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Suppose I have 2 lemons each with a zinc nail and a penny for electrodes and I wire them up in series so that Nail-A passes through a load and connects to Penny-B and Nail-B connects to Penny-A.

I think I understand that the lemon juice puts electrons into Nail-A as it creates zinc oxide and Penny-B is losing electrons as the lemon juice creates hydrogen gas and therefore a voltage occurs because of an excess of electrons on Nail-A and a deficiency of electrons at Penny-B. I assume that something similar is occurring between Nail-B and Penny-A.

In the case of a single lemon, I can get roughly 1 volt between Nail-A and Penny-A. But I get double that between Nail-A and Penny-B. What exactly is the connection between Nail-B and Penny-A doing to increase the supply of electrons at Nail-A and therefore increase the voltage?

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I took the liberty of creating a diagram, illustrating the current flow.

Two Lemons:

<------Stuck together---->   <-----Stuck together----->
Nail-A---Lemon-A---Penny-A---Nail-B---Lemon-B---Penny-B
  |                                                |
  ------------------------Load----------------------

One Lemon:

<------Stuck together---->  
Nail-A---Lemon-A---Penny-A
  |                  |
  --------Load--------

If I understand correctly, your question is not about lemons specifically, or about the electrochemistry going on here, but about Kirchhoff's circuit laws, specifically the Voltage Law. It implies that voltages add up if you connect voltage sources in series.

In your case, two sources connected in series with 1V each lead to a "combined" 2V voltage source.

This would happen with any voltage source, even mixed sources (e.g. lemons and oranges).

After looking at voltages, let's look at the current. Let's assume that the load follows Ohm's law and has a resistance $R=1 \Omega$, i.e. a 2V voltage drop across the load leads to 2A, a 1V voltage drop across the load leads to 1A. Because current is constant in a closed circuit, this leads to a change in the electrochemistry of each sources:

  • 1 lemon case: each lemon creates 1V potential difference, 1A current flows through it
  • 2 lemon case: each lemon creates 1V potential difference, 2A current flows through it

This "just" increases the number (happening in parallel) of electrochemical reactions as long as the current is low. With increasing current, other effects take over:

  • The lemons themselves have non-zero electrical resistance, i.e. they heat up and "eat up" some of the voltage, leading to a decrease of the potential difference.
  • The number of parallel reactions is limited (e.g. too little electrode surface or ions in the electrolyte). If the ions can't be transferred from one electrode to another as effectively, the current will still increase but the voltage difference lowers. An some point, the current increase stops.

Short circuiting the lemon would be the extreme case of the described situation. In that case, the output voltage drops to 0V (because there is no load) and the current is at the lemon-specific maximum.

The behavior at large (or maximum) current seems to be complex and there are quite a few results when you google it. The so-called Newman's model is used here and here.

I guess if life gives you lemons, connect them in series.

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  • $\begingroup$ I obviously constructed the question poorly because I actually was trying to get at the electrochemistry of what was going on. I'm aware that voltages add when cells are put in series, but why they do is a mystery to me. I was hoping to constrain the question by using a 2 lemon battery as I think I understand the chemistry (in a layman sort of way) that generates voltage using 1 nail and 1 penny, but I don't understand what chemistry/physics causes double the electrons to appear at LemonA/NailA when the connection between LemonA/PennyA and LemonB/NailB is made. $\endgroup$
    – JohnBR
    May 13, 2020 at 18:52
  • $\begingroup$ My answer still stands. By connecting in series, the voltage (=potential difference) is doubled, not the number of electrons (=current). It's like dropping a lemon from second floor instead of first floor - the individual parts don't change at all. Depending on the load, it may allow double the current at double the voltage and that changes what happens inside the lemons. Note: You could create the same behavior with one lemon and a load with half the resistance. $\endgroup$
    – zonksoft
    May 13, 2020 at 19:09
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    $\begingroup$ I expanded my answer regarding the difference in current and added a connection to the electrochemistry, I learned a lot myself. $\endgroup$
    – zonksoft
    May 16, 2020 at 11:22

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