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I'm a moleculary biology student and have to do a detailed presentation about SDS-PAGE for my next lab.

So I was currently trying to understand the physics of electrophoresis and electrolysis. Since I was never very good at physics I wanted to make sure I understand it correctly:

Am I right in saying, that if I have two electrodes in a non-conducting fluid, that no current will flow and that thus the electrodes are not actually charged but neutral? Since if you have a normal current with a metal wire, current also does not flow, if you have a gap in the wire. The electrones are not "waiting" and the ends of the wire, if you know what I mean.

So the electrodes are uncharged, right? But if I would take salt water as the fluid, the charge can be balanced by ions, and the electrodes themselves will get charged, if we assume that no Redox reaction occurs. So that there is e.g. the cathode with an excess of electrons, and sodium ions moving towards it and then surrounding it. And the more electrons flow over time, the more sodium ions gather there. Am I picturing this correct? But since water is present, electrolysis could occur and the electrons in the cathode would get less (and more in the anode). But why can it still attract proteins and nucleic acids in electrophoresis, if the electrodes are constantly held neutral? Is it because there are constantly electrones flowing and thus the electrodes are never actually neutral? I must say, trying to understand things in detail is very confusing, especially in physics. I hope someone can help and/or maybe recommend a book or other source where this is explained. Most books and videos only explain it superficially and jump right to equations. Thanks :))

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    $\begingroup$ I’m voting to close this question because it concerns electrochemistry and is therefore better suited to a chemistry or physics forum. The application of electrophoresis to the separation of biological molecules is tangential to the core of this question, which concerns the charge of the electrodes. $\endgroup$
    – acvill
    Oct 15 '21 at 14:26
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Electrodes are neutral, but they are held at different potential. This is similar to the current flow in a wire: although current flows, the wire remains neutral, i.e., no charge is accumulated (unless we have a capacitance somewhere in the circuit).

Analogy: flow of water
A good analogy to current flow is a flow of water in a river. The water itself is charge, while difference in height corresponds to the potential difference. It is clear that water will flow from higher ground to lower ground, but the density of water at a particular place is not changing, because, as water flows, more water is supplied from the source, while the excess water at lower ground does not stagnate, but moves further towards the sea.

Electrolysis
In electrolysis cathode supplies electrons, while the anode collects them. They are held at different potentials, so, if ions are present in the liquid, the negative ions will drift towards the anode, whereas the positive ions drift towards the cathode.

When the negative ions reach the anode, they give it their excess electrons, contributing to the current flow. The ions themselves transform in neutral molecules and condense on the anode. Similarly, when the positively charge ions reach the cathode, they take come electrons and become neutral. As long as there are ions that can come and take/give electrons from the two electrodes, the current will flow. The excess electrons are supplied and taken back by the buttery that supplies the potential difference. Buttery is akin to evaporation in the water cycle in the nature in the above analogy, which takes water from the sea and back to the river source.

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  • $\begingroup$ Thank you for the quick reply. Just to be sure: Let's say the power source is a capacitator (so just seperated charge, which creates a potential difference). Would that mean, that if the fluid is not conducting, that no electrons flow from the capacitator to the electrode? Or wait, do electrons even "flow"? And since you said the electrode is always neutral - why are positvely or negatively charged ions attracted to them? $\endgroup$
    – Felix H.
    Oct 15 '21 at 9:29
  • $\begingroup$ If the fluid is non-conducting, the current will not flow. Note however, that a capacitor is not a buttery - if a current flows, it gets discharged, i.e., the potential difference will decrease and the current would eventually stop. The gravitational potential that make a river flow is very similar (even mathematically) to the electric potential - why does water in a river flow, even though there is no empty space ahead? $\endgroup$ Oct 15 '21 at 9:33
  • $\begingroup$ Yes, that makes sense. Can you have a current without electrolysis? $\endgroup$
    – Felix H.
    Oct 15 '21 at 13:23

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