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Author Topic: What happens to electrons that leave batteries?  (Read 5380 times)

Offline colarris

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What happens to electrons that leave batteries?
« on: 14/10/2013 23:27:44 »
If batteries provide the 'force' to get electrons in a circuit moving what happens to them when they reach the end of the circuit? What if all the electrons in the circuit have reached the end of the circuit won't it have 'run out' of them? Does the battery actually provide electrons??  :o
« Last Edit: 15/10/2013 14:43:24 by chris »


 

Offline alancalverd

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Re: Batteries and electrons
« Reply #1 on: 14/10/2013 23:29:41 »
Yes.
 

Offline CliffordK

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Re: Batteries and electrons
« Reply #2 on: 14/10/2013 23:41:01 »
An electrical circuit must have the positive and negative poles meeting. 

With a battery, the + & - poles are the two terminals on the battery.  If your circuit starts at the battery, then goes off to nowhere, then you will have no current flow.

Your battery has chemical changes that force the electrons to move from the negative pole towards the positive pole.  As the battery wears down, the chemical changes continue until there is nothing left to change to a lower energy form, and the current flow from the battery stops.
 

Offline colarris

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Re: Batteries and electrons
« Reply #3 on: 14/10/2013 23:46:51 »
What happens to the electrons, do they go back into the battery?
 

Offline CliffordK

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Re: Batteries and electrons
« Reply #4 on: 15/10/2013 00:23:19 »
What happens to the electrons, do they go back into the battery?
Yes

The electrons ultimately travel from one ion to another ion within the battery.
 

Offline flr

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Re: Batteries and electrons
« Reply #5 on: 15/10/2013 08:39:13 »
Let's consider the discharging of a battery (such as Zn/ZnSO4 || CuSO4/Cu )
At the negative electrode an electrochemical reaction takes place such as Zn --> Zn2+ + 2e-
The electrode is negative because of those electrons released from the above [oxidation] reaction.
The e- formed on the negative electrode will go through an external circuit to the positive electrode while the excess of positive ions near (-) electrode will diffuse in solution.

Note that some of the energy of the electrons could be used to get useful work (power up a laptop or an iphone). 

Once the electrons get to the opposite (positive) electrode they will participate to a second electrochemical reaction where a positive ion will accept these electrons and became neutral or less positive. (i.e. Cu2+ + 2e- --->Cu)

The electrons are fully accounted for: what came out of from the oxidation reaction (taking place at anode) is 'consumed' in the reduction reaction taking place at cathode.   

-----

 
 

Offline CliffordK

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Re: Batteries and electrons
« Reply #6 on: 15/10/2013 09:58:54 »
You also don't have positive ions without negative ions.

In the above example,

Zn2+ is part of Zinc Sulfate, (Zn2+)(SO42-)
And Cu2+ is also part of Copper Sulfate (Cu2+)(SO42-)

So, every time Copper Sulfate is gains 2 electrons to form elemental copper, a sulfate ion (SO42-) is released.  This sulfate ion must then have a pathway to reach the Zinc to form the Zinc Sulfate.

Thus, you can't have two disconnected half-batteries.
 

Offline alancalverd

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Re: Batteries and electrons
« Reply #7 on: 15/10/2013 10:54:48 »
What happens to the electrons, do they go back into the battery?

Yes, but not the same ones in the first instance. Imagine the circuit as jammed full of free electrons. You stuff some in at one end, they all move along a bit, and some others come out at the other end. The drift rate of a single electron in copper wire is about walking speed, but the transmission rate (time between one electron entering the wire and another leaving it) is close to the speed of light.   
 

Offline colarris

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Re: Batteries and electrons
« Reply #8 on: 15/10/2013 13:47:42 »
What happens to the electrons, do they go back into the battery?

Yes, but not the same ones in the first instance. Imagine the circuit as jammed full of free electrons. You stuff some in at one end, they all move along a bit, and some others come out at the other end. The drift rate of a single electron in copper wire is about walking speed, but the transmission rate (time between one electron entering the wire and another leaving it) is close to the speed of light.   

 Wow, thanks for the info. I assumed that ALL the electrons moved as the speed of light! :)

Is it possible that the all the electrons produced by the battery won't even make it through to the end of the circuit during its life and in fact its just the electrons already in the wire that are 'pushed on' through it? *hope that makes sense!*
« Last Edit: 15/10/2013 13:54:58 by colarris »
 

Offline chris

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Re: What happens to electrons that leave batteries?
« Reply #9 on: 15/10/2013 14:45:46 »
Yes; the electrons drift along at a fairly low speed, meaning that, in even a short wire, electrons leaving the battery might not make it back to the battery before the circuit is disconnected. But because another electron further along the wire has re-entered the battery, there is no net charge difference between battery and wire; essentially it's a one-in, one-out effect.
 

Offline Bored chemist

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Re: What happens to electrons that leave batteries?
« Reply #10 on: 15/10/2013 21:51:19 »
It's more complicated. In a typical circuit the drift velocity of the electrons might be something like one meter each hour. On the other hand, that is superimposed on their random thermal motion which is of the order of 2000000 metres per second.
http://en.wikipedia.org/wiki/Drift_velocity#Numerical_example
 

Offline chris

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Re: What happens to electrons that leave batteries?
« Reply #11 on: 15/10/2013 22:59:44 »
Although, BC, thermal motion is unlikely to have a net direction assuming no net thermal gradient in the wire.
 

Offline Pmb

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Re: What happens to electrons that leave batteries?
« Reply #12 on: 15/10/2013 23:25:45 »
Although, BC, thermal motion is unlikely to have a net direction assuming no net thermal gradient in the wire.
When an electric field is applied like there is in a wire with a current in it, there is a net drift. This is not caused by thermal motion.
 

Offline Pmb

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Re: What happens to electrons that leave batteries?
« Reply #13 on: 15/10/2013 23:33:15 »
If batteries provide the 'force' to get electrons in a circuit moving what happens to them when they reach the end of the circuit? What if all the electrons in the circuit have reached the end of the circuit won't it have 'run out' of them? Does the battery actually provide electrons??  :o

As stated above the battery causes an electric field to be established inside the wire. This field causes a net electron drift along the wire. Electrons leave one and go back into the other terminal, just like water in a hose.
 

Offline Bored chemist

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Re: What happens to electrons that leave batteries?
« Reply #14 on: 16/10/2013 21:20:33 »
Although, BC, thermal motion is unlikely to have a net direction assuming no net thermal gradient in the wire.
I described the thermal motion as "random". Did you think I thought that it had any net direction?

Pete, I think (though I'm a long way from sure) that the field is set up in the space round the wire.
Certainly the speed at which a change in voltage moves down a wire is dependent on the properties of the insulation surrounding the conductors.
 

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Re: What happens to electrons that leave batteries?
« Reply #14 on: 16/10/2013 21:20:33 »

 

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