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Unread 05-14-2004, 07:32 PM
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A question about current....

Current flows from positive to negative and is not the flow of electrons right? Its the flow of positive charge. Im so confused. I have two books that sort of give conflicting information.
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Unread 05-14-2004, 07:35 PM
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Re: A question about current....

Quote:
Originally Posted by Adam Y.
Current flows from positive to negative and is not the flow of electrons right? Its the flow of positive charge. Im so confused. I have two books that sort of give conflicting information.
Conventional current is in the direction that positive charges would flow. "Real" current can be either the flow of positive or negative charges. Remember, sometimes its ions like (Ag+) that are positively charged and are flowing. So, the information is not conflicting, as long as negative current is opposite than positive current.
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Unread 05-14-2004, 07:43 PM
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Re: A question about current....

Like silverstar said, Conventional current is the idea that current flows from positive to negitive.

In reality, charge flows from negitive to positive (electrons being negitively charged and all). But, for many reasons, it is easier to think of it the otherway. I found this extreamly confusing at first to. I couldn't figure out why my multimeter was giving me negitive voltages for everything.

*sigh* conventions can be so tiresome.

-Andy A.
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Unread 05-14-2004, 07:46 PM
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Re: A question about current....

Quote:
Originally Posted by Andy A.
Like silverstar said, Conventional current is the idea that current flows from positive to negitive.

In reality, charge flows from negitive to positive (electrons being negitively charged and all). But, for many reasons, it is easier to think of it the otherway. I found this extreamly confusing at first to. I couldn't figure out why my multimeter was giving me negitive voltages for everything.

*sigh* conventions can be so tiresome.

-Andy A.
No, in reality, charge flows from positive to negative or negative to positive (see my explanation in the post above).
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Unread 05-14-2004, 08:36 PM
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Re: A question about current....

Quote:
Originally Posted by SilverStar
Conventional current is in the direction that positive charges would flow. "Real" current can be either the flow of positive or negative charges. Remember, sometimes its ions like (Ag+) that are positively charged and are flowing. So, the information is not conflicting, as long as negative current is opposite than positive current.
hmmm, that brings up an interesting question. Does current flow the same way regardless of wether ions or electrons are carrying the charge? ie, are there ion resistivity constants for each ion and medium material (the "wire") that can be used to calculate resistances and such? Will current flow at (approximately) the speed of light as with electrons (i'd think so)?
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Unread 05-14-2004, 09:00 PM
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Re: A question about current....

I remember we spent a week on this topic in physics class, but that was 3 years ago so I don't remember too much of it. What I do remember is the teacher explaining the differences between conventional current and actual current...

Conventional current is what most people recognize- electricity flowing from positive to negative, as it makes logical sense- higher concentrations of things/surplus (positive) moving towards a lower concentration/deficit (negative) to balance out.

On a technical level though, the actual charge is negative- as electrons are generally negatively charged. Electrons are much much less massive than protons, and hang out on the fringes of the atom, and so are much easier to toss around. Therefore, it's actually the negative charge that's traveling through the wire.

If that makes no sense, or is completely wrong, please correct me... this sort of thing has always fascinated me.
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Unread 05-14-2004, 09:06 PM
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Re: A question about current....

ACTUALLY, in a galvanic or electrolytic cell (yeah for AP Chem + AP Physics) the positive ions flow also.

Current flows in opposite directions for positive ions than for electrons. Current never travels even close to the speed of light (remember, current is in Columbs per second, not meters per second, so that's an incongruous dimensional comparison). EMF travels close to the speed of light. And yes, you can find resistance using the following equation:
R=rho * length/ Area
Just replace "rho" with the resistivity of the ion/wire combo.
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Last edited by SilverStar : 05-15-2004 at 02:32 PM.
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Unread 05-14-2004, 09:38 PM
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Re: A question about current....

Yes, you are right, i should have said emf instead of current, that is what i really meant. I always find it kind of surprising how slow electrons actually travel through a wire. (on the order of mm/s, if im not mistaken)

here's a quick little explanation
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Last edited by Max Lobovsky : 05-14-2004 at 09:42 PM.
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Unread 05-16-2004, 03:00 AM
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Re: A question about current....

Hello All,
There are generally two conventions for electrical current. "Electron Current" follows the flow of electrons that emit from the negative terminal of the battery and are attracted to the positive terminal. This current is more easily understood in terms of vacuum tubes where electrons are generated at the cathode and flow to the plate. The plate is always more positive than the cathode when current is flowing. Unfortunately, meters that measure current flow, indicate the opposite polarity when measuring current compared to electron flow. Most electrical people are referring to "conventional current" when they speak about current flow. This current appears to flow from the positive terminal to the negative terminal of the battery but is in fact electron current. An ammeter with the positive lead connected to the positive terminal of a battery (when inserted into a working circuit) will show positive current.
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Unread 05-16-2004, 03:37 PM
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Re: A question about current....

Quote:
Originally Posted by SilverStar
ACTUALLY, in a galvanic or electrolytic cell (yeah for AP Chem + AP Physics) the positive ions flow also.

Current flows in opposite directions for positive ions than for electrons. Current never travels even close to the speed of light (remember, current is in Columbs per second, not meters per second, so that's an incongruous dimensional comparison). EMF travels close to the speed of light. And yes, you can find resistance using the following equation:
R=rho * length/ Area
Just replace "rho" with the resistivity of the ion/wire combo.
Calarification for my earlier post (above):

The formula for resistance I gave is only applicable for metal wires. In metal wires you'll probably never have positive ions moving. Since pos. ions can only move in solutions, gel, and salt bridges, there is no valid "length" or "area" of the solution.
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Unread 05-17-2004, 07:53 AM
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Re: A question about current....

Ok its time for a little historical honesty in this thread. The concepts for voltage, current, power... were all standardized on before we really knew about electrons and the subatomic particles.

I dont know who established them, but we had things like batteries and static charge generators long before we knew that electrons even existed. Its impossible to look at an arc of electricity and see which way its flowing, so at some point some scientist called one end of a battery the positive side, and using magnets to sense current flow, using the N pole of the earth as the basis for which end of a magent is the North end, we ended up with amp meters that would deflect to the right when connected to the "postive" side of a battery

so all this equipent started to be manufactured, the conventions were already in place, and it wasnt until much later that the structure of atoms was theorized and verified that we learned that, well.... we got it backwards

in 99.999% of the cases, current is the movement of electrons, and they move from - to +. It was too late to change it.

BTW current and voltage travel at the speed of light - when you connect a voltage source to a long wire you dont have to wait for the 1st electron to make it all the way to the other end before current flows - its not like an empty hose

its like a full hose. Current responds at the speed of light, the individual electrons however do not travel down the wire at that speed.
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Unread 05-17-2004, 09:24 AM
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Re: A question about current....

Actually, current moves quite slowly... It's electromagnetic radiation that moves at the speed of light (radio waves, light waves, microwaves, etc).

Electricity is very analagous to water in a hose. Only difference is wires are not empty like a hose would be- the metals inside are chock full o' electrons (water) already. When electricity is applied, electrons sitting there are just pushed through the wire (as water would be if a hose were already filled), giving the instant response.

This site has some good info on how fast current actually moves.
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Unread 05-17-2004, 01:27 PM
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Re: A question about current....

Quote:
so all this equipent started to be manufactured, the conventions were already in place, and it wasnt until much later that the structure of atoms was theorized and verified that we learned that, well.... we got it backwards
Franklin was the person that invented the convention of positive to negative. Joseph Thomson discovered that it was backwards. Unfortunately, formulas by that time had been using Franklin's standard.
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Unread 05-18-2004, 01:35 PM
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Re: A question about current....

Quote:
Originally Posted by SilverStar
ACTUALLY, in a galvanic or electrolytic cell (yeah for AP Chem + AP Physics) the positive ions flow also.

Current flows in opposite directions for positive ions than for electrons. Current never travels even close to the speed of light (remember, current is in Columbs per second, not meters per second, so that's an incongruous dimensional comparison). EMF travels close to the speed of light. And yes, you can find resistance using the following equation:
R=rho * length/ Area
Just replace "rho" with the resistivity of the ion/wire combo.

Okay, so can someone explain a bolt of lightning to me?

-dave
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Unread 05-18-2004, 01:44 PM
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Re: A question about current....

Quote:
Originally Posted by dlavery
Okay, so can someone explain a bolt of lightning to me?

-dave
I'm not quite sure, but i think it's the flow of electrons from earth to the sky.
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