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Really basic electronic question

by sameeralord
Tags: basic, electronic
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sameeralord
#1
Apr11-08, 04:41 AM
P: 640
Hello guys

I got a small question with voltage and charge. If voltage provides potential energy for charges why doesn't the voltage decrease all the time when the charges are moving. I mean I only see voltage change when their are resistors and stuff but my question is shouldn't the voltage change all the time because the charges are losing potential energy as they are moving. Also can you explain potential difference to me. I'm bit confused with that. My electronic undertstanding is not really good and your help would be really appreciated
Thanks!!
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DavidWhitbeck
#2
Apr11-08, 07:33 AM
P: 352
You are implicitly holding a picture in your head of a conga line of electrons zipping through the circuit. However, the drift speed is very small. What really happens is the electromagnetic wave passes through disturbing electrons in it's wake. The mean free path is small, they will not move all the way through the wire (they won't get that far at all) before they scatter and lose their energy to molecular collisions (which is why circuits lose so much energy to heat).

I think that you should think more about what the wave is doing and less what the electrons are doing.
chroot
#3
Apr12-08, 11:40 AM
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Voltage is not potential energy. Charges do not gain voltage, nor do they lose voltage. Voltage is applied across a conductor in the same way that a pump creates pressure in a pipe. The electrons move as a result of this pressure.

Also, please note that real wires are not actually zero resistance, even though they are idealized in class. If you connect a wire across the terminals of a battery, a large current will flow, but the voltage will change smoothly from one end of the wire (1.5V, say) to the other (0V, say).

- Warren

tiny-tim
#4
Apr12-08, 12:45 PM
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Smile Really basic electronic question

Hi sameeralord ! Welcome to PF!

A battery is usually an exact voltage.

The potential difference between the thingies of a 1.5 volt battery stays at 1.5 volts.

Think of it like water flowing out of a pipe in a tank.

If the tank is always full (because it is supplied from another tank), then the pressure at the pipe exit will be constant and that's how batteries should behave.

But if the tank gradually empties, then the pressure will fall but batteries aren't generally like that!

Though I suppose it depends on how the voltage is being produced

erm have I got that right, guys?
chroot
#5
Apr12-08, 12:51 PM
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As you discharge a battery, its voltage will decrease. That's how the battery monitor on your cell phone knows when the battery is almost dead. Different battery chemistries have different profiles. Alkaline batteries being losing voltage from the time you begin using them, while lithium ion batteries often lose only very little voltage until they are almost completely depleted.

- Warren
LURCH
#6
Apr12-08, 01:02 PM
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This confused me a little bit...
Quote Quote by chroot View Post
Voltage is not potential energy. Charges do not gain voltage, nor do they lose voltage. Voltage is applied across a conductor in the same way that a pump creates pressure in a pipe. The electrons move as a result of this pressure.

- Warren
It starts by saying Voltage is not potential energy. But then it goes on to explain that voltage is like pressure in a pipe, which is potential energy. Now, voltage is often referred to as "electrical pressure," or "electrical potential." Please clarify; what do you mean by the statement that "Voltage is not potential energy"?
-Thanks
DavidWhitbeck
#7
Apr12-08, 06:47 PM
P: 352
Quote Quote by chroot View Post
Voltage is not potential energy. Charges do not gain voltage, nor do they lose voltage.
This really needs to be commented on. Voltage is electric potential difference. Electric potential is potential energy divided by charge. Voltage is a measure of potential energy, contrary to what you said.

Charges move because they are exposed to an electromagnetic force that accelerates them from rest. Not because they feel some kind of "pressure".

The fluids analogy is only an analogy and is not supposed to replace the correct explanation.
sameeralord
#8
Apr12-08, 07:18 PM
P: 640
Thanks everyone who replied.. Thanks for the welcome as well. I like the pressure idea it makes it easier to understand. I was all this time thinking of potential energy turning into kinetic energy lol. Thanks once again for everyone who helped
DavidWhitbeck
#9
Apr12-08, 07:31 PM
P: 352
No wait stop! You were right to think that, because it's true!

Batteries supply electric fields that accelerate electrons, that is they move to lower electric potential energy (and higher potential difference or voltage) transforming that energy into kinetic to conserve total energy. What you said is completely correct.

The pressure idea is only an analogy to use for high school students because the theory is too abstract for them to immediately grasp.
sameeralord
#10
Apr12-08, 07:42 PM
P: 640
Quote Quote by DavidWhitbeck View Post
No wait stop! You were right to think that, because it's true!

Batteries supply electric fields that accelerate electrons, that is they move to lower electric potential energy (and higher potential difference or voltage) transforming that energy into kinetic to conserve total energy. What you said is completely correct.

The pressure idea is only an analogy to use for high school students because the theory is too abstract for them to immediately grasp.
Well then I'm back where I started. Shouldn't the volatage change all the time then. I'm confused now

EDIT: Oh I reread your first post. It does makes sense. So then what does the resisotor. Does it decrease the current
DavidWhitbeck
#11
Apr12-08, 08:21 PM
P: 352
No the voltage doesn't change all the time.

The change in the electric potential energy of an electron is due to the voltage difference of the battery, which does not change. They might be mathematically proportional to each other, but physically one causes the other but not the other way around.

It's the concept that you're confused about so let's step back and consider an electron in between a parallel plate capacitor instead--

You have two big sheets, one charged - and the other +. An electron in between the plates would be attracted towards the + sheet (and be repelled from the - sheet) and move towards the + sheet right? That is because of the electric force on the charge.

Now the electron accelerates because the electric field produced by the capacitor is doing work on it. So the potential energy of the electron changes with time. *BUT THE VOLTAGE DIFFERENCE ACROSS THE CAPACITOR REMAINS CONSTANT.* Why? The electric field the plates exert on the electron and thus the voltage difference had nothing to do with the electron.

And if the electron was not there, that electric field would still be there.
Nabeshin
#12
Apr12-08, 08:37 PM
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Voltage is strictly a spacial phenomenon. It is not a noun. There exists a point in space on one end of the battery at a potential of, say, 9V with respect to the other end of the battery. Again, this is a spacial phenomenon and is reduced when charge moves to equalize this potential. However, the chemicals inside a battery work to restore the original potential difference.


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