0
$\begingroup$

In this image taken from the manual of the NaRiKa/Nada Scientific EM-4N e/m apparatus, you can see that the positively-charged electrode (the one electrons are attracted to) is labeled "anode." e/m apparatus circuit diagram This leads to some student confusion, because they are used to the anode of the battery being the negative terminal.

Some cursory research (for example this chemistry stack exchange question) makes it clear that this is a complicated issue. It appears that fundamentally (at least in chemistry), the question of which terminal is labeled "anode" has to do with where oxidation occurs. But in this case, we don't have an electrolytic or galvanic cell... just a 200-500V DC power supply. So why is the positive terminal the anode here?

I have found some conflicting info:

  • "The anode is the positively charged electrode; The anode attracts electrons or anions" (source)
  • "the anode is the source of electrons to the external circuit and the cathode is the sink." (source)

Can anyone provide an explanation suitable for intermediate level physics undergrads that explains why the anode is positive in this case?

$\endgroup$
2
  • $\begingroup$ The two are not entirely contradictory. The anode is at positive voltage with respect to the cathode to accelerate electrons (or anions). From downstream, you see electrons or anions coming through the anode to the rest of your equipment. $\endgroup$ – Jon Custer Dec 17 '19 at 19:16
  • $\begingroup$ @JonCuster Hmmm, I see what you're saying. The second half of that sentence though, "the cathode is the sink," would seem to contradict that though. If we're looking at where the electrons are "coming from" and "going to," it could make sense to say that the electrons are coming from the anode, but I don't know how that beam of electrons could end up back at the cathode. $\endgroup$ – Bunji Dec 17 '19 at 22:26
1
$\begingroup$

In any device (for example battery, electrolytic cell or diode) the anode is the electrode towards which, inside the device negative charge carriers flow, or away from which positive charge carriers flow. In other words, inside the device conventional current is from anode to cathode (so outside the device, in accordance with Kirchhoff's first law, it is from cathode to anode).

In the electron gun shown, if the accelerating voltage is connected the right way round, the heated cylinder is the cathode, and the perforated electrode through which the electron beam emerges is the anode. If you connect the voltage the wrong way round, the gun simply won't work (but we still go on calling the heated electrode the cathode and the perforated one the anode).

Similarly, with a pn diode the p-type material is the anode and the n-type material is the cathode. [We are not considering the case of breakdown for a large reverse voltage.]

In a water electrolysis cell with two platinum electrodes, which is anode and which is cathode depends only on which way round we connect the battery. The electrode to which we connect the positive terminal of the battery is the anode, because inside the electrolysis cell, negative ions will move towards this electrode.

In a battery on discharge the anode is the terminal to which negative ions travel, so this electrode acquires a negative charge. Again, conventional current inside the cell is from anode to cathode, and outside the cell from cathode to anode. If we are charging a battery, we use an external power supply to drive a current through the battery in the reverse direction, so the electrode that was the cathode on discharge now becomes the anode and vice versa.

[As pointed out in the comments, chemists define the anode of a galvanic or electrolytic cell as the electrode at which oxidation occurs, that is electrons are released. This results in a conventional current outside the device from cathode to anode and $through$ the device from anode to cathode! The advantages of defining anode and cathode in terms of conventional current are that (a) no knowledge of the chemistry of the device is needed (b) it would be odd to discuss the operation of a thermionic diode or of an electron gun in terms of oxidation and reduction.]

$\endgroup$
8
  • $\begingroup$ So by saying, "the anode is the electrode towards which negative charge carriers flow, or away from which negative charge carriers flow," are you simply saying that the company could have labeled either terminal the anode? As currently labeled, negative charge carriers flow toward the anode, and away from BOTH the anode and cathode! $\endgroup$ – Bunji Dec 18 '19 at 15:51
  • $\begingroup$ Sorry. There was a typo in my first para. It was supposed to read "the anode is the electrode towards which negative charge carriers flow, or away from which positive charge carriers flow." $\endgroup$ – Philip Wood Dec 22 '19 at 18:06
  • $\begingroup$ Actually in galvanic cells and electrolytic cells (electrochemistry) the anode is where oxidation occurs (electrons are produced) and the cathode is where reduction happens (electrons are consumed). The mneumonic is cations are reduced at the cathode and anions are oxidised at the anode. But in other fields of physics the anode and cathode are reversed $\endgroup$ – ChemEng Dec 22 '19 at 21:57
  • 1
    $\begingroup$ @ChemEng With respect, you haven't answered my question – at least not in a way that I can understand. Are you saying that within galvanic or electrolytic cells negative charge carriers don't travel towards the anode? Or are you saying that within magnetrons or Bunji's electron gun, negative charge carriers (electrons in this case) don't flow towards the anode? $\endgroup$ – Philip Wood Dec 23 '19 at 22:33
  • 1
    $\begingroup$ Thank you for clarifying your position. Just to make quite sure that I understand you, are you claiming that $within\ the\ cell\ itself$ negative charge carriers flow away from the anode? $\endgroup$ – Philip Wood Dec 23 '19 at 23:11
1
$\begingroup$

In any complete DC circuit (DC so that the potential $\phi$ is defined), there will be places where charge is flowing in the direction that reduces $q\phi$ ("downhill"), and there will also be places where charge is flowing in the direction that increases $q\phi$ ("uphill"). You can't have one without the other. That would be like the joke about how when I was a boy, we had to walk 5 miles to school, and it was uphill both ways.

This means that we can never have a consistent system for labeling cathode and anode based on the direction of flow of charges. Consider a battery lighting a flashlight bulb. If I focus my attention on interior of the battery (as a chemist probably would), I get one answer. If I focus my attention on the external circuit, I get another.

You also quote a second definition, which talks about where the charge is, and "charge" is also what you refer to in the title of the question. That would have the advantage of removing the ambiguity. But nobody ever measures the charge on the terminals of a battery. The things we measure are currents and potential differences.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.