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Quoted from BU-104b: Battery Building Blocks:

The electrode of a battery that releases electrons during discharge is called anode; the electrode that absorbs the electrons is the cathode.

Based on the definition for cathode and anode above, why is the cathode filament in an x-ray tube negatively charged (and discharges electron) rather than being positively charged. Alternatively, since the filament is negatively charged, should it instead be called the 'anode' filament?

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  • $\begingroup$ A Negative ion is pulled to the ANode. So is an electron. Can’t talk to electrochemistry norms, but that is how it is for ion/electron beams. $\endgroup$
    – Jon Custer
    Sep 25, 2020 at 1:58
  • $\begingroup$ @Jon Custer thanks for the input, may I ask where did you read the premise that ‘a negative ion/electron is pulled to the anode’? Does this premise relate to the concepts in galvanic cell or electrolysis by any chance? $\endgroup$ Sep 25, 2020 at 2:17
  • $\begingroup$ As noted, this is in the context of ions and electron beams. More broadly, if the cathode were positive relative to anything else, why would the electron leave? $\endgroup$
    – Jon Custer
    Sep 25, 2020 at 12:04
  • $\begingroup$ @Jon Cluster About "if the cathode were positive relative to anything else, why would the electron leave?", I was thinking the filament should be called the 'anode' filament as it's negatively charged and electrons exit from it - and this is in line with the definition that cathode=reduction (gain of electrons) and anode=oxidation (loss of electrons). $\endgroup$ Sep 25, 2020 at 13:34
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    $\begingroup$ chemistry.stackexchange.com/questions/68533/… may help $\endgroup$
    – Jon Custer
    Sep 25, 2020 at 13:37

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The definition of the cathode and anode don't depend on which electrode is at a higher or lower potential, but on which direction the current flows.

The cathode is the the electrode by which electrons enter the device from outside. Put another way, it is the electrode that conventional current flows out of.

In the case of a cell providing power to a circuit, it is the terminal with more positive potential, from which conventional current flows into the circuit.

In the case of a pn-junction diode it is the n-side of the junction, which will be at a less positive potential when the diode is in its conducting state. (Very pedantically, we might reverse which terminal we call cathode and anode when the diode is reverse biased, but practically we always call the n-side of the junction the cathode)

In the case of the x-ray tube, electrons must enter the device at the cathode terminal in order to be emitted into the tube and eventually strike the anode to produce x-rays. This means that conventional current will be flowing out of the cathode.

The point of commonality is that conventional current flows out of the cathode, and into the anode.

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  • $\begingroup$ Thanks for the answer, although I managed to figure the answer from Jon's link, it was nice to see someone confirm my understanding and add another layer to it. I guess for my question, it's about recognising there is an external circuit attached to the cathode filament and anode plate, and cathode filament got its name bc it gained electrons from the external circuit and anode got its name bc it lost electrons to the external circuit $\endgroup$ Sep 25, 2020 at 16:12

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