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Problems with voltage

  • #1
Another question I have and i noticed my teacher didn't know how to answer is...


if you have a battery of say 12v, i understand that this battery does 12 joules of work per coulomb to move and separate the electrons.

now if the electrons are moved from one pole to the other, this creates the current right/?

Now i know the electrons will be attracted to the positive side of the battery right? but why do they travel all the way through the wire until they reach the positive side, instead of acumulating themselves on the negative pole, since the positive pole is much closer to them, my intuition tells me that they would try to reach the positive side because its very close to them, and so they woould accumulate.

I dont get this, please help me?
 

Answers and Replies

  • #2
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They travel that way because thats the direction of the electric field and that is what pushes them.
 
  • #3
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In a battery chemical energy is converted into electrical energy; current flows NOT because of an initial electric field but because different materials immersed in electrolytes either give up or absorb ions. Its essentially a chemical process that we use to make electricity.

The electric potential (energy) results from chemical energy conversion. The ions in the electrolyte migrate as they do because of chemistry...they cannot move the other way inside the battery because chemical energy opposes it.


For example, if you immerse zinc (which becomes a cathode) in salt water (a weak electrolyte) and connect it via wire to another material, say a bronze shaft or propeller, the zinc corrodes in such a way that it prevents the bronze from deterioriating....via a flow of ions in the electrolyte and electrons in the wire.....that type of "battery" is called cathodic protection. It's widely used to protect valuable underwater metals...in boats in my example.


Maybe a rough, very rough analogy, would be "why does iron rust?" Why can't we make air rust? Because the natural chemical process causes oxygen to combine with iron to form iron oxide; there is not a natural reverse process under normal conditions.

You can read more here for another type battery:

http://en.wikipedia.org/wiki/Battery_(electricity)#Wet_cell
 
  • #4
Redbelly98
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To put it simply:

The chemical reaction occuring at the negative terminal produces excess electrons. The reaction at the positive terminal consumes electrons. This creates an electric field that pushes electrons from the negative to the positive terminal.
 
  • #5
To put it simply:

The chemical reaction occuring at the negative terminal produces excess electrons. The reaction at the positive terminal consumes electrons. This creates an electric field that pushes electrons from the negative to the positive terminal.
right but if you have a wire around the earth, the electrons will choose to travel all the way, instead of accumulating on the negative terminal since the positive terminal is just next to the negative and the electric field there should be stronger and attract the electrons all the way back through the battery.

Why is it not the case?
 
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  • #6
Matterwave
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Er, the electrons in the wire don't actually travel across the wire all that much. The drift velocity is like less than a millimeter a second. The build up of charges puts a net electric field in the wire which makes the charges in the wire acquire a net average velocity in one direction, but that velocity is pretty small.

So, you shouldn't really be thinking that the electrons actually go around the wire from the negative node to the positive node.
 
  • #7
Er, the electrons in the wire don't actually travel across the wire all that much. The drift velocity is like less than a millimeter a second. The build up of charges puts a net electric field in the wire which makes the charges in the wire acquire a net average velocity in one direction, but that velocity is pretty small.

So, you shouldn't really be thinking that the electrons actually go around the wire from the negative node to the positive node.
Wow but my teacher told us that the electrons travel at almost the speed of light in the wire, if current is DC
 
  • #8
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Your teacher is wrong , the electrons travel very slowly in the wire , but because they have an electric field associated with them , when we move the first electron its electric field pushes the one in front of it an so on . So the electrical interaction of the electrons travels pretty quick . Also maybe read about the Drude model it talks about how the electrons move in the material .
 
  • #9
Redbelly98
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Wow but my teacher told us that the electrons travel at almost the speed of light in the wire, if current is DC
Electrical signals, that is changes in the electric field or voltage, do travel at close to the speed of light, even though the electrons don't. You teacher either misspoke -- or was misheard :smile:
 
  • #10
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Wow but my teacher told us that the electrons travel at almost the speed of light in the wire, if current is DC
You can print this explanation and bring it to class to clarify DRIFT VELOCITY with your teacher:

http://en.wikipedia.org/wiki/Electric_current#Drift_speed

Another way to think about this when a battery is the source would be to note that ions (charged particles) in an electrolyte must move through that electrolyte.....as a result of chemical reactions......not easy to do at lightspeed!!!!!
 
  • #11
To put it simply:

The chemical reaction occuring at the negative terminal produces excess electrons. The reaction at the positive terminal consumes electrons. This creates an electric field that pushes electrons from the negative to the positive terminal.


Nice, I get that.

But think about this, why do the electrons travel all the way through the wire to reach the other end, why don't they kind of accumulate at the negative end of the battery, since the positive pole, which has a lack of electrons would attract those electrons ?
 
  • #12
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But think about this, why do the electrons travel all the way through the wire to reach the other end
Inside the battery, the concentrations will matter too. Because of the electric field, an ion may want to travel in one direction, but because of a concentration gradient it may want to go in the opposite direction.

It's a bit like the atmosphere. Gravity acts on every molecules in the air, but that doesn't mean that the air falls flat on the ground. All that happens is that the air becomes a little denser the nearer the ground it is, because the gravitational forces are countered by the pressure that builds up when the concentration of air molecules increases. The gravitation forcing the air down is balanced by a pressure gradient forcing air up.

Inside the battery the electric forces will compete with concentration gradients, and that's why the charges end up going the "wrong way" in the electric field inside the battery. The chemical reactions forces positive ions up to a higher potential, and negative ions to a lower potential, and because of that, the battery can be used as an energy source.
 
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  • #13
Inside the battery, the concentrations will matter too. Because of the electric field, an ion may want to travel in one direction, but because of a concentration gradient it may want to go in the opposite direction.

It's a bit like the atmosphere. Gravity acts on every molecules in the air, but that doesn't mean that the air falls flat on the ground. All that happens is that the air becomes a little denser the nearer the ground it is, because the gravitational forces are countered by the pressure that builds up when the concentration of air molecules increases. The gravitation forcing the air down is balanced by a pressure gradient forcing air up.

Inside the battery the electric forces will compete with concentration gradients, and that's why the charges end up going the "wrong way" in the electric field inside the battery. The chemical reactions forces positive ions up to a higher potential, and negative ions to a lower potential, and because of that, the battery can be used as an energy source.


Thank you very much for your reply.

What so you mean by concentrations or gradient?

Do you mean that on the negative side there are so many electrons that they repulse each other and end up going through the longest path?


If you extend the wire around the earth the current would still go through this longest path wouldnt it? I thought so because it travels all the way from the electric generator to our houses.

Man it's so complicated, i wonder how ever man has managed to understand all this, and sometimes i even may think that we may have understood it only superficially to some extent.
 
  • #14
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If you extend the wire around the earth the current would still go through this longest path wouldnt it?
No single electron will travel around the earth on this wire.

Picture the electrons all lined up touching each other around the earth like billiard balls... these billiard balls extend from the positive pole... all the way around the earth... and end up at the negative pole.

Now if you tap the ball on the (say) negative pole, the energy is transferred all the way through all of the billiard balls and the one on the positive pole moves.

The electrons barely moved, but the energy transferred all the way through to the other end and we measured a current...

Does that help?
 
  • #15
No single electron will travel around the earth on this wire.

Picture the electrons all lined up touching each other around the earth like billiard balls... these billiard balls extend from the positive pole... all the way around the earth... and end up at the negative pole.

Now if you tap the ball on the (say) negative pole, the energy is transferred all the way through all of the billiard balls and the one on the positive pole moves.

The electrons barely moved, but the energy transferred all the way through to the other end and we measured a current...

Does that help?
that completely makes sense, but wow, each person tells me one thing... what about the electric field which pulls the electrons? :S
 
  • #16
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The electric field is what "taps" the ball on the negative pole in the billiard example. Instead of someone tapping it (with force from your hand), it was pulled there by the attraction of the charges. That is the force that moved it...

Now I think what you really want to know is why they don't attract inside of the battery instead of outside of the battery. That is more of a chemistry problem, which some of the above posts try to address... and which I know very little about.
 

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