# Why does electricty flow?

1. Jun 10, 2004

### 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?

2. Jun 10, 2004

### Integral

Staff Emeritus
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.

3. Jun 10, 2004

### 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 +?

4. Jun 10, 2004

### Gokul43201

Staff Emeritus
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.

5. Jun 10, 2004

### 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.

6. Jun 10, 2004

### 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?

7. Jun 10, 2004

### 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

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)

8. Jun 10, 2004

### 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.

9. Jun 10, 2004

### krab

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.

10. Jun 10, 2004

### krab

+ 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.

11. Jun 10, 2004

### Math Is Hard

Staff Emeritus
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!

12. Jun 10, 2004

### TALewis

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.

13. Jun 10, 2004

### Gza

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.

14. Jun 10, 2004

### Math Is Hard

Staff Emeritus
thanks, TA!

15. Jun 10, 2004

### Math Is Hard

Staff Emeritus
and thanks, Gza!

16. Jun 10, 2004

### 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.

17. Jun 10, 2004

### Integral

Staff Emeritus
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.

18. Jun 10, 2004

### ZapperZ

Staff Emeritus
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.

19. Jun 10, 2004

### Staff: Mentor

nit pick?

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.

20. Jun 11, 2004

### Gokul43201

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

21. Jun 11, 2004

### 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.

22. Jun 11, 2004

### Math Is Hard

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

23. Jun 12, 2004

### Gza

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 whats 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.

Last edited: Jun 12, 2004
24. Jun 12, 2004

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.

25. Jun 14, 2004

### robphy

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

Last edited by a moderator: Apr 20, 2017