Fri 27 Feb 2009
I just got this question from a teacher on Webconnect (which lets teachers ask science questions):
“In the past when I taught electricity I always understood that it flows from the negative terminal to the positive. The CPO books and materials have the opposite – from positive to negative. This doesn’t make sense to me in how you generate the flow of electrons, pulling to the opposite charge. Is the book wrong or have I forgotten stuff? 8th grade teacher”
It depends on what you define as “electricity”. Do you mean the flow of “electrons” or the flow of “current”? Because, due to an unfortunate quirk of history, the direction of *current* flow is opposite to the direction of *electron* flow. Take a moment and re-read that, because it’s not what you would expect. If electrons are flowing to the right across this screen, then we say that current is flowing to the left.
So, let’s say that the left hand side of this screen is the positive terminal and the right hand side is the negative terminal
+ -
*Electrons* will flow towards the opposite charge, as you say. That’s which direction? Right to left
<—- electrons
But *current* is the opposite direction. Left to right.
—-> current
So *current* does flow from positive to negative, like your books say. And electrons do get pulled towards the negative charge, like you say. But we define electric current to be the opposite direction of electron flow.
There’s some good history on why it’s defined this way, but I’m too busy to find it right now — if someone has a good link, stick it in the comments, thanks!
UPDATE 4/27
Here’s a relevant comic from xkcd
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16 Responses to “Electron flow vs. current flow”
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Kenneth Finnegan
Posted: Feb 27th, 2009 at 8:15 pm1Reply to this comment.Another thing to note is that the speed of current is dependent on the speed of the electric field, which is C (3E8 m/s), where the speed of the electrons is based on the current density (current per area of wire). The “drift velocity” of the electrons ends up being mind-blowingly slow: normal household values run in the range of 0.15 mm/s.
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Stacy Kissel
Posted: Mar 7th, 2009 at 1:51 am2Reply to this comment.We can blame Ben Franklin for this one. He coined the terms positive and negative and named the charges that move from object to object positive. It wasn’t until much later that we discovered electrons are what move in solids. See http://www.allaboutcircuits.com/vol_1/chpt_1/7.html Students get a kick out of learning that our definition of current in circuits is backward! By the way, I happen to be one of the CPO writers.
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sciencegeekgirl
Posted: Mar 8th, 2009 at 3:58 pm3Reply to this comment.Thanks, Stacy, for the history note!
By the way, I happen to be one of the CPO writers.
I’ve been wracking my brain and am pulling a blank — what does CPO stand for?
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sciencegeekgirl
Posted: Mar 10th, 2009 at 5:08 am4Reply to this comment.And Chemgeek sent me another history link from NASA about Franklin and the twisted route of history: http://www-istp.gsfc.nasa.gov/Education/woppos.html
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David E Darragh
Posted: Mar 20th, 2009 at 8:17 pm5Reply to this comment.Okay, now, in welding, we can change polarity. DCEN is referred to as “straight polarity.” (work positive, electrode negative) DCEP, or “reverse polarity” is work piece negative, electrode positve. Which direction(s) are the electrons and current flowing? Now, I’m really confused.
Dave
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sciencegeekgirl
Posted: Mar 25th, 2009 at 4:58 pm6Reply to this comment.Okay, now, in welding, we can change polarity. DCEN is referred to as “straight polarity.” (work positive, electrode negative) DCEP, or “reverse polarity” is work piece negative, electrode positve. Which direction(s) are the electrons and current flowing?
Dave
The direction would depend on the polarity. Electrons always go from negative to positive (and current from positive to negative). If you’ve “reversed polarity” then all you’ve done is switch + and -, and then the electrons (and current) will switch direction. In my fancy schmancy diagram in the post above, that would amount to switching the + and -, so electrons will now flow to the right.
In other words, in DCEN your electrons are flowing away from the electrode (and current towards the electrode) and in DCEP the electrons are flowing towards the electrode (and current away from the electrode).
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chetan kumar
Posted: Dec 7th, 2009 at 9:12 am7Reply to this comment.we can that current flows from positive to negative because of current flows from higher potential to lower potential so that we that current flows from positive to negative.
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chetan kumar
Posted: Dec 7th, 2009 at 9:14 am8Reply to this comment.we can that current flows from positive to negative because of current flows from higher potential to lower potential so that we may say that current flows from positive to negative.
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swati
Posted: Jan 8th, 2010 at 8:34 am9Reply to this comment.Thanks, that was very useful.
I got confused too!
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siddharth
Posted: Feb 19th, 2010 at 12:57 pm10Reply to this comment.actually it is a just a game of energy….
look….we know opposite charges cant stay apart ..this is wat was founded naturally…so if ++++++ (–>>>>>>>) ——-
charges r separated…so assume potential diff between.these be ‘V’..after some time say ‘t’situation will be..+++ (–>>>>>) — … now potential has dropped to say ‘v’..
beacuse most of -ve electrons have gone to +ve side..to decrease the energy of system..(that is the law..they try to come to each other to decrease the energy of system ..of each other..)…so to derive a mathematical equation for this..we gave a convention first that +ve side will be treated as higher potential and -ve side low potential….so v(+)-v(-) is a +ve quantity…and this difference is found to be proportional to resistance and flow of current…now resistance is +ve thing(because hindrance to flow is +ve thing)…..so current is given +ve thing…
so when we go from…v(+) to v(-)……we consider something going from ++ (–>>>>) –….so in an eq.. both sides must be +ve…thats y a convention was set that current flows from ++ to — direction..it was just a convention set up during its invention..so that w can study it mathematically…and after that every thin was set according to this convention…every law is studied lik that…books were written in this way…so mits just a convention…and its just an abstract thing….ok…got the point….just a convention…nothin real..in it…in reality its just the electron which flow nothin else…. -
siddharth
Posted: Feb 19th, 2010 at 1:03 pm11Reply to this comment.we want the equation….v(+)-v(-).. be +ve must be +ve thing…so in an equation….we relate it with product of resistance(a +ve thing) and cuurent(must be +ve)…so current is given convention from ++ terminal to — terminal….
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Bellatrix Bly
Posted: Mar 1st, 2010 at 9:38 pm12Reply to this comment.Ohmigawd, thanks! I’ve been having a bad day, and science class didn’t make it any better, so I think you’ve just saved my sanity!
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Jojo Bizarro
Posted: Jun 17th, 2010 at 5:23 am13Reply to this comment.Given that electrons flow from negative to positive, and that electrons are only things that move along the entire current path, it should make perfect sense that electric current flows from negative to positive. We say that the Danube River flows eastward out of Germany and into the Black Sea because the carriers of that flow, water molecules, go that way. To say that the Danube flows westward out of the Black Sea and into Germany sounds downright absurd, and to say that current flows from positive to negative sounds equally idiotic. Whenever I see current arrows going from positive to negative on a diagram, I can’t help but think that whoever drew them is ignorant.
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Anurag Mishra
Posted: Jun 17th, 2010 at 2:16 pm14Reply to this comment.Okay, now, in welding, we can change polarity. DCEN is referred to as “straight polarity.” (work positive, electrode negative) DCEP, or “reverse polarity” is work piece negative, electrode positve. Which direction(s) are the electrons and current flowing?
DaveThe direction would depend on the polarity. Electrons always go from negative to positive (and current from positive to negative). If you’ve “reversed polarity” then all you’ve done is switch + and -, and then the electrons (and current) will switch direction. In my fancy schmancy diagram in the post above, that would amount to switching the + and -, so electrons will now flow to the right.
In other words, in DCEN your electrons are flowing away from the electrode (and current towards the electrode) and in DCEP the electrons are flowing towards the electrode (and current away from the electrode).Well, this is a simple fundamental why DCEP is called reverse polarity and DCEN is called straight polarity. In the case of DCEN, the electrons are flowing from the electrode to the workpiece being welded and the flow of weld metal is also from the electrode (or filler wire) to the workpiece, hence straight polarity. In the case of DCEP, flow of electrons is from the workpiece to the electrode while the flow of weld metal is from the electrode (or filler wire) to the workpiece, hence reverse polarity. Further approximately 70% of the heat is generated at the positive side. This is the reason why we get faster welding while using DCEP.
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Anurag Mishra
Posted: Jun 17th, 2010 at 2:31 pm15Reply to this comment.I do agree to Siddharth. A current is defined as the flow, be it a flow of river or flow of electrons! Whenever there is difference in the quantum of charge, there will be a resultant flow from the higher side to lower side. Ofcourse, after overcoming the barriers. (Imagine the case of lightening, there is always charge in the clouds, but lightening occurs only when it has sufficient potential difference to overcome the resistance provided by the atmosphere) So, if there is an accumulation of 10000+ charges at one side and 100+ on the other side, there will be a flow from the 10000 side to 100 side. This was the thing taken into consideration when the current was defined. Further, there is nothing in this world so-called NON CONDUCTOR. A material may be called as a BAD CONDUCTOR of electricity, but it cannot be called as NON CONDUCTOR. These are called bad conductors because they provide resistance to the current flow and that resistance has a limit, that is, the material can withstand certain amount of potential difference until it gives way. That is why for working on very high voltage equipment we need thicker rubber mat to stand on, rather than a thinner rubber mat which would have been sufficient for low voltage. Another thing, every material becomes a super-conductor at absolute zero, that is 0 Kelvin. This means that at zero Kelvin, there is no resistance to the flow of electrons.
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sciencegeekgirl
Posted: Jun 17th, 2010 at 3:41 pm16Reply to this comment.Given that electrons flow from negative to positive, and that electrons are only things that move along the entire current path, it should make perfect sense that electric current flows from negative to positive…. and to say that current flows from positive to negative sounds equally idiotic. Whenever I see current arrows going from positive to negative on a diagram, I can’t help but think that whoever drew them is ignorant.
You’re calling all physicists ignorant?
Unfortunately, even though it seems to make so much sense that current should go from negative to positive, since that’s the direction of electron flow, the convention that current flows from positive to negative was created by Franklin, long before we knew what the charge carriers were (or, in your example, before we could see the water molecules in the Danube — or, even, the direction of the flow of the water. All we knew was that the “Danube” would turn a flywheel, but didn’t know which way it was moving to do so). So, even though it’s counter-intuitive, we’re stuck with the convention. The way that current is shown on current diagrams is opposite to the direction of electron flow. It’s a quirk of history, rather than that we don’t know what we’re doing.
