How does voltage affect particles in current

In summary: What is the difference between potential energy and kinetic energy?3) Does gravity have any affect on electricity?
  • #1
Screwy
12
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So, I have a problem visualizing effect of voltage on the current in a circuit. Here goes:

-Intensity (I) of a current is a ratio of charge traveling through a cross section of a conductor, and time. We can visualize it as a flow of water through a pipe: It is all about quantity of particles.

-Voltage (V) of a circuit is a difference in potentials between two poles. We can imagine this as a height of a chamber from which we can release water into pipes.

So, power of a current is I times V, in other words - to preserve power we can "increase quantity of water and reduce the height of a chamber" or "increase height of a chamber and release smaller quantities of water".

In this analogy, we can easily see that we either have less water moving faster or more water moving slower to preserve total power of a system. The higher chamber results in FASTER water!

Now this is my problem. What property of electrons does voltage affect, to make their energy higher?
Is it their speed?
Is it the drift speed, or just a speed of any electron in particular?

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Additional question, very much related:

Let's imagine a small charge (-) between two parallel plates of a capacitor. It is near (-) plate of a capacitor.

Now, the charge has a certain Electrical potential energy (voltage times charge).
If we release the charge and let it move towards (+) plate of a capacitor, its potential energy is reduced as it approaches (+) plate, and all of its potential energy at this point is converted to... to WHAT?

There is not much talk about this in textbooks, and it would help me visualize electricity if I could correctly realize what property of a particle, besides it's charge, plays a part in its energy.
 
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  • #2
Don't consider the speed of electrons in circuits, this just leads to all sorts of misconceptions. The average speed of those electrons is really small (of the order of millimeters per second).

What property of electrons does voltage affect, to make their energy higher?
It determines the potential difference for the electrons. In a resistor (where more voltage usually leads to more current), you can view this as a higher force on the electrons, but be careful with conclusions based on that view.

and all of its potential energy at this point is converted to... to WHAT?
In the flight, it gets converted to kinetic energy. At the plate, it is converted to heat, electromagnetic radiation, or other forms of energy you don't care about in circuits.
 
  • #3
Now this is my problem. What property of electrons does voltage affect, to make their energy higher?
Is it their speed?
Is it the drift speed, or just a speed of any electron in particular?
Voltage changes the potential difference ... like any potential difference.
Consider gravity - if the gravity is stronger, the gravitational potential difference between some height and the ground would be bigger. The affects the resulting motion of objects moving in the gravitational field.

You should see this same effect in your example of a parallel plate capacitor. Change the voltage and the potential-difference between the plates changes (this is actually a tautology) - changing the way the electron between the plates accelerates..

The parallel plate example works just like a mass falling in gravity - the electron "falls" in the electric field toward the positive plate trading potential energy for kinetic energy. When it hits the plate it becomes part of the plate like the mass becomes part of the ground when it lands. In both cases, what happens next depends on the situation on the ground/plate: it can be quite complicated.

As mfb points out - the water-pipe model for electricity in circuits is only an analogy - like any analogy it can only be carried so far. It is commonly introduced at early levels so students have something to help visualize simple circuits ... it's a "lie to students". One that often annoys college professors who have to undo the misconceptions that result.

The different (imperfect and incomplete) models that are used to describe stuff when you are starting out can be confusing. As you progress in your studies, the different models will get merged, and the less useful ones discarded.

They won't all get merged though - because we don't have a grand unified theory (not yet). It is best to think of them as models rather than actuality - and understand that different situations will call for a different picture.
 
  • #4
Thank you for your responses!

Simon Bridge said:
The parallel plate example works just like a mass falling in gravity - the electron "falls" in the electric field toward the positive plate trading potential energy for kinetic energy.

Yes! All right.

Just help me wrap my mind around these three problems:

1) Does higher voltage mean electrons in a CIRCUIT accelerate faster (thus increasing their kinetic energy)?

if so,

2) Does the overall (Brownian) speed of electrons increase (which would be weird, because that would mean that the temperature of electrons increases)

or does the Drift speed of a current increase?

and, finally, if so

3) how does this property affect the Power of a current?
I am not asking for formulae, but for a visual explanation... In other words, if a current moves "faster", why does it make it stronger.

NOTE: 2) and 3) are based on assumptions that I made not knowing answers to 1) and 2) so ignore these questions if they make no sense :smile:
 
  • #5
Screwy said:
1) Does higher voltage mean electrons in a CIRCUIT accelerate faster (thus increasing their kinetic energy)?
Again, acceleration and kinetic energy are not useful quantities of electrons in circuits.
Between parallel plates: Sure.

or does the Drift speed of a current increase?
Drift speed in resistors increases if you increase the voltage.

3) how does this property affect the Power of a current?
I am not asking for formulae, but for a visual explanation... In other words, if a current moves "faster", why does it make it stronger.
The electrons have more potential difference - similar to a different height for gravity.
 
  • #6
mfb said:
The electrons have more potential difference - similar to a different height for gravity.

Dropping an object from a height starts a process of converting potential energy to kinetic energy.

The bigger the drop, faster the ball will eventually go.THAT'S what I'm asking - what will the difference in potentials in a circuit convert to? What kind of energy?

Because, by definition of P=I*V, ONE "High Voltage" electron has more power than ONE "Low Voltage" electron. Why? Because of what property?

Another possibility is that it is not an electron that changes, but the nature of the flow. Either way, this is where I hit a wall so far.

Thank you for your cooperation mfb and Simon, we're getting there!
 
  • #7
First think what energy exactly does pressurized water have. Then see below.
Some electrons in a circuit are affected by an electromotive force. Those electrons may have potential energy, for example if the voltage source is a capacitor.

An electron in a wire can use the energy of the electrons inside the voltage source.

What I'm saying is that those electrons affected by an EMF may have some energy. Other electrons don't have energy.

Water up in a water tower has potential energy. Water that is under pressure has no energy. If pressurized water does work, a pump or a water tower is doing the work.
 
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  • #8
Screwy said:
THAT'S what I'm asking - what will the difference in potentials in a circuit convert to? What kind of energy?
In a resistor? Usually heat.
Other options are light, mechanic energy, chemical energy and some more.

Because, by definition of P=I*V, ONE "High Voltage" electron has more power than ONE "Low Voltage" electron. Why? Because of what property?
Voltage IS this property.
 
  • #9
Voltage is a property of a system or part of a system. It isn't a property of an electron
 
  • #10
Screwy said:
THAT'S what I'm asking - what will the difference in potentials in a circuit convert to? What kind of energy?

Heat, motion, light, etc. Stick a light bulb in a circuit and you have light and heat generated. Stick a motor in there and you have motion.

Because, by definition of P=I*V, ONE "High Voltage" electron has more power than ONE "Low Voltage" electron. Why? Because of what property?

Single electrons are not described by such rules and are not within the scope of this thread. It is the circuit as a whole that is affected by voltage and it is the circuit as a whole that is within the scope of the normal EE equations such as Ohms law.

Another possibility is that it is not an electron that changes, but the nature of the flow. Either way, this is where I hit a wall so far.

You are correct, it is the current that changes when you increase or decrease voltage. Specifically, more charges per second will flow by a certain point in a circuit when you increase voltage.

Realize that electrons in a conductor are NOT like electrons in free space. You can't push an electron and have it whirl around the circuit. It is subject to all kinds of electromagnetic forces within the circuit which rob it of whatever energy you gave it nearly instantly. It takes a sustained difference in potential, aka voltage, to make the charges flow in a net direction. (Even with applied voltage you still have some that flow the opposite way)
 
  • #11
Because, by definition of P=I*V, ONE "High Voltage" electron has more power than ONE "Low Voltage" electron. Why? Because of what property?
Not to take anything away from Drakkith here, but my immediate responce was that power is not something an electron has in the sanse that it has charge and energy, it is a process: power is the rate of change of energy.

An electron released a high voltage (read - high "potential difference") potential will change it's potential energy faster than one released in a low voltage potential.

"Voltage" is a problematic term here - the electric potential has units of "volts" and so does the potential difference and fair enough. Either can be called "voltage", but the voltage of an electric circuit is usually the applied potential difference of a "voltage source" - i.e. a battery. It may refer to the change in potential across a component like a resistor, and even the "rating" of a load - like a light-bulb rated for 240V 10A. For this discussion we probably need to distinguish - or just drop the term "voltage" completely for the sake of clear communication.

I suspect that saying what kind of voltage is being referred to will clear up a lot of the confusion.
 
  • #12
So, how would you describe a "visual" difference between a state of high voltage - low intensity circuit, and low voltage - high intensity circuit (with same power)?

let's just describe 3 situations:

1) voltage is zero. there is no flow of free electrons in the conductors.

2) voltage is rather low, and the current is quite intense. So, the flow of electrons is there, and quite a lot of electrons are flowing in a unit of time. (that's what high intensity says, isn't it?)

3) this same current gets transformed to high voltage, with no losses.
Now, the number of particles that are flowing in a unit of time is smaller than before (that's what low intensity is, right?). Also, the flow of electrons is still here, voltage isn't zero. Actually, it's high. And we have less electrons taking part in a current. And we have the same power of current.

So, there has to be some other difference in the behaviour of a circuits, besides number of charges bustling around the wires. Something that compensates for the drop in number of particles.

And don't say "that IS voltage, this other property!", because that is like saying that "HEIGHT OF A WATERTOWER" is a property of water! That is the CAUSE, voltage is the CAUSE, and I'm after the EFFECT.

Please think about this.
 
  • #13
how would you describe a "visual" difference between...
Who are you asking?

By "intensity" do you mean intensité de courant or "currant intensity" which is just "current" these days? Or do you mean the square energy per unit area?
Don't know what you mean by "visual" either - you don't normally see electrons or electric currents directly and what you see indirectly depends on the method used.

Taking "intensity"="currant"
1. the current is zero - so there must be an open circuit in DC or no charge carriers.
2. for a low potential difference high current in an electric circuit means that the resistance is low.
3. higher applied potential difference, same current, then the resistance was also increased.
For a given circuit, the voltage and the currant have a relationship depending on the exact makeup of the circuit. They cannot take on just any old values.
In a simple DC circuit, they are related by the load resistance through Ohm's law.
there has to be some other difference in the behaviour of a circuits, besides number of charges bustling around the wires. Something that compensates for the drop in number of particles.
The resistance is the feature you are looking for.
Like drakkith was saying before
an electron ... [in a] circuit ... is subject to all kinds of electromagnetic forces
... the load resistance is not the half of it.
 
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  • #14
Screwy said:
So, how would you describe a "visual" difference between a state of high voltage - low intensity circuit, and low voltage - high intensity circuit (with same power)?

There is no "visual" difference. Attempting to visualize it will fail because you cannot see what is happening in the circuit. If you need something to visualize I'd recommend the water pipe analogy, where current is water flow. It isn't correct, but it's the closest thing you're going to get to visualization.

let's just describe 3 situations:

1) voltage is zero. there is no flow of free electrons in the conductors.

There is no NET flow. Charges, electrons in this case, move all over the place thanks to thermal energy. This is one reason objects emit thermal radiation, they have moving charges which emit radiation upon acceleration. But they do not flow in any specific direction, as a whole, on average.
2) voltage is rather low, and the current is quite intense. So, the flow of electrons is there, and quite a lot of electrons are flowing in a unit of time. (that's what high intensity says, isn't it?)

Then the circuit has little resistance and little power.

3) this same current gets transformed to high voltage, with no losses.
Now, the number of particles that are flowing in a unit of time is smaller than before (that's what low intensity is, right?). Also, the flow of electrons is still here, voltage isn't zero. Actually, it's high. And we have less electrons taking part in a current. And we have the same power of current.

The same current gets transformed to high voltage? Then you have the same current as before, but with a higher voltage, meaning higher power. This means that the resistance has increased and you will dissipate more power in the circuit.

So, there has to be some other difference in the behaviour of a circuits, besides number of charges bustling around the wires. Something that compensates for the drop in number of particles.

There is. Voltage and resistance.

And don't say "that IS voltage, this other property!", because that is like saying that "HEIGHT OF A WATERTOWER" is a property of water! That is the CAUSE, voltage is the CAUSE, and I'm after the EFFECT.

Please think about this.

You are getting stuck trying to understand and visualize what voltage and current are. You WILL stay confused if you keep trying to visualize it. You simply cannot. It isn't possible to visualize what happens in the circuit. You cannot boil this down to "what happens to a single electron" because standard circuits have trillions of electrons doing all sorts of things all at once that results in the observable effects we see.

It takes energy to keep a voltage applied to a circuit. This energy is lost as current flows. Either in the form of heat, light, or motion.
 
  • #15
First, Simon:

I was talking about the current, yes. "I" comes from "Intensity" and I tought that I would use the word Intensity for the current, and "Current" for adressing electricity. Sorry for confusing you, after reading your answers, I have realized that I messed up by using word "Current" inappropriately in question 2).

Actually, my idea was that if we have I1 and V1, making together for P1=I1*V1, and if we transform this system, without losing power, to I2=0.5*I1 and U2= 2*U1, then P2 would be P1 of course.

BUT, since I2= 0.5*I1, that means that dQ2=0.5dQ1, only half the charge would flow through some cross section of a wire.
And power remained the same.
This would mean that an average particle in a second case somehow contributes more to the total power of a system. Average particle is somehow stronger, don't mind me using the layman's terms. Do you see what I'm getting at?
And Drakkith,

Thank you for adressing the real problem, and that's my intention to describe phenomenons to myself from "the ground level up".

However, this is really something that shook my world.

Drakkith said:
It isn't possible to visualize what happens in the circuit

It is impossible to know what electrons are doing? It's impossible to even describe it? Is this a certainty or your point of view, Drakkith?

Could you maybe point me to and article explaining why I can't imagine what is happening in an electrical cord?

That is pretty discouraging for me, I must say. How to passionately study physics, if there is no chance of understanding the principle? I tought that was the whole point of science...
 
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  • #16
Screwy said:
Actually, my idea was that if we have I1 and V1, making together for P1=I1*V1, and if we transform this system, without losing power, to I2=0.5*I1 and V2= 2*V1, then P2 would be P1 ofcourse.

BUT, since I2= 0.5*I1, that means that dQ2=0.5dQ1, only half the charge would flow through some cross section of a wire.
And power remained the same.
This would mean that an average particle in a second case somehow contributes more to the total power of a system. Average particle is somehow stronger, don't mind me using the layman's terms.
Each charge starts out at twice the potential energy, yes.
So each has more to convert to something else.

Notice I am being careful to talk about "charges" rather than electrons - this goes to the difficulty in talking about what happens on an individual electron basis in a material object like a wire. We understand these things in terms of emergent properties like current and charge instead. That way we can make sense of specific situations like electric circuits.

Perhaps you can visualize the difficulty:
It is difficult to study a crowd by watching just one person in it. Each person is moving according to the various forces of their individual circumstances. But from a distance, everyone is basically moving randomly. We know it isn't really random - ask anyone and you'll see they have reasons for what they do. But that's what it looks like.

Now if something were to attract the crowds attention - say they are a crowd at a ferry terminal, and the ferry just arrived, then the crowd gets some directionality to it. Most people are still mostly milling about but there is a general motion towards the ferry. That's what an electric current would look like. If the crowd were football fans waiting outside the stadium and someone just opened the gates - then there would be a more definite directionality - this would be a higher voltage. But we are still daling with only 100s-1000s of people. That's still small enough to see the crowd and the individual dynamics at the same time.

Now make the number of people very large - say 100-million people are in this crowd ... and the regeon you are looking at is a largish island like Zanzibar... so everyone has about 3 square meters? To stand back far enough to see the whole crowd, you can no longer see the individual people in order to see how their individual motion contributes to the whole. Complicated things can happen that you won't see because you are either too far away or because it's happening because of someplace you are too close into see. Instead you have to deal with the crowd as a whole.

In a cubic centimeter of copper, there are around 10^24 free electrons ... trying to figure what is happening in the copper in terms of individual electrons would be impossible. Imagine trying to understand a crowd of 10 000 000 000 000 000 000 000 000 people?
However - in special situations, a lot of the randomness averages out. If we deal with all those electrons in terms of what they look like when we stand back, some sort of understanding is possible.

The methods form part of solid state physics ... which electric circuits are a small part of.
Here you can think of clouds of particles moving around under the influence of forces - but only if you concede that these cannot be the same as electrons in a vacuum.
They usually have the wrong mass for example - and can be positively charged. We call the positive charges "holes" and the weird masses are "effective mass"s because we understand that these things are emergent properties of a bulk material ... just like we understand that each person in the crowd has their own reasons for doing stuff.

How to passionately study physics, if there is no chance of understanding the principle?
You have read too much into the statement. The principles can be understood - just not by visualizing the behavior of individual electrons in a wire.

There are a whole bunch of handy visualizations that get used as teaching aides - but they are all inadequate in some important way.
At some point you will have to discard them. Be prepared. Discarding bad ideas is part of what science is about after all.
 
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  • #17
Great analogy and great effort, Simon.

So here's a picture I have so far:

average kinetic energy of charged particles in a wire has little to do with voltage and current. It has more to do with temperature, probably.

But what happens is when voltage is introduced, charged particles have a tendency to direct their motion in a particular direction. So, voltage doesn't add energy to particles, it only arranges their behaviour to a certain degree, which results in a net flow of energy?
 
  • #18
We'll find out if it is a good analogy when people start criticizing it ;)

Thinking statistically is the way to go.
Applying a potential difference shifts the statistics.
The result, averaged over a lot of electrons is the electric circuit math you have been learning in terms of currents and power and so on.
 
  • #19
Screwy said:
That is pretty discouraging for me, I must say. How to passionately study physics, if there is no chance of understanding the principle? I tought that was the whole point of science...

You can understand the principles. You just can't visualize them. You can only visualize the analogies or something else you make up in your mind that you associate with the principles.
 
  • #20
If only we adopted Steinmetz' suggestion of defining voltage by the electrostatic (he called it the dielectric) field around the conductor, then all of these misunderstandings would not exist.

The concept of particles hurts our understanding of electricity, it is much easier defined in fields and waves.
 
  • #21
Screwy said:
Could you maybe point me to and article explaining why I can't imagine what is happening in an electrical cord?

That is pretty discouraging for me, I must say. How to passionately study physics, if there is no chance of understanding the principle? I tought that was the whole point of science...

You have my sympathy but you are asking too much, I'm afraid. What you are asking for is to feel you 'understand' something by only using the terms and ideas that you are totally familiar with. You just cannot assume this is possible.
Let me give you an analogous situation - but in Maths.
When you were in lower School, you learned Arithmetic and then how to solve simple equations - like 2x=6, what is x? You could work this out (but even then, you had to know about how to 'do' algebra)..
Then someone told you about quadratics like x2 = 4, what is x? There were two solutions to this -not too hard to grasp.
THEN, they gave you an equation x2 = -4, what is x?
There is no way you could answer this, just using the Maths knowledge you already had. You just HAD to learn about complex numbers to take the next step. The old stuff you already knew just couldn't help you.
In the same way, you can't expect that the cuddly, concrete stuff you started off Science with will get you all the way. There are no true answers to any of the 'Why' questions that people come up with - just deeper and deeper layers. You need to accept some things and get familiar with them - familiar enough to take them with you and attack the next layer.
"It's turtles all the way."
 
  • #22
sophiecentaur said:
What you are asking for is to feel you 'understand' something by only using the terms and ideas that you are totally familiar with.


I don't think that I asked such a preposterous question, nor do I think that my understanding of physics is on such a low level for this problem.

All I asked is how do you people imagine a diffrence in a high voltage flow of electrons, compared to a low voltage one. What difference would you expect to see when you could "zoom in".

I can solve high school level problems regarding charges, current, circuits and such. However, I want to get more intuitive with the concepts, not just problem solving.

If you think that I am missing some knowledge required for understanding this subject, feel free to suggest some material...
 
  • #23
Screwy said:
I don't think that I asked such a preposterous question, nor do I think that my understanding of physics is on such a low level for this problem.

All I asked is how do you people imagine a diffrence in a high voltage flow of electrons, compared to a low voltage one. What difference would you expect to see when you could "zoom in".

I can solve high school level problems regarding charges, current, circuits and such. However, I want to get more intuitive with the concepts, not just problem solving.

If you think that I am missing some knowledge required for understanding this subject, feel free to suggest some material...

One way that helped me "visualize" current flow in a conductor was to look at Ohm's law while holding various terms constant. Remember, for a simple conductor, V = IR. In some simplified sense you can imagine the conductor crystal not being clear so the electrons keep hitting obstacles. This is what resistance is.

For example, imagine we want a given current flow (call it I, recall current is just the number of charges on average passing through a given plane in the conductor).

If you have a lot of resistance in the wire (remember resistance is caused by impurities in the conductor crystal or due to thermal agitation of the lattice sites) then you need to push hard on the conductor to get your current through. So, when R is high, V must also be high to get a given I.

If you have low resistance (because either in the particular conductor it is easy for electrons to free themselves from the lattice or because you have a thick wire and your reference plane is larger) then you don't have to push as hard to get the charges through. So, when R is low, V doesn't need to be so high to get a given I.

There is a lot of handwaving here. To really understand this at a level to give quantitative predictions you need to understand a bit of quantum mechanics. But just Ohm's law itself can give some intuition.

Also, remember, each electron is actually moving quite slowly, but there are an almost unbelievable number of electrons in the current moving every which way due to drift and diffusion. The net current due to drift (caused by your applied voltage) can be large and move quickly.
 
  • #24
Screwy said:
I don't think that I asked such a preposterous question, nor do I think that my understanding of physics is on such a low level for this problem.

All I asked is how do you people imagine a diffrence in a high voltage flow of electrons, compared to a low voltage one. What difference would you expect to see when you could "zoom in".

I can solve high school level problems regarding charges, current, circuits and such. However, I want to get more intuitive with the concepts, not just problem solving.

If you think that I am missing some knowledge required for understanding this subject, feel free to suggest some material...
I'm sorry if I upset you but, in fact, your question may be a bit more naive than you appreciated. Electrons in a condensed medium do not behave as individuals. What happens is at a quantum level and to talk of electrons as if they are little charged balls, buzzing about in a network of positive, fixed charges is, in fact, a very 'High School Level' of looking at it. "Zooming in" is not really an appropriate thing to do. The elementary treatment of conduction fails to explain superconductivity, for a start.
Go to any textbook on solid state Physics if you want to see a more useful treatment of conduction in solids.
 
  • #25
Well, quantum mechanics is definitely on my list of things to learn, but I tought that I would first explain everything to myself in a "classical" way, and than upgrade that picture once I get some aquaintance with a quantum way of things (some time in the future).

Would you suggest doing things other way around?

I took up my old high school textbooks and started learning the whole bundle from the scratch, with an intention to install some really good knowledge in my mind for the rest of my life. I got to the second grade so far, and have to go a lot of distance first to catch up with what I was familiar with earlier. Than I would attack relativity and quantum mechanics. Is this the suggested way to learn this stuff these days? My textbooks are 10 years old.

BTW, Calgrace: Thank you, that fits nicely with a model that I got in my head so far, thanks to this thread and some talk with a couple of friends who study of physics. I feel pretty confident handling voltage now, and I hope I can be of some help to someone on this forum in the future like everyone's been to me :smile:
 
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  • #26
Screwy said:
Well, quantum mechanics is definitely on my list of things to learn, but I tought that I would first explain everything to myself in a "classical" way, and than upgrade that picture once I get some aquaintance with a quantum way of things (some time in the future).

Would you suggest doing things other way around?
As I have implied before, you seem to be seeking something that doesn't really exist - a classical, pictorial, 'zoomed in' explanation will fail.
My point is that you cannot look too deeply into the classical model for any phenomenon - there is no point and it leads to wrong conclusions. Let's face it, if you could do that, there would be no point in bring QM into it. This is where the advantage lies in 'going with the Maths' and simply using the classical model to get results. It will not let you down in problems involving the usual scale of quantities.
Everyone carries mechanical type models in their heads when dealing with complex situations but these models are inherently flawed. We have to learn to keep them deliberately 'fuzzy' and not let them intrude too much. You will find page after page of 'personal views' about electrons, Photons, Fields etc. etc. on PF. People make amazing assertions about the 'true nature' of things, all based on a pet picture that they carry in their heads. The sad thing is that they (I'm sure I have to include myself in this, at times) think they're being helpful.
The conclusion from 'the Maths' is always more likely to be near the truth than the conclusion from the 'dodgy model'. Make your way slowly and rigorously through Physics, in the conventional way (QM, SR, GR come way after the classical stuff) and you will eventually reach the limit of what you can cope with.
 
  • #27
Well, to my point of view, when we have a potential energy difference U1-U2 between two points 1,2 a charged particle traveling from 1->2, will gain kinetic energy T=U1-U2.
Now, if you increase the voltage (U1-U2) the particle will gain more kinetic energy.
In a wire, particles move thermally but if you apply a voltage they will gain a small amount of kinetic energy (they will move like the slowest of slugs :-) ) towards the lower potential. That's why we have an electric current. Due to the collective directed motion of the electrons.
The speed that each of the electrons will gain is minuscule. But we have MANY electrons in the wire. Thus, the current density J=ρ*u (ρ= the charge density of the wire) will increase and so will the current because I=∫Jda , a the cross section of the wire.
Therefore this is why the current increases.
Small increase in velocity per particle times many particles-> Current increases
 
  • #28
Wrong on two counts, I'm afraid. There is NO PD across a superconductor and electrons do not gain KE as they move through a conductor - like they do in a CRT.
Any serious treatment of electron in conductors needs to involve QM or it is suspect.
 
  • #29
First of all I speak of a c o n d u c t o r.
Now, about the quantum mechanical part.
In order to understand the drift of electrons in a conductor, what you need to know is the fermi velocity of electrons in room temperature (~2*10^6m/s) and the mean free path between two collisions (~400 Angstroms for copper).
Electrons in a conductor are not free. They are in the valence band . In order to jump to the conduction band they need infinitessimal energy since the band gap is almost zero. This energy they retrieve from the applied electric field .
Let's calculate the drift velocity now in order to see if this approach is 'suspect'.
The assumptions one takes are the following:
1) The electron moves free within its mean free path ( L=400 A). During the time of flight inside this path it is accelerated by the applied voltage.
2)After the mean free path it loses all its kinetic energy and starts from scracth due to collisions.

I am going to calculate with my approximations above the mean drift velocity of electrons in copper in room temperature:
We assume an electric field of E=1V/m

The electron is accelerated . It's acceleration is a=F/m=eE/m=e*1/m=
= (1.6*10^-19C)*(1V/m)/(9.11*10^-31 kg)=> a=0.2*10^12m/s^2 (1)

The time between two collisions, a.k.a the time within which the electron is free to accelerate, is the ratio of the mean free path to the Fermi velocity.

t = L/uF = (400 A)/(2*10^6m/s) = 2*10^-14s (2)

The max speed attained within the mean free path is :
(1)
umax= a*t = (0.2*10^12 m/s^2)*(2*10^-14 s)=4mm/s (3)
(2)

The mean velocity within the mean free path is 0.5*umax = 2mm/s
You can check the numbers in the bibliography for consistency.

The outcome of all these is that the mean velocity of electrons within the mean free path is analogous to the applied field . Each succesive mean free path has the same mean velocity. (Remember that electrons lose all their kinetic energy after one mean free path). Thus, the mean drift velocity is constant within the conductor and equals:
u = 0.5*(L/uF)*(e/m)*E

Therefore , if you increase the applied voltage, the electric field increases and thus the mean velocity increases. If you multiply this by the charge density of electrons and integrate by the cross section of the wire, you obtain the increase in current due to the increase in the applied voltage.
Therefore, each electron gains kinetic energy within one mean free path due to the increase in the applied voltage and this results in increase in the current.
 
  • #30
e.chaniotakis said:
each electron gains kinetic energy within one mean free path due to the increase in the applied voltage

Great, this is actually very visual :smile:

One question though, maybe this goes without saying or you even said it but you used some terms I didn't understand :smile:

If there is no current, free electrons are buzzing around in all directions. They have some kinetic energy depending on the temperature. So if we observe a conductor with no potential difference at its ends at 0 degrees Kelvin, electrons are not moving at all. So when we apply voltage we can imagine that their only motion is the one coming out of electrical force being applied to them.
But how do you overcome this problem at some other temperatures? Can we talk about kinetic energy of an electron as an effect of voltage at say 20 degrees celsius? I know that their net velocity is equal to the drift velocity, but energy is a scalar so there is no point in averaging it, since parts of it wouldn't cancel out...
 
  • #31
Dear Screwy,
I don't really get the point of your question!
In room temperature, when the fermi velocity is ~10^6m/s while the drift velocity ~1mm/s, the kinetic energy due to the electric field is negligible compared to the fermi energy.
What happens in the conductor is, to me, that the electrons gain collectively a small amount of velocity in the direction of the potential difference and that's what causes the current.
Now about the cooling part, zero degrees Kelvin cannot be reached (3rd law of thermodynamics), and as we approach it quantum behavior becomes prominent..- energy is not zero.
As you cool a conductor, resistivity drops . This can be translated as the fact that since the temperature drops, fermi speed drops and the collision rate drops two. Therefore there will be more 'time' for the electron to speed up due to the electric field. Therefore mean velocity increases and thus the current. Is that what you want?
Cheers

P.S: Here is an interesting link about quantum behaviour http://www.asu.edu/news/Science-2012-Ferry-45-6.pdf
 
Last edited:
  • #32
Screwy said:
So if we observe a conductor with no potential difference at its ends at 0 degrees Kelvin, electrons are not moving at all.
The Fermi distribution at 0K has many electrons with significant momentum - this can be interpreted as "movement", and it is nearly the same as for room temperature.
 
  • #33
Menaus said:
If only we adopted Steinmetz' suggestion of defining voltage by the electrostatic (he called it the dielectric) field around the conductor, then all of these misunderstandings would not exist.
That would only add yet another use of the word "voltage" to get confused over. ;)
The concept of particles hurts our understanding of electricity, it is much easier defined in fields and waves.
Restricting to a single simplified model can only hurt ... doesn't matter if it is particles, waves, or something more exotic. You use the model that best fits the situation. The kinds of electric circuits being thought about here are very special cases and models which work for other cases will be inefficient here and t'other way 'round too.

But I think you are basically correct: it is OPs insistence on imagining electricity a certain way that is producing the problems. The exploration so far looks a lot like adding epicycles ad infinitum rather than abandoning the model in favor of something easier to use, doesn't it?
 
  • #34
the electrons gain collectively a small amount of velocity in the direction of the potential difference and that's what causes the current

In average, right? Not each and every one of them. Most of the motion is brownian motion, but it is biased in the direction of the current, so when you sum up all the vectors of velocity all that remains is drift speed.
 
  • #35
Simon Bridge said:
But I think you are basically correct: it is OPs insistence on imagining electricity a certain way that is producing the problems. The exploration so far looks a lot like adding epicycles ad infinitum rather than abandoning the model in favor of something easier to use, doesn't it?

Nicely put. There is a terrible temptation to hang on, to the bitter end, with a familiar sounding model rather than make a leap to embrace a new one.
 
<h2>1. How does voltage affect the movement of particles in a current?</h2><p>Voltage is the force that drives the movement of particles in a current. Higher voltage means a greater force, which leads to faster movement of particles.</p><h2>2. Can voltage change the direction of particle movement in a current?</h2><p>Yes, voltage can change the direction of particle movement in a current. This is because voltage determines the direction of the electric field, which in turn determines the direction of particle movement.</p><h2>3. Does increasing voltage increase the number of particles in a current?</h2><p>No, increasing voltage does not increase the number of particles in a current. The number of particles in a current is determined by the material and the circuit, not by the voltage.</p><h2>4. How does voltage affect the speed of particles in a current?</h2><p>As mentioned earlier, higher voltage means a greater force, which leads to faster movement of particles. Therefore, increasing voltage will increase the speed of particles in a current.</p><h2>5. Is there a limit to how much voltage can affect particles in a current?</h2><p>Yes, there is a limit to how much voltage can affect particles in a current. This limit is determined by the material and the circuit, and exceeding it can cause damage or failure of the circuit.</p>

1. How does voltage affect the movement of particles in a current?

Voltage is the force that drives the movement of particles in a current. Higher voltage means a greater force, which leads to faster movement of particles.

2. Can voltage change the direction of particle movement in a current?

Yes, voltage can change the direction of particle movement in a current. This is because voltage determines the direction of the electric field, which in turn determines the direction of particle movement.

3. Does increasing voltage increase the number of particles in a current?

No, increasing voltage does not increase the number of particles in a current. The number of particles in a current is determined by the material and the circuit, not by the voltage.

4. How does voltage affect the speed of particles in a current?

As mentioned earlier, higher voltage means a greater force, which leads to faster movement of particles. Therefore, increasing voltage will increase the speed of particles in a current.

5. Is there a limit to how much voltage can affect particles in a current?

Yes, there is a limit to how much voltage can affect particles in a current. This limit is determined by the material and the circuit, and exceeding it can cause damage or failure of the circuit.

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