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If you have two batteries with different voltages, let's 12 and 9 volts and you connect the negative terminals while you put a LED or whatever between the positive terminals, there will be current flow. I reason that is because there is a potential difference even thought both terminals are positive. However I'm not sure I understand how electron current would work here.

If there is current then that must mean that electrons actually flew out of the lower positive terminal into the higher one? is that what's going on? if not, could someone clarify it?

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  • \$\begingroup\$ Related old question: Is current flowing backwards a bad thing?. \$\endgroup\$ – The Photon Feb 2 '16 at 5:02
  • \$\begingroup\$ Summary: current flowing in to the positive terminal is exactly how you recharge a rechargeable battery. On the other hand, if your battery chemistry doesn't allow recharging then doing this could cause problems. \$\endgroup\$ – The Photon Feb 2 '16 at 5:03
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It sounds from your question that you aren't asking about the effects of electron flow in the "wrong" direction on battery chemistry, but rather asking if a "positive" voltage can "act like" a negative or a ground. If so, then the answer is yes, it can.

All voltage is relative, so your reasoning is correct. If you connect two positive but non-equal voltage nodes together, current will flow between them.

Calling something "positive" only means that it has a higher voltage potential than something else which you are using as a ground reference.

You can think of it as pressurized cans of air. Let's call 1 atm (which is air pressure at sea level) our reference pressure. If you pressurize one can to a pressure of, let's say, 2 atm, you might say it has a "positive pressure". If you then pressurize another can to 3 atm, it also has a positive pressure. Connecting either can to a 1 atm pressure (i.e. by opening it) will cause air to flow out of it (this is analogous to electrons flowing in the wire) until the pressure equalizes. But if you connect the two cans to each other, air will flow out of the 3 atm can and into the 2 atm can until the pressure equalizes.

So it is with voltages. The electroncs that are under more "pressure" (voltage) will push harder against the ones that are under less pressure (but still "positive", vs. some arbitrary reference).

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  • \$\begingroup\$ great thanks for the answer, a bit of a more annoying questions then, could you tell me how is it that electrons usually flow out of the negative terminal? i.e. when you close a circuit if they 'physically' can flow out other side, how is it they usually go from the - to + terminals, why not make the circuit go the other way? \$\endgroup\$ – Daniel Feb 2 '16 at 5:41
  • \$\begingroup\$ well that last one is beside the point, it would have to do merely with the chemistry and the design of the battery I would think. \$\endgroup\$ – Daniel Feb 2 '16 at 5:52
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    \$\begingroup\$ Same reason that water flows out of a faucet and not in, and why things fall down rather than up...because that's the way the force vector points. That's what voltage does. And a battery, by its chemistry, creates voltage. \$\endgroup\$ – rothloup Feb 2 '16 at 6:26

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