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In gel electrophoresis, there needs to be an electric field created in the gel. I realized that I have some gaps related to my understanding of electromagnetism because I cannot fully understand in what conditions a field gets created. Are the electrode plates on either side in contact with the gel or insulated, and if so, how does this affect the field? My understanding is two insulated plates with different voltages would still create a field between them. Is the issue that the surface charges in the gel would align themselves to cancel out this field on the gel inside? Thus, do you need to actually pass a current through the gel in order to create an electric field inside? I think this is related to the nature of the electric field in a current carrying wire (where I treat the gel as a really thick rectangular wire), which I also realize I don't understand. Namely, do the electrodes need to span the full width of the gel rectangle to create a uniform field across the width?

More generally, what are the possible ways to create an electric field on the interior of a conductive object such as a gel? Do you have to pass a current through it from one side to the other? My other thought is that a changing magnetic flux would also create an electric field, although it would be of a odd shape. But would this induced electric field exist on the interior or also be subject to some cancelling effect as is the electrostatic case?

B. Lau
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  • This may help: What is gel electrophoresis? - YourGenome https://www.yourgenome.org/facts/what-is-gel-electrophoresis/ – Stevan V. Saban Sep 26 '23 at 15:51
  • The gel used in electrophoresis, is semi permeable allowing the charged particles in the gel to migrate through the gel. The gel does not respond (or minimally) to the field. – Stevan V. Saban Sep 26 '23 at 15:55
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    The best analogy is a leaky capacitor. The gel is the dielectric and the charged particles in the gel (that move when a voltage is applied between the two electrodes) are the "leak" – Stevan V. Saban Sep 26 '23 at 16:05
  • Are you familiar with the concept of a dielectric medium? – Michael Seifert Sep 27 '23 at 11:35
  • @StevanV.Saban What I don't understand is: Is there current passing between the electrode metal and the gel it is in contact with? And how does the situation change if I insulate the electrodes from the gel but apply the same voltage. – B. Lau Sep 27 '23 at 20:57
  • The charge carriers are the ions in the gel. Once the charge carrier reaches an electrode, electrons are passed between the electrode and the charge carrier depending on how ionized the charge carrier is. The gel is a passive medium for transport. – Stevan V. Saban Sep 27 '23 at 21:12
  • So if I insulate the electrode the setup no longer works? – B. Lau Sep 27 '23 at 21:16
  • I recommend googling "electrolyte" or looking in a basic electrochemistry to learn more about how current is carried through aqueous solutions. – Stevan V. Saban Sep 27 '23 at 21:17
  • Yes if you insulate the electrode, you will still create a field for transport (though reduced depending on the insulator), but you are blocking the oxidation and reduction at the electrodes. – Stevan V. Saban Sep 27 '23 at 21:18
  • Right, and my understanding is you just need to create a field to move the dna molecules, so it seems that electrodes could be insulated. – B. Lau Sep 27 '23 at 21:20
  • Yes, I believe so. But I'm not an expert in gel electrophoresis so I would double check. – Stevan V. Saban Sep 27 '23 at 21:22

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Disclaimer: I have modeled this situation before according to the model below. However, this is not the only way to model gel electrophoresis, so there may be more accurate techniques. I believe this explains the gist in a way that makes sense of the biological application sufficiently.

This situation can be modeled using a parallel plate capacitor with a dielectric. The gel acts as the dielectric, and the terminals are charged anodes and cathodes that act as the parallel plates. One way to model this situation is to require that the applied electric field must be uniform, created by two parallel plate electrodes with some voltage difference. We also assume that the gel does not change in response to the field. Finally, we assume that the electrodes span the full width of the terminal for ease of the parallel plate capacitor calculation, which allows for the creation a uniform field across the width. Details of this are worked out on the wikipedia pages for electrophoresis and capacitors.

This answer does a good job of explaining how physics of the the gel electrophoresis apparatus affords the desired biological application.

Relativisticcucumber
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