soooo....electrons and electromagnetics...

[element]

Ok which causes which? Does electron flow cause the E-field or does the E-field cause electron flow?

 

[JohnCU]

Electrons move extremely slow in circuits. It's the energy that moves at the speed of light. It's like, when I yell at you across the room, the energy travels over the medium which is air, and the air molecules vibrate, and move a little, but the energy moves extremely quickly. In AC circuits, they just vibrate back and forth (electrons, that is).

 

[Heisenberg]

Both really. Electrons produce an electric field, but the can also be made to move by an external field.

 

[JohnCU]

Yeah, when you move a magnet near a circuit, it produces a current. That's how a generator works if I'm not mistaken. And there is an electric field surrounding a wire carrying a current.

 

[element]

Originally posted by: Heisenberg
Both really. Electrons produce an electric field, but the can also be made to move by an external field.

Ok i understand that, but let's say you've just completed a simple DC circuit, say a constant DC source and a resistor across it. You have a constant current flowing through the resistor. What caused those electrons to flow? An E-field?

 

[PowerEngineer]

Originally posted by: element
Ok which causes which? Does electron flow cause the E-field or does the E-field cause electron flow?

Electrons, whether stationary or in motion, create an electric field. Electrons in motion create a magnetic field.

 

[element]

ok I see, so it's an intrinsic property of electrons that they always have this e-field, which is what causes them to flow, due to the repulsive properties of the e-field, do I have that right?

the electric field of 1 electron pushes on another one and that causes them to flow? so what made the first one move in the first place?

basically, how does current flow begin? i know I'm missing something here I just don't know what it is.

 

[JohnCU]

In a wire, let's say a copper wire, there are electrons that are free to move around. They move randomly, when there is nothing pushing them in a certain direction. Put a little voltage on a good conductor, and the free electrons begin moving.

 

[Heisenberg]

Originally posted by: element
ok I see, so it's an intrinsic property of electrons that they always have this e-field, which is what causes them to flow, due to the repulsive properties of the e-field, do I have that right?

the electric field of 1 electron pushes on another one and that causes them to flow? so what made the first one move in the first place?

basically, how does current flow begin? i know I'm missing something here I just don't know what it is.

The potential difference supplied by the source is what makes them all move more or less uniformly. As they start to move, the repulsion between electrons causes the current as the first one pushes on the second, then the second on the third, etc.

 

[element]

Originally posted by: Heisenberg

Originally posted by: element
ok I see, so it's an intrinsic property of electrons that they always have this e-field, which is what causes them to flow, due to the repulsive properties of the e-field, do I have that right?

the electric field of 1 electron pushes on another one and that causes them to flow? so what made the first one move in the first place?

basically, how does current flow begin? i know I'm missing something here I just don't know what it is.

The potential difference supplied by the source is what makes them all move more or less uniformly. As they start to move, the repulsion between electrons causes the current as the first one pushes on the second, then the second on the third, etc.

ah of course, the e- want to go from a high potential to a low potential, like water down a hill. but water down a hill is caused by gravity and the high potential is a higher elevation.

what then is a higher electrical potential? just more electrons piled up and wanting to seperate due to repulsive forces between like charges?

 

[f95toli]

Higher potential=More potential energy.
The voltage "tilts" the potential which means that the electrons can start "rolling down the hill"; voltage is after all just the difference in height between the "start" and "stop" of the tilted potential.

 

[element]

Originally posted by: f95toli
Higher potential=More potential energy.
The voltage "tilts" the potential which means that the electrons can start "rolling down the hill"; voltage is after all just the difference in height between the "start" and "stop" of the tilted potential.

what hill? there is no hill, what height? there is no height. if i turn a battery upside down it works just the same.

 

[Jeff7181]

I don't think anybody really knows for sure WHY electrons move. Most people just accept that electrons move from positive to negative for the same reason the positive side of a magnet sticks to the negative side of another magnet... it just does. But there's also the electron hole theory that basically states on the positive side, there is a lack of electrons (lack of electrons = "hole") and since all things in nature seem to seek balance, electrons simply migrate to an area where electrons are less concentrated. This theory says that electrons move from negative to positive, which is opposite from what most people believe. I think some people claim to have disproved the electron hole theory, but I haven't really read up on that.

But as for your original question, it's sorta like the chicken and the egg. The flow of electrons cause an electromagnetic field to form, and passing a conductor through an electromagnetic field causes the flow of electrons.

 

[CycloWizard]

Originally posted by: element
what hill? there is no hill, what height? there is no height. if i turn a battery upside down it works just the same.

The way I look at it is in terms of fluid flow (as you yourself already pointed out). A better way to look at it may be water flowing through a pipe. The water will flow from the point of high pressure to the point of low pressure. The high pressure may be induced by pumping, elevation, or other things - the manner in which pressure is generated isn't important, since it's a state variable. Thus, the fluid seeks to balance out the pressure gradient over the flow path - it wants to equalize pressure at all points.

Flowing electrons are the same way. It doesn't matter how the initial voltage occurs, but the electrons must flow in an effort to couteract the difference in potential between the two points. The electric potential is directly analagous to pressure gradient in fluid flow. Pressure-induced flows are even called 'potential flows'. If you understand what I'm saying, stop here! The next paragraph is potentially very confusing, depending on your background.

If that doesn't help, try this: any system always wants to be at equilibrium. The simplest way to depict this is through entropy - the measure of disorder of a system. At equilibrium, a system is as disordered as it can be: it has no preferred direction and no potentials exist to force it in either direction. Thus, when a potential (either electric or pressure) is applied to the existing equilibrium, fluid or current will flow from high potential to low potential in an effort to remain as disorderly as possible.

 

[unipidity]

The idea of height comes from attempting to draw an analogy between an electric and gravitational field. There is no 'height' to a circuit, but between any two points, there is a potential difference, as there is between the top of a hill and a valley (albeit a gravitational, not electrical, potential).

WHY is... an impossible question really. You can just keep on asking it until you reveal the axioms of the system involved. Like charges repel. Unlike attract. It just IS... a fundamental force.

 

[TuxDave]

Originally posted by: CycloWizard

Originally posted by: element
what hill? there is no hill, what height? there is no height. if i turn a battery upside down it works just the same.

The way I look at it is in terms of fluid flow (as you yourself already pointed out). A better way to look at it may be water flowing through a pipe. The water will flow from the point of high pressure to the point of low pressure. The high pressure may be induced by pumping, elevation, or other things - the manner in which pressure is generated isn't important, since it's a state variable. Thus, the fluid seeks to balance out the pressure gradient over the flow path - it wants to equalize pressure at all points.

Flowing electrons are the same way. It doesn't matter how the initial voltage occurs, but the electrons must flow in an effort to couteract the difference in potential between the two points. The electric potential is directly analagous to pressure gradient in fluid flow. Pressure-induced flows are even called 'potential flows'. If you understand what I'm saying, stop here! The next paragraph is potentially very confusing, depending on your background.

If that doesn't help, try this: any system always wants to be at equilibrium. The simplest way to depict this is through entropy - the measure of disorder of a system. At equilibrium, a system is as disordered as it can be: it has no preferred direction and no potentials exist to force it in either direction. Thus, when a potential (either electric or pressure) is applied to the existing equilibrium, fluid or current will flow from high potential to low potential in an effort to remain as disorderly as possible.

lol... yay for water pipe analogies! :thumbsup:

 

[DrPizza]

Originally posted by: Jeff7181
I don't think anybody really knows for sure WHY electrons move. Most people just accept that electrons move from positive to negative for the same reason the positive side of a magnet sticks to the negative side of another magnet... it just does. But there's also the electron hole theory that basically states on the positive side, there is a lack of electrons (lack of electrons = "hole") and since all things in nature seem to seek balance, electrons simply migrate to an area where electrons are less concentrated. This theory says that electrons move from negative to positive, which is opposite from what most people believe. I think some people claim to have disproved the electron hole theory, but I haven't really read up on that.

But as for your original question, it's sorta like the chicken and the egg. The flow of electrons cause an electromagnetic field to form, and passing a conductor through an electromagnetic field causes the flow of electrons.

First, I also love the water pipe analogy... it works great in class.

Now, per this post.
Magnets have a North and a South pole, not a positive and negative.
And,
most people don't accept that electrons move from positive to negative. They move from negative to positive. What you're probably thinking of is conventional current which is thought of as the movement of holes - areas missing an electron. It's sort of like a blinking marquee... the lighted lights are moving one way - these would be like electrons. But, the unlit bulbs are moving the other way - these are the holes. So, traditional current is from positive to negative - it's the flow of these "holes."

I'm reaching a bit here, because I haven't researched it 100%, but, before we knew the exact nature of these charges, we recognized that there were different charges. Take 2 pieces of scotch tape, put them together, then pull them apart. Then rub some fur on a plastic rod (or otherwise produce two more charged object.) One piece of the tape will go toward the charged object. The other piece of tape will be repelled by it. This type of experimenting was simple to do and produced the conclusion of 2 charges.

Now, Benji Franklin (IIRC), decided that the positive charge is the side that gained something... gained = extra = positive. When, in reality, it's the electrons that moved, hence the positive now has a deficit of electrons, resulting in what we now know is a net positive charge. But, before we knew what was moving, whether it was +'s or -'s, humankind was screwing around with electricity... getting keys shocked on kite strings, inventing lightbulbs and sh!t, etc. Direction of current was defined - positive to negative. To make it make sense with today's knowledge, we refer to it as the flow of holes.

 

[DrPizza]

before I get bashed... the flow of holes is really discussed in semiconductors... transistors, etc. I'm not sure that there really are "holes" when electrons are moving in metallic wire. The analogy to semiconductors works though - it helps make sense why conventional current is backwards.

 

[f95toli]

Originally posted by: Jeff7181
I don't think anybody really knows for sure WHY electrons move

I think I have at least a pretty good idea about HOW they move, I also understand why they move in an E-field. The problem is that in order to understand the details you need a working knowledge of quantum mechanics.
However, once you understand the basics of quantum mechanics it is actually not that complicated as long you only consider good conductors sich as copper where the electrons are more or less free.
We can also write down equations for more complicated materials but then you need a computer to solve the problem.

 

[jagec]

Originally posted by: DrPizza
before I get bashed... the flow of holes is really discussed in semiconductors... transistors, etc. I'm not sure that there really are "holes" when electrons are moving in metallic wire. The analogy to semiconductors works though - it helps make sense why conventional current is backwards.

why would you get bashed? It's common knowledge that the "positive to negative" convention was decided before people knew electrons had a negative charge. Now that we know, it is also common knowledge that electrons actually flow from negative to positive, but since the old convention was already in use people stuck to it.

 

[element]

Originally posted by: jagec

Originally posted by: DrPizza
before I get bashed... the flow of holes is really discussed in semiconductors... transistors, etc. I'm not sure that there really are "holes" when electrons are moving in metallic wire. The analogy to semiconductors works though - it helps make sense why conventional current is backwards.

why would you get bashed? It's common knowledge that the "positive to negative" convention was decided before people knew electrons had a negative charge. Now that we know, it is also common knowledge that electrons actually flow from negative to positive, but since the old convention was already in use people stuck to it.

I think because as he knows, holes don't really flow. the term holes means a lack of electrons, am I right?

Speaking of which can someone please answer what a higher electrical potential is? just more electrons piled up and wanting to seperate due to repulsive forces between like charges?

 

[CycloWizard]

Originally posted by: TuxDave
lol... yay for water pipe analogies! :thumbsup:

Hehe... I've never been taught anything about circuits but I managed to learn quite a bit by simple analogies with things I do understand. Chemical engineers are taught analogies between momentum, heat, and mass transfer. I'm not sure why our classes never draw parallels to electricity, other than we have enough to cover as it is.

 

[f95toli]

An electric potential is no different that a gravitational potential, it just a difference in potential energy. The potential energy of an electron in a material is its energy compared to the ground state in equilibrium; if you want to go into details you need to understand comcepts like Fermi surfaces etc so I suggest you look that up in a book.

Holes ARE different from electrons, they have different properties (holes and electrons do not have the same mass for example). Of course there are no "real" holes, just "absent electrons", so in a way this is just a model, but it is a very usefull model.

 

[unipidity]

f95... just what exactly ISNT a model? Its not like were down with any fundamental truths yet, so anything that makes a few usefull predictions is as good as anything else.

 

[deveraux]

Originally posted by: PowerEngineer

Originally posted by: element
Ok which causes which? Does electron flow cause the E-field or does the E-field cause electron flow?

Electrons, whether stationary or in motion, create an electric field. Electrons in motion create a magnetic field.

Couldn't have put it better myself. As to your question about why electrons move, I think the rest have explained it quite well in their "water pipe"/"hill" analogy. Unfortunately, there are certain things that you must accept (i.e. axioms). It is a property of matter than unlike charges attract and like charges repel. I don't think there's anyway of explain that unless you get into very heavy quantum mechanics.

But if you accept that it is a property of matter, then, it becomes much easier to understand why an E-field is generated, and also why a H-field (magnetic field) is generated.

As to your question about potential. It really is no different from standing on top of a high hill and hence having a higher potential. It is the amount of energy a "charge carrier" (i.e. electron) has when it "begins" its journey around the electrical circuit and loses it to resistance due to various components.

By Kirchoff's Voltage Law, we can say that by the time the charge carrier gets to the end of the circuit, it MUST have used up ALL of its potential energy.

My $0.02