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When current starts flowing in a circuit, voltage of the battery drops from EMF value. There are three sources of this however here I am only interested in ohmic resistance of the battery.

EMF of the battery is created because of electrochemical processes on both electrodes which create potential difference on electrode/electrolyte interface due to charge separation.

When talking about ohmic resistance of the battery I am not sure how does ohmic resistance of battery components affect charge separation on the interface. What does ohmic resistance of electrolyte or electrodes have to do with equilibrium of electrochemical reactions on the electrodes?

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  • $\begingroup$ I would tend to say that ohmic resistance problems relate to the kinetics of electrochemical reactions and are not related to electrochemical equilibrium (which is studied at zero current) ? $\endgroup$ Jul 7, 2021 at 10:52
  • $\begingroup$ Kinetics of electrochemical reaction as a factor affecting voltage drop of the battery is called activation resistance and it is described by Butler - Volmer equation. Since we are at physics stack exchange I wouldn't go to much in this topic as it is more chemistry related. However, ohmic resistance also affects voltage drop. As I said in the post there are three sources of the voltage drop: ohmic, activation and concentration resistance. Here I am interested in ohmic resistance as I do understand other two. $\endgroup$ Jul 7, 2021 at 11:01
  • $\begingroup$ This is also what I think. But in your question you ask about the link with electrochemical equilibrium ? And I think there is no link because equilibrium is studied at zero current (and therefore without ohmic effect) $\endgroup$ Jul 7, 2021 at 11:25
  • $\begingroup$ Yes, I agree. However, when current starts flowing voltage of the battery drops from EMF because of internal resistance. Since voltage of the battery originates from potential difference on electrode/electrolyte interfaces and voltage of the battery drops this means there are some changes on electrode/electrolyte interface when current starts flowing. I am not sure how does potential difference on interfaces suddenly change when current starts flowing and what does ohmic resistance have to do with that. $\endgroup$ Jul 7, 2021 at 11:33
  • $\begingroup$ Since potential difference is tied to the interfaces, how can suddenly some of it shift to electrolyte, electrode or any other component when current starts flowing. $\endgroup$ Jul 7, 2021 at 11:35

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Nothing. The Ohmic resistance does not change the electrochemical reactions on the electrodes, it changes the electric field between the electrodes. By Ohm’s law, if there is a current through a resistor there is also a voltage across the resistor and the two are related by $V=IR$.

Suppose that the circuit is grounded at the conductor attached to the negative terminal of the battery and that the half cell at the negative side is 0.5 V and the half cell at the positive side is 1 V.

Now, in an open circuit condition there is no current so there is no Ohmic voltage drop. So the negative terminal is at 0 V, the electrolyte is at 0.5 V, and the positive terminal is at 1.5 V.

Suppose the circuit is closed and there is enough current to produce a 0.1 V Ohmic voltage drop (and neglect the other electrochemical effects at the electrodes). Now, the negative terminal is at 0 V and the electrolyte immediately adjacent is at 0.5 V. However, because of Ohm’s law the electrolyte is not all at 0.5 V, but instead the electrolyte adjacent to the positive terminal is at 0.4 V, and thus the positive terminal is at 1.4 V.

So Ohmic resistance gives a voltage drop without changing the electrochemistry at the electrodes, but instead by changing the E field between the electrodes.

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