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It is my understanding that in a Galvanic or electrolytic cell the electrode from which the electrons flow is known as the anode. Yet it is also my understanding that in a vacuum tube the electron source is known as the cathode, i.e. "cathode ray tube" and that electrons flow toward the anode.

  1. Is my application of these terms backward in one of the examples?
  2. Do the terms anode and cathode fail to uniformly distinguish electron flow?
  3. Do I misunderstand the function of a vacuum tube?
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  • \$\begingroup\$ Have you read the Wikipedia article about it? \$\endgroup\$ – Arsenal Nov 9 '15 at 18:31
  • \$\begingroup\$ Basically it's like you say en.wikipedia.org/wiki/Anode#Flow_of_electrons And it's probably so due to en.wikipedia.org/wiki/Passive_sign_convention \$\endgroup\$ – Fizz Nov 9 '15 at 18:42
  • \$\begingroup\$ @Respawned I think passive sign convention is the part I was missing. I was thinking about the inner workings of the vacuum tube when I should have been thinking about it only as a black box with external contacts, for the purpose of labeling. Is that correct? \$\endgroup\$ – Mr.Wizard Nov 9 '15 at 19:42
  • \$\begingroup\$ As far as I know. Now it's possible someone came up with the cathode/anode assignment variations [on battery vs. diode] for a different reason, but it is consistent with PSC. I can't say I want to investigate further how the terminology might have come up... \$\endgroup\$ – Fizz Nov 9 '15 at 19:46
  • \$\begingroup\$ Actually, PSC is only consistent with the anode/cathode naming of a regular diode, but not with a Zener one (when used as such; nothing prevents a Zener from being used as regular diode as well). \$\endgroup\$ – Fizz Nov 9 '15 at 20:10
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The anode is where current flows into the device (that is electrons flow out of device); the cathode is where current leaves the device (that is electrons flow into the device). You can use the ACID mnemonic to help you remember (Anode: Current Into Device).

I think there are two possible sources of confusion:

  1. The direction of electron flow is actually opposite to the direction of current (electrons are negatively charged).
  2. Electrons flow through both the anode and cathode, the question is what direction? I.e. are the electrons flowing from the circuit into the device or are they flowing from the device into the circuit?

For example when you say electrons flow from the anode, do you mean that they flow into the circuit and out of the device (i.e. cell)? If so you are correct. Note the usage of the terms is consistent in CRTs, within the actual tube (the device) electrons flow out of the cathode, but from the perspective of the circuit driving the tube, the electrons are going into the cathode.

Note that which terminal is the anode and which is the cathode is defined by the direction of current flow not polarity, so in power sources (e.g. discharging batteries) the positive terminal is the cathode, but in power sinks (e.g. charging batteries) the positive terminal is the anode.

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1) There is a chance that you might not have noticed the difference when talking about Galvanic cells. In your statement "...in a Galvanic cell the electrode from which the electrons flow is known as the anode." This statement would be correct if you say, current flows, instead of electrons flow. This is due to "old" terminology that used positive current flow, instead of the more "modern" electron flow.

2) No, electrons always flow towards a more positive level.
3) No, the cathode is the source of the electrons in a vacuum tube.

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