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# Why doesn't connecting 2 batteries drain it?

Thu 31 May, 2012 10:30 pm
Hi everyone.
For some time now I had this question on batteries, but nobody I know could answer it for me.

The question is: Why doesn't connecting 2 batteries drain it?

By 'connecting', I just mean touching a battery's + end with another battery's - end, i.e. I am not completing the circuit.
Obviously, this doesn't drain either battery.
But shouldn't the electrons in the - end of a battery move into the + end of the second battery nevertheless? And thus (half) drain both of them?
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contrex

1
Fri 1 Jun, 2012 12:02 am
If there is no circuit, no current will flow. If you put two batteries together, end to end, like in a flashlamp, then the bottom of the first one is "negative" with respect to the top of that same battery. Therefore If you joined them with a wire then current would flow. However the bottom of the first battery is not "negative" with respect to the top of the second battery. There is no "potential difference". Look this term up.

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BillRM

1
Fri 1 Jun, 2012 05:20 am
Now if you hooked up two batteries in a parallel connection and the voltages of the batteries are not identity there will be a small flow of current from the battery with the greater voltage to the one with the lesser voltage until the voltages match.

The current flow depending on the degree the voltages does not match and the internal resistance of the batteries.

With car batteries where one is discharge and the other is fully charge that current can be many amps for a short time period.
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1
Fri 1 Jun, 2012 06:53 am
I am not completing the circuit.

It's the same reason that putting a spoon in a bowl of cereal doesn't make the cereal disappear.

Or walking to the diving board doesn't make you wet.

You have the potential for something to happen, but you didn't take all of the necessary steps.
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Ragman

1
Fri 1 Jun, 2012 08:56 am
no completed circuit = no drain

For example no ground or no potential difference. There is nothing pushing the flow of electrons.
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Ragman

1
Fri 1 Jun, 2012 09:07 am
This exact question was asked (more or less) elsewhere in a Physics forum:

"The confusion here is from the initial poor description of how a battery works:

A battery consists of three things: a positive electrode, a negative electrode, and an electrolyte in between. The electrodes are made of materials that strongly want (are predisposed) to react with each other; they are kept apart by the electrolyte.

The electrolyte acts like a filter that blocks the flow of electrons, but allows ions (positively charged atoms from the electrodes) to pass through. If the battery is not connected to anything, the chemical force is pulling on the ions, trying to draw them across the electrolyte to complete the reaction, but this is balanced by the electrostatic force-- the voltage between the electrodes. Remember-- a voltage between two points means there is an electric field between those points which pushes charged particles in one direction.

When you add a wire between the ends of the batteries, electrons can pass through the wire, driven by the voltage. This reduces the electrostatic force, so ions can pass through the electrolyte. As the battery is discharged, ions move from one electrode to the other, and the chemical reaction proceeds until one of the electrodes is used up.

Thinking about two batteries next to each other, linked by one wire-- there is no voltage between the two batteries, so there is no force to drive electrons. In each battery, the electrostatic force balances the chemical force, and the battery stays at steady state.

(I kind of glossed over what it means for two materials to "want" to react with each other. Google "Gibbs free energy" for more details on that. You might also google
Nernst equation.)
"
33export

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Fri 1 Jun, 2012 09:36 am
Flashlight batteries are connected in series and do not drain until you switch the light on. Three 1.5 v. cells produce the amount of current to light the bulb.
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1
Sat 2 Jun, 2012 05:52 pm
@Ragman,
I don't think I'm getting this.

You said "Thinking about two batteries next to each other, linked by one wire-- there is no voltage between the two batteries, so there is no force to drive electrons."

Why isn't there voltage between the two batteries? I am connecting the positive end of a battery with the negative end of another one, so isn't there a potential energy difference?

Thank you.

p.s. Could you give me the link to the question in the physics forum?
contrex

1
Sat 2 Jun, 2012 11:09 pm
I am connecting the positive end of a battery with the negative end of another one, so isn't there a potential energy difference?

No, there isn't. I already told you that. If you stack 2 batteries one on top of the othere there is a potential difference between the positive cap of the top one and the negative base of the bottom one. Your question could only arise because you don't understand how batteries or electric circuits work. You need to do some studying. If you are still at school ask your teacher.

1
Sat 2 Jun, 2012 11:19 pm
@contrex,
I am not getting it; could you be more explicit? As far as I can see, the only difference between a completed circuit and my hypothetical incomplete circuit is the layer of conductive material(electrolyte). How does this affect the voltage?
I.e. what do the Ions have got to do with this?
p.s. please give me the physics forum link if you have it
roger

1
Sat 2 Jun, 2012 11:32 pm
Maybe a different approach to the same thing would help. If you stack two, three, or more dry cells, positive to negative you aren't doing anything different than if you had a single cell. What you have done is to essentially creat a longer battery with a higher voltage potential

That longer battery acts exactly the same as a single cell. It's just sitting there, waiting to go to work. If they are connected by wire, they act exactly the same as if they were stacked, as in a flashlight.

0
Sun 3 Jun, 2012 12:40 am
@roger,
That is part of my question; why does it act like a single battery?
contrex

1
Sun 3 Jun, 2012 12:45 am

That is part of my question; why does it act like a single battery?

That's what voltage sources that are connected in series do.

0
Sun 3 Jun, 2012 01:07 am
@contrex,
Why?
roger

1
Sun 3 Jun, 2012 01:39 am
Try the old water analogy, then. You have two cylinders full of water. The water is going nowhere. Now connect them. What you now have is one longer cylinder of water - going nowhere.

Please don't come back and tell us the water would run out the end, unless you're just being deliberately obtuse.
contrex

1
Sun 3 Jun, 2012 02:20 am
@roger,
roger wrote:
unless you're just being deliberately obtuse.

I think you may have something there.
0 Replies

0
Sun 3 Jun, 2012 12:27 pm
@roger,
Firstly, I am not being obtuse: saying the water would run out in the end doesn't even make any sense.
Secondly, I think the water analogy is a bit oversimplified in this case.

Anyways, it seems to me that the main difference between two 1.5V batteries connected in a series and a 3v battery is the extra separator.

So, before I say anything else, this is my new question; what do ions have got to do with voltage?

p.s. This is a question on Physicsforum which seems similar to my question:

2
Mon 4 Jun, 2012 08:02 am
The ions have to get their electrons from somewhere.

If you set the battery on the counter, and don't connect it to anything, the metal ends will have slight electrical charges, because the electrolyte mixture will have "pulled" some free electrons from one end, and "pushed" them to the other end.

When you place to batteries in contact, there will be a very tiny flow of electrons where the terminals touch, but not enough to really worry about.
0 Replies

Ragman

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Mon 4 Jun, 2012 12:09 pm