a zener diode change depending on the direction of current flow??

    Questions, questions .....
     
  10. Jun 2, 2014 #9

    sophiecentaur

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    The problem is that the electrons flow in a circular path round a circuit and Kirchoff1 applies. So electrons actually enter and leave both the Cathode and the Anode. the effect of this is to confuse people as the rule is context sensitive. In a battery, electrons enter the cathode plate from the electrolyte but in a TV tube, they leave the Cathode into the vacuum. This is a great example of where trying blindly to learn a rule, without understanding, is deadly.
     
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