Why do electrons flow in a wire?

[aychamo]

So, I guess I have two questions :)

The first, is why does electricity flow? I mean, what makes it go from one part of a wire to another?

And the second, do electrons flow "in" the wire, or on the surface of the wire?

 

[Integral]

Electrons are charged particles therefore they experience a force when in the presence of an electric field. It is this force which creates the flow in a conductor.

All free charge must reside on the surface of conductors, thus all free electrons are on the surface, thus the current flow is surface charge. There may well be more to it then that.

 

[aychamo]

What creates the electric field? I remember something from my second physics class about electric fields and the right hand rule. Is this what makes electrons flow form - to +?

 

[Gokul43201]

The electric field itself is created by a separation of charge - eg : electrodes of a battery. The field is created in a direction "generally" pointing away from the +ve electrode and towards the -ve electrode. So, if you dropped a positive charge into the region of this field, it would get pushed in that direction, ie. towards the -ve electrode. But electrons are -vely charged. So they are pushed in the opposite direction, ie. towards the +ve electrode.

This has nothing to do with the right hand rule, which is used when you throw a conductor into a magnetic field.

 

[JohnDubYa]

Electricty flows because one area of a circuit has too many electrons, and another area of the circuit doesn't have enough. Since electrons repel, they will travel from the former area to the latter. Their motion is called current.

Invoking electric fields to explain current is not going to help those that are struggling with fundamental concepts, in my opinion.

 

[jdavel]

Integral said, "All free charge must reside on the surface of conductors, thus all free electrons are on the surface"

Yikes! Want to try that again?

 

[Gza]

Free charge only resides on the surface of a conductor in the absence of an internal electric field(since the charges themselves arrange their configuration to make this true.) When free charge is moving through a wire, there is for sure an electric field acting, and we can't really say where the charge is at any time, except its in the wire moving. To get back to the question

Invoking electric fields to explain current is not going to help those that are struggling with fundamental concepts, in my opinion.

To me at least, the electric field is the fundamental concept. From integrating this field on a distance, we obtain the electric potential. If there is a difference in potential across two points in space, there is an electric field in between, and we call this difference the voltage. Now since there is a voltage across the + and - terminals of the battery, when this battery is connected to, say, a wire, what do you think will go on, based on what I said above?(aside from killing your battery)

 

[JohnDubYa]

Gza, I am not saying you are incorrect. But the electric field IS an abstract notion that a person struggling with the very fundamentals of electromagnetism is going to have a hard time understanding.

Exactly what IS an electric field? Try explaining that to the average Joe. If you revert to using the concept of charge attraction and repulsion (which the average Joe CAN understand), then why not just use those concepts and refrain from mentioning the field altogether?

Let me provide a simpler example.

Suppose you have a positively charged ion. And suppose nearby there is a free electron. You could invoke an electric-field explanation as to why the electron will migrate to the ion. You can say that the ion creates an electric field at the position of the electron, and the electron interacts with this field.

But do we have to make it so complicated? Why not just say that the electron is attracted to the ion because opposite charges attract? To me, that notion is every bit as fundamental as the electric field, and a Helluva lot easier to understand.

Besides, we created the electric field to provide a mechanism for determining the force acting on the electron. As far as introductory physics is concerned, it is a purely man-made construct, not a fundamental principle. I am not sure anyone has even settled on whether it really exists or is just purely mathematical. In the region between the two charges, is there really something there physically? I think the philosophers have to try and answer that question.

 

[krab]

thus the current flow is surface charge. There may well be more to it then that.

That would predict that the conductance of a wire is proportional to its radius. It's not; it's proportional to radius squared, or area. Current is confined to the surface in the case of high frequencies, when skin depth is small compared with radius. But for DC, current flows throughout the wire. Current is not after all excess charge; the wire is still neutral.

 

[krab]

What creates the electric field? I remember something from my second physics class about electric fields and the right hand rule. Is this what makes electrons flow form - to +?

+ and - in a sense is the electric field. They reflect the places where electric potential is resp. more positive and more negative. A potential difference from place to place is the same as saying there is an electric field.

 

[Math Is Hard]

hi - I am kinda barging in here but...
why does the current move in the opposite direction that the electrons are moving? that's what my book states, but it doesn't explain why. thanks!

 

[TALewis]

hi - I am kinda barging in here but...
why does the current move in the opposite direction that the electrons are moving? that's what my book states, but it doesn't explain why. thanks!

The *convention* for the direction of positive current corresponds to the direction of the flow of positive charges. However, in metal, the positive charges (protons), are fixed in place and cannot move. Only negative charges (electrons) are free to move. So, if electrons are inclined to move in one direction, protons would be inclined to move in the opposite. That's why, according to *convention,* current is opposite electron flow.

 

[Gza]

why does the current move in the opposite direction that the electrons are moving? that's what my book states, but it doesn't explain why. thanks!

The physics is the same reguardless of who is carrying the charge. Negative charge moving forward is completely analogous to positive charge moving backwards. When this stuff was being figured out, they had to go with one of the two, and guessed it was positive charge moving. It wasn't until the discovery of the Hall effect that we found out it was electrons that carry the charge. Funny thing is, that doesn't even matter when it comes to analyzing the behavior of a current in a circuit.

 

[Math Is Hard]

thanks, TA!

 

[Math Is Hard]

and thanks, Gza!

 

[JohnDubYa]

Essentially, the problem comes from the triboelectric chart. When two materials are rubbed together, one accumulates one type of charge, the other accumulates the oppositve type of charge. Unfortunately, when they decided to assign one of the materials as positive, they got it wrong. It turns out that the material they assigned as positive was the one that LOST the electrons. So when it was later discovered that the electrons are the particles that actually move during the process, it was too late.

In other words, science called heads, and the coin turned up tails. And we've been stuck ever since.

At least that is how I understand the story.

 

[Integral]

Integral said, "All free charge must reside on the surface of conductors, thus all free electrons are on the surface"

Yikes! Want to try that again?

Opps! I should know better then trying to make a quick post at the end a 12hr shift!

I like JohnDubYa's excess electron explanation.

When I learned circuits, by the US Navy,we were taught electron flow as the main current carrier, it is not clear to me why academia insists on positive current flow.

 

[ZapperZ]

When I learned circuits, by the US Navy,we were taught electron flow as the main current carrier, it is not clear to me why academia insists on positive current flow.

Does that mean then that (i) if I have a stream of protons moving in a particular direction, then the US Navy's definition of "current" points in the opposite direction of the proton flow, and (ii) the Navy redefined Maxwell equations, in particular, Ampere's Law, with an extra negative sign for the curl of B?

Zz.

 

[Doc Al]

nit pick?

That would predict that the conductance of a wire is proportional to its radius. It's not; it's proportional to radius squared, or area. Current is confined to the surface in the case of high frequencies, when skin depth is small compared with radius. But for DC, current flows throughout the wire. Current is not after all excess charge; the wire is still neutral.

I agree with everything said, except for a slight nit with with the last statement. I believe that when a wire is connected to a battery a non-uniform surface charge is created along the wire. It is this non-uniform surface charge that creates the electric field inside the wire that drives the current. The amount of surface charge needed is extremely small. Thus I think that the wire is very slightly charged. If I'm wrong, please correct me.

 

[Gokul43201]

hi - I am kinda barging in here but...
why does the current move in the opposite direction that the electrons are moving? that's what my book states, but it doesn't explain why. thanks!

Hmmm...so that really was your "borderline stupid quetion", eh ?

 

[jdavel]

Doc Al said, "Thus I think that the wire is very slightly charged. If I'm wrong, please correct me."

No, I think you're right on all counts. In fact, you can put a static charge on a wire by just touching it with one end of a battery and then removing it. Ignoring leakage, the wire now has a (very small) net charge.

 

[Math Is Hard]

Hmmm...so that really was your "borderline stupid quetion", eh ?

And I was quite satisfied with Motai's explanation of electrons swimming upstream against the current like salmon!

 

[Gza]

it is not clear to me why academia insists on positive current flow.

If I'm correct, this is done arbitrarily. The symmetry of Maxwell's equations under all transformations must be preserved, since nature doesn't insist on manmade coordinate systems. (who's to say what's left, right, +, -)? When getting down to the nitty gritty of solving the current in a ciruit, yes, by all means pick a coordinate system now and stick with it, making sure you remain consistent throughout the problem. I guess physicists just have a bright outlook on life, and chose positive to be the charge carriers. Some engineers I speak to who freak out about me talking about positive charge moving through wires demonstrate that they don't undertand the above-mentioned symmetry. Which is why I'm a physicist, and they're an engineer.

 

[BoulderHead]

I understood this confusion was traceable back to Benjamin Franklin, who simply took a guess that current flow was from positive to negative. Later, when it was determined to be otherwise the “damage” had already been done.

 

[robphy]

Electricty flows because one area of a circuit has too many electrons, and another area of the circuit doesn't have enough. Since electrons repel, they will travel from the former area to the latter. Their motion is called current.

Invoking electric fields to explain current is not going to help those that are struggling with fundamental concepts, in my opinion.

Besides, we created the electric field to provide a mechanism for determining the force acting on the electron. As far as introductory physics is concerned, it is a purely man-made construct, not a fundamental principle. I am not sure anyone has even settled on whether it really exists or is just purely mathematical. In the region between the two charges, is there really something there physically? I think the philosophers have to try and answer that question.

Avoiding the electric field may lead to problems later explaining why an induced current flows in a conducting ring in the presence of a changing magnetic field. [In standard texts, the [radial] electric field [of a point charge] is introduced to explain electrostatic attraction and repulsion of point charges. Later on, one learns that one can have [curly] electric fields without any charges present. Thus, we learn that the electric field has its own existence.]

Does that mean then that (i) if I have a stream of protons moving in a particular direction, then the US Navy's definition of "current" points in the opposite direction of the proton flow, and (ii) the Navy redefined Maxwell equations, in particular, Ampere's Law, with an extra negative sign for the curl of B?

Zz.

Since protons are positively charged, the electric current (defined by convention as the flow of positive charge) flows in the same direction of the protons' flow.

Ampere's Law still reads (modulo your choice of units) $$\vec\nabla\times\vec H = \vec J + \frac{\partial \vec D}{\partial t}$$
http://wwwppd.nrl.navy.mil/nrlformulary/maxwells_equations.pdf

 

[Integral]

Avoiding the electric field may lead to problems later explaining why an induced current flows in a conducting ring in the presence of a changing magnetic field. [In standard texts, the [radial] electric field [of a point charge] is introduced to explain electrostatic attraction and repulsion of point charges. Later on, one learns that one can have [curly] electric fields without any charges present. Thus, we learn that the electric field has its own existence.]

Since protons are positively charged, the electric current (defined by convention as the flow of positive charge) flows in the same direction of the protons' flow.

Ampere's Law still reads (modulo your choice of units) $$\vec\nabla\times\vec H = \vec J + \frac{\partial \vec D}{\partial t}$$
http://wwwppd.nrl.navy.mil/nrlformulary/maxwells_equations.pdf

I think you missed the point of ZZ's post, this is sort of a back handed explanation to me of why academia used positive flow convention. The Navy, used to, and probably still does, teach NEGATIVE current flow to Technicians. This is a bit more physical for the SPECIAL case of current flow in an electronics circuit, where electrons are the current carrier, in spite of the current definition given by Maxwell and and Ben Franklin. We were made aware that there was also a positive current flow convention. Hard not to since we were always flowing against the arrows in the schematic of a transistor. This is the first time that I have been made aware of the obvious, that is, negative current flow is not the same current as defined by Maxwell.

 

[ZapperZ]

Since protons are positively charged, the electric current (defined by convention as the flow of positive charge) flows in the same direction of the protons' flow.

Ampere's Law still reads (modulo your choice of units) $$\vec\nabla\times\vec H = \vec J + \frac{\partial \vec D}{\partial t}$$
http://wwwppd.nrl.navy.mil/nrlformulary/maxwells_equations.pdf

How can you get away with saying that? If the Navy DEFINES current flow in the SAME direction as the flow of negative charges, then if I have a flow of positive charges and I ALSO use that direction as the flow of current, don't you think there is a BIG, major screw up here waiting to happen?

Now use the right-hand rule (as defined mathematically when we do a curl of a vector field) for a straight wire, for example. You will notice that once you define how you want the direction of current to move, you will get the OPPOSITE direction for the direction of the H field vector. If I define current flow to be the direction of positive flow, this will produce an opposite field than if I define the direction of negative flow as the direction of current.

I will express my amusement that this has become such a complicated issue. I guess that having to deal with the transport problem in solids, I truly find such issue rather trivial, very much like defining where the "zero" of a potential is. The fact that all our "measurement" can easily be "renormalized" to a different gauge to transform an "electron flow" into a "positive hole flow" without having to make any significant changes to our laws means that maybe we're wasting time on something that does not have much significance. Is there really a major conceptual problem with defining current flow as the flow of positive charge? If there is, then there should be a similar conceptual problem with defining electron charge as "negative".

Zz.

 

[Integral]

I will express my amusement that this has become such a complicated issue. I guess that having to deal with the transport problem in solids, I truly find such issue rather trivial, very much like defining where the "zero" of a potential is.

You are absolutly correct. It is a trivial issue. It seems that it is you who is the most concerned with it. Why bother?

Read my reply to robphy.

 

[robphy]

I see that I missed (glossed over) the Navy convention for current.

 

[ZapperZ]

You are absolutly correct. It is a trivial issue. It seems that it is you who is the most concerned with it. Why bother?

Read my reply to robphy.

I think now it is you who missed the point. I'm concern when people decide to change the convention on how we define things for no reason other than a matter of "taste". It is one thing to change how we define things when there is a valid reason for it. It is another simply because it didn't "feel" right. That is why I asked you if Ampere's law is defined differently when you are taught to define current as the direction of electron flow in a conductor.

What I find "trivial" is the apparent difficulty in having the direction of current flow opposite to the direction of negative charge flow. Why is this even an issue?

Zz.