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Electric circuit question

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bnlacava
#1
Aug21-13, 04:00 PM
P: 2
Hi. So we all know that electricity requires a complete circuit in order for electrons to flow in a wire. It's my understanding that this is due to the charge differential at the positive and negative, creating an electric potential. So, given that (or correct me if my understanding is wrong):

Why is there no electrical potential between the positive terminal of one battery and the negative terminal of another battery? This obviously does not create a circuit, but why does that charge differential between those two terminals not create an electric potential from which a current can be generated? How is the negative terminal of one battery "different" from the negative terminal of another battery?

Thanks, this is puzzling me!
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phinds
#2
Aug21-13, 04:16 PM
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Voltage is relative within a circuit. Voltages from two circuits that are not connected together (and assuming no common ground, which WOULD connect them) have no relationship with each other.
stevenb
#3
Aug21-13, 04:49 PM
P: 697
There can be (and is) a potential difference between those terminals. The issue is measuring it. Any voltmeter has a finite source resistance built in. Although large, it is typically comparable to the resistance between the open terminals. So, any measurement will either appear to drift, or may look like zero depending on the actual values of the meter resistance and the resistance between open terminals.

We typically call such voltages "floating" because we can not identify a clear value. It will drift and change depending on the surrounding conditions such as temperature, humidity and distances between contacts. And, it will change when you put the meter on it, which creates the classic measurement problem, "how do you measure something without changing the value you are trying to measure?".

bnlacava
#4
Aug21-13, 08:05 PM
P: 2
Electric circuit question

My meaning was more along of the lines of putting a wire between the terminals, not just through space. If one terminal is charged + and one terminal is charged -, why is there no flow of electrons? A similar analogy would be if you chopped a battery in half, laterally, so you end up with two "monopoles" (for lack of a better term) instead of one dipole battery. My hunch is that, if you put a wire between those two monopoles, for one tiny brief instant there would be a current, but it would stop as soon as the charges were equalized, since there is nothing acting to keep the charges unbalanced. Does this seem right?
davenn
#5
Aug21-13, 09:59 PM
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My meaning was more along of the lines of putting a wire between the terminals, not just through space. If one terminal is charged + and one terminal is charged -,
that's not what you initially said ...
Why is there no electrical potential between the positive terminal of one battery and the negative terminal of another battery? This obviously does not create a circuit,
you answered your own question in the bold part, then phinds confirmed that for you

there must be a circuit between the negative and positive terminals of the battery for the current to flow. The only time you will get current flowing across a gap in a circuit is when electric field is high enough for electrons to flow across a gap ... eg
a lightning strike across the huge gap from cloud to ground.
On a smaller scale in an electron Tube ( valve) where electrons flow from the cathode heater to the anode plate.
On an even smaller scale in a spark gap gaseous arrestor here the gap may only be a 2 to 4 mm

cheers
Dave
stevenb
#6
Aug21-13, 10:14 PM
P: 697
Quote Quote by bnlacava View Post
My meaning was more along of the lines of putting a wire between the terminals, not just through space. If one terminal is charged + and one terminal is charged -, why is there no flow of electrons? A similar analogy would be if you chopped a battery in half, laterally, so you end up with two "monopoles" (for lack of a better term) instead of one dipole battery. My hunch is that, if you put a wire between those two monopoles, for one tiny brief instant there would be a current, but it would stop as soon as the charges were equalized, since there is nothing acting to keep the charges unbalanced. Does this seem right?
Well, batteries are not quite storing charge, but is a chemically based electromotive force. But, if we think of a capacitor, we can think of real charges stored on plates, and get to the same question.

So, if you don't create a closed circuit, by using yet another wire on the other two terminals, you won't get a current flow. You said it yourself. You need a complete circuit.

Without a complete circuit, for the electrons to transfer from the - terminal to the +terminal, they first would need to overcome the attraction from the other positive plate .

For a circuit configuration, where the two capacitors are physically uncoupled, then no current will flow. However, if the 4 plates are close enough to each other to have capacitive coupling between to the capacitors, then you might see a brief current flow, as you suggest. The charges would not fully equalize however, but would find a new equilibrium distribution.

This is a very subtle thing you are talking about, and it is usually ignored in circuit theory, but is important in electrostatics.
carlo27
#7
Oct3-13, 02:13 AM
P: 1
Your circuit is simply called an "OPEN CIRCUIT", no current or potential difference that will generate on it.
darkwood
#8
Oct7-13, 11:21 AM
P: 24
I think your approaching this in the wrong way.... when you connect the positive terminal of one battery to the negative of another battery you are just extending the cells (making a longer battery albeit with an external link). This will increase the PD across the remaining open terminals of the batteries to the value of the sum of the 2 batteries.

Consider a battery with 10 cells stacked to give 10v if you split this battery down the middle you will get 2 batteries of 5v - 5 cells each so like your question asks putting the 2 separate 5 cell stack back together simply increases the voltage it can deliver it doesn't create a circuit between 2 batteries it simply becomes a larger battery.
meBigGuy
#9
Oct7-13, 10:44 PM
P: 1,083
There is no "absolute" potential associated with a battery. It's positive terminal has potential relative to its negative terminal, and to nowhere else (unless there is a leakage path). All it can do is force current from its positive to its negative. Does that answer your question, or do you want to know more?

Note that the current might go through lots of things and lots of transformations to get there, but it is still a matter of current goes out +, same amount of current comes in -.


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