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I know that if you have two batteries, battery A and battery B, and connect the negative terminal of battery A to the positive terminal of battery B, there is no potential difference. If you consider the negative and positive terminals of the same battery, the potential difference exists (say 1.5V).

I've learned that the potential difference exists because the atoms at the positive terminal have few electrons compared to the negative terminal and so electrons flow to equalize the two terminals. Why does this flow only occur when it is the terminals of the same battery and not opposite terminals of two different (identical in all characteristics) batteries?

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  • \$\begingroup\$ The thing you say about electrons is definitely not true. You appear to be trying to grapple with the fact that there is no such thing as absolute voltage. You will make the best predictions about the behavior circuits by imagining that "absolute voltage" does not exist, only "relative voltage", and only WITHIN a connected circuit. So for example, if you have two batteries both unconnected, there is no such quantity as "the voltage between opposite terminals of two different (unconnected)" batteries, because relative voltage only exists among points connected into a circuit. \$\endgroup\$ – Glenn Willen May 25 at 20:39
  • \$\begingroup\$ Of course, you bristle at this model, because it requires the current to somehow "know" what is happening far away from it. You are right to bristle, and at the level of physics it's not quite as simple as this model where the current at one end of a wire magically knows what's connected at the other end of a wire. But for all practical purposes the model is correct. \$\endgroup\$ – Glenn Willen May 25 at 20:40
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Measuring over two separate batteries won't work as they have no reference between each other. Connecting two separate batteries with a single wire does not complete a circuit so current does not flow. It only makes the connected battery terminals to be at same potential, so you can build a 3V battery from two 1.5V batteries.

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So, what electrons do isn't really important unless you're studying physics. In reality current actually flows from the negative to the positive, but we didn't know that when we made up the circuit conventions and it doesn't actually matter how it works (again unless you're a physicist). Everything is drawn backwards to reality!

What you need to keep in mind is that current flows between different potentials when it has a path to do so.

If you join the positive and negative terminals of two batteries, all you did was create a bigger battery. It is now a battery with two individual cells (although the batteries may already be made up of multiple smaller cells connected in this way in order to achieve their given voltage). A typical battery is actually assembled in exactly this way, with multiple lower voltage cells strung together.

But until you create a path which forms a complete circle (circuit), there is no where for anything to go. You are changing the potential, but you aren't creating a circuit.

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Because batteries are neutral. Poles are equal in opposite charges for the both batteries, so you can't free the charge out of either battery without applying power.

Also, notice that it is a circuit with a great impedance between other poles of batteries.

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I know that if you have two batteries, battery A and battery B, and connect the negative terminal of battery A to the positive terminal of battery B, there is no potential difference. If you consider the negative and positive terminals of the same battery, the potential difference exists (say 1.5V).

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. What you are describing. There will be no voltage between the connection point of two batteries as they are directly connected.

I've learned that the potential difference exists because the atoms at the positive terminal have few electrons compared to the negative terminal and so electrons flow to equalize the two terminals.

You have learned incorrectly. All the atoms have the required number of electrons. The potential difference exists to to the chemical reaction of the cell. Current is the movement of electrical charge - either positive or negative - so don't keep thinking of it as "electrons" although they are most common mobile charge carriers. Current can only flow when there is a conduction path from one potential to another.

Why does this flow only occur when it is the terminals of the same battery and not opposite terminals of two different (identical in all characteristics) batteries?

Current can only flow in a circuit and only if there is potential difference (voltage) driving it. By just connecting the negative of BAT1 to the positive of BAT2 you have not created a circuit. It is still an "open circuit" so no current will flow. If you look at Figure 1, what path could the current possibly take?

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