WHY does CURRENT attain STEADY state?

[tan99]

When charges are flowing (current) through a combination of conductors of different resistances in series, then why don't the charges flow faster(more current) in the material with lower resistance and slower(less current) in high resistance material? Why is the current same(steady) in the whole path?
Like, in fluids, flow is faster when area of cross-section is less and vice-versa.

 

[Relena]

there is a mistake with your analogy
you have to compare the current I with the flow rate or the discharge Q, and you'll find it is constant whatever was the velocity of flow.

 

[K^2]

Because the voltage isn't constant throughout. If we keep to your fluids example, the pressure differential that's pushing the fluid through differs from section to section. It is higher in the narrow sections than the wide sections.

 

[tan99]

Thanks guys!...@K^2: Is the pressure differential is less across the larger area section because the flow is steady or the other way?

 

[AJ Bentley]

You have to watch out for analogies. They can suddenly throw up the wrong answer.

Can I point out that electric flow isn't the same as the flow of matter?
It's easy to get the idea of electrons pouring down a wire (those little black ones that usually spin around the outside;-)

It's actually more in the nature of a movement of electromagnetic field. It travels at or near the velocity of light. In the case of normal electron current in a conductor, the actual electrons move quite slowly but their fields butt up against each other which sends an e-field shockwave down the wire at the speed of light.

A better analogy is sound traveling down a solid rod when you strike the end. No matter how hard you hit it, the speed is the same.

 

[tan99]

nice insight! But i guess ur talking about the setting up of electric field as soon as the potential difference is created. And, by definition, current is the volume flow rate of the charge carriers across a cross section, which remains constant. By the way, the energy that is transferred as heat, etc. to say, a light bulb is carried by the electrons, right? correct me if I'm wrong.

 

[brainstorm]

A better analogy is sound traveling down a solid rod when you strike the end. No matter how hard you hit it, the speed is the same.

The problem with this analogy is that it's not empirically observable because you can't see how fast sound travels through a rod. There's also no analogies for resistance, voltage, and amperage as there is with water flowing through pipes. You can't observe the amount of sound energy flowing through a solid rod or the intensity of the sound waves. You could theorize them in terms of amplitude and frequency but you might end up back with the same water-flowing-through-pipes analogy to make intuitive sense of how it works. You want to use theoretical knowledge as an analogy for other theoretical knowledge. That's like the blind leading the blind.

You do make a good point about electricity not changing speed like water through pipes; and it is also a common misnomer that electrons actually flow through a conductor to transmit electricity instead of acting as a medium for energy transfer in the form of waves. I think many people think that when they flip a light switch the electrons at the switch actually travel to the light bulb the way water flows through pipes. So in that sense, you're right that the analogy needs critical qualification.

 

[AJ Bentley]

Can't really answer that one, it's a really deep question.

Electrons are quantum particles. Inside a conductor they exist as waves. They lose individuality like ripples on water. They share the electric field too, in a fundamentally intimate way -there's no saying 'this bit of field belongs to this or that electron'

When electricity flows down a wire, a particle electron enters the wire (say it's been fired from an accelerator or jumped across as a spark). It instantly ceases to exist and becomes part of the 'sea' of electron-stuff. It's entry may disturbs the distribution of matter/electrodynamic waves in such a way to cause an electron to 'pop' out of the other end.
Is it the same one - or a different one? It carries charge - that is - it has an electromagnetic field - we call that charge. Is it carrying the same charge it went in with - or the charge of the electron that just entered? or what? What IS charge anyway?

For convenience we say that electrons carry' charge in the same way that they 'carry' mass.
What that actually means is beyond me.

 

[brainstorm]

For convenience we say that electrons carry' charge in the same way that they 'carry' mass.
What that actually means is beyond me.

"charge" means that a particle has a tendency to repel or attract other particles depending on those particles' charge. Negative repels negative and attracts positive.

I don't know exactly what you mean by electrons "carrying" mass, but maybe you are claiming something about inertia being the product of electron momentum? I believe there are different models and theories that claim different things about the existence of mass. I think that at the quantum level there is not difference between matter and energy, that it's all field-energy. In that sense, the material qualities intuitively associated with atoms could be seen as patterned energy-field behavior, I think.

 

[AJ Bentley]

No, I'm not claiming anything.

I use the term 'carry' dissociate the concept of an electron (or any other particle) from it's properties as a way of pointing up the possible distinction. It serves to remove the idea that an electron somehow 'owns' it's charge or it's mass. Or that either property is an intrinsic part of electron-ness.

As for mass, at any level, mass and energy are the same thing.
Almost, that is what I am trying to say, matter is not mass. It possesses mass (including what we call rest mass) but matter itself is something else that isn't really understood.
If you could take an electron and somehow remove it's rest-mass and then remove it's charge, would there be something left?

Actually I should be really talking about quarks in this context - but life's too short...

 

[brainstorm]

No, I'm not claiming anything.

I use the term 'carry' dissociate the concept of an electron (or any other particle) from it's properties as a way of pointing up the possible distinction. It serves to remove the idea that an electron somehow 'owns' it's charge or it's mass. Or that either property is an intrinsic part of electron-ness.

As for mass, at any level, mass and energy are the same thing.
Almost, that is what I am trying to say, matter is not mass. It possesses mass (including what we call rest mass) but matter itself is something else that isn't really understood.
If you could take an electron and somehow remove it's rest-mass and then remove it's charge, would there be something left?

Actually I should be really talking about quarks in this context - but life's too short...

What do you mean by "owns?" What is an electron except its properties and behaviors? For whatever reason, electrons have been used to explain numerous phenomena including light-generation, electricity-transmission, chemical behavior, and the volume of atoms.

Coming up with plausible models for how electrons work in these various situations and subjecting these models to testing according to empirical measurements and observations is the cause of "electron-ness." If someone discovers that it's not electrons but some other type of energy or field that is responsible for transmitting electricity, then that part of the concept's explanatory value would fall away, no?

 

[Dale]

I use the term 'carry' dissociate the concept of an electron (or any other particle) from it's properties as a way of pointing up the possible distinction. It serves to remove the idea that an electron somehow 'owns' it's charge or it's mass. Or that either property is an intrinsic part of electron-ness.

What do you mean by "owns?" What is an electron except its properties and behaviors?

I'm with brainstorm on this. The mass and charge of an electron is not just an intrinsic part of electron-ness it is the definition of what an electron is. An electron is not just some shopper trying on different shoes to find out if the positive charge or negative charge shoe fits better. An electron is the fundamental fermion which has a negative charge and .5 MeV mass. If something had different properties then it wouldn't be an electron. The disassociation of a particle from its properties is at best a useless philosophical device as far as I can tell.

 

[AJ Bentley]

Apologies all round, I'm in danger of hijacking the thread. Last post - honest!

I don't think anyone would argue have argued in the 18th century that the kinetic energy of a particle is the particle itself.
These days we know that the kinetic energy is mass, which might be considered an intrinsic part of the particle. We don't know where the .5MeV rest mass of an electron comes from but it may well have an external basis similar to kinetic energy.

I feel the same about charge - the charge of an electron doesn't seem to me to be the same thing as the electron itself - I can imagine an electron stripped of it's charge.
(You might argue that it's no longer an electron - but that's just nomenclature)

Is it unreasonable to assert that mass and charge and the other fundamental properties have a separate existence from each other? Some particles have one or another, or more. Given the question over the origin of rest mass can one absolutely state that matter consists entirely and solely of these phenomena?

What I am suggesting is that there is another fundamental property that you could simply call matter or 'existence', which is common to all of these.

I'm willing to concede that it might be a useless concept but I feel uncomfortable without 'something' as a base to which I can add the observed properties.

 

[Dale]

I feel the same about charge - the charge of an electron doesn't seem to me to be the same thing as the electron itself - I can imagine an electron stripped of it's charge.
(You might argue that it's no longer an electron - but that's just nomenclature)

No, it is not just nomenclature, it is physics. That object you are describing, a fermion with .5 MeV mass but no charge, would not behave like an electron physically. Such an object would be experimentally distinguishable from a fermion with .5 MeV mass and -1 charge, regardless of the nomenclature.

 

[brainstorm]

Apologies all round, I'm in danger of hijacking the thread. Last post - honest!

I don't think anyone would argue have argued in the 18th century that the kinetic energy of a particle is the particle itself.
These days we know that the kinetic energy is mass, which might be considered an intrinsic part of the particle. We don't know where the .5MeV rest mass of an electron comes from but it may well have an external basis similar to kinetic energy.

I think it is true that an object-orientation dominated and continues to dominate materialist consciousness in many ways. I don't know how many people still think of an electron as a tiny piece of solid matter, but I would guess lots. How could energy account for rest mass, though? The best I can come up with is that inertia could be a kind of gyroscopic resistance to motion, but a gyroscope only resists motion perpendicular to its axis; it doesn't increase its inertia in any direction.

I feel the same about charge - the charge of an electron doesn't seem to me to be the same thing as the electron itself - I can imagine an electron stripped of it's charge.
(You might argue that it's no longer an electron - but that's just nomenclature)

This part I don't get. If you said that charge is just the product of a direction of motion of an rotating field or something similar, I could see your point - but I don't see the point of simply saying that charge is completely separate. Surely it must be a product of some aspect of the electron's fundamental construction/mechanics?

What I am suggesting is that there is another fundamental property that you could simply call matter or 'existence', which is common to all of these.

I have never not been called a crank for suggesting this, but I have the opinion that recursivity of energy/field motion is the cause of energy behaving as matter. It seems to me that linear energy is radiation therefore non-linear/recursive energy would behave as a particle with inertia/mass.

 

[AJ Bentley]

I don't see the point of simply saying that charge is completely separate. Surely it must be a product of some aspect of the electron's fundamental construction/mechanics?

The concept of charge is an abstraction that allows you to put a mathematical handle on certain aspects of the behaviour of matter. Without charge as a separate idea, I think it would be difficult to make any sense of the world.

Electrodynamics deals with the concept of charge and it's interaction entirely separately from any other properties of matter. You can go through the entire subject without once mentioning any particular charged particle.

Similarly, mass is a concept that allows mathematics to be applied to other aspects of the world.

My point is this. Some of the 'stuff' in the universe has charge, some has mass, some has both, some neither. What I find intriguing is the fact that in stuff that has both, the two go round together.
When you apply an electric field to a charged, massive particle, the charge moves - that's understandable - but why does the mass go right along with it?
There's nothing in the whole subject of electrodynamics that requires mass and charge to be associated.
You can argue that that is how the world IS - but that's a cop-out.

To me, matter is a concept, similar to but separate from mass and charge that has the property of 'being' - it's what holds the charge and mass together.
It's also the subject of quantum mechanics - it's to the Schrödinger Equation what Q is to Coulombs law.

 

[K^2]

Thanks guys!...@K^2: Is the pressure differential is less across the larger area section because the flow is steady or the other way?

As it has been mentioned, it doesn't work exactly the same way in hydrodynamics and electricity. It's just an illustration.

The reason you get constant current is because if you have a divergence in flow, more charges flow into a segment of wire than flow out. That segment becomes charged, and the potential at that point rises, which works to reduce current going in and increase current going out.

The steady state, of course, is when the potential at a point stops changing, and that happens when the currents on both sides are equal.

There is something similar happening in hydrodynamics with buildup of pressure, but there are a lot of additional considerations there, so don't stretch the analogy too thin.

 

[AJ Bentley]

Relena gave the correct answer.

there is a mistake with your analogy
you have to compare the current I with the flow rate or the discharge Q, and you'll find it is constant whatever was the velocity of flow.

Current is amount-per-second not velocity. Charges-per-second compare to litres-per-second
There's really no equivalent concept for velocity in electricity flow.

 

[brainstorm]

Electrodynamics deals with the concept of charge and it's interaction entirely separately from any other properties of matter. You can go through the entire subject without once mentioning any particular charged particle.

You're right about just claiming that particles have charge or not because that's the way things are is a cop out. But how would you explain the existence of an electric field without disequilibrium caused by positive and negative battery terminals, or some other differential state between an electron-rich and electron-hungry side? What would cause something to draw electricity if not it's charge?

 

[AJ Bentley]

What would cause something to draw electricity if not it's charge?

Electricity flows round a loop if you wave a magnet over it.

Not fair - electromagnetism is just a relativistic effect of moving charges.

Seriously though, I didn't say anything about electricity flowing without charge - exactly the opposite - charge IS electricity.

What I am pointing out is that charge is a completely independent 'thing' in it's own right - it isn't caused by, or dependent on something or anything else except it's own properties.

As far as I know, there are no particles that have been discovered that have charge but no mass - although there are those that have mass but no charge - the neutron for one. But I maintain that it's possible that someone might one day find a particle with 'bare' charge (zero mass)

If someone says that is impossible, I challenge them to prove it by logical means.

 

[brainstorm]

I maintain that it's possible that someone might one day find a particle with 'bare' charge (zero mass)

If someone says that is impossible, I challenge them to prove it by logical means.

Light traverses paths defined by the contours of spacetime, supposedly, and those contours are defined by gravitational fields. So I wonder if you could say that light is energy that is charged in the sense that it must travel along paths defined by gravitational "poles" or "nodes."

I realize this is a stretch, but could light proceed in a direction where there was no further mass to extend the gravity field of the last node the light passed in its trajectory? Presumably the light would "loop around" and ultimately proceed in the direction of whatever massive object determined the continuity of spacetime in that direction, no?

 

[AJ Bentley]

Gravitation is mass-related not charge-related. A photon of light carries no charge and has zero rest-mass although it does possesses energy which equates to mass.
It IS affected by gravitation - that's how the General Theory of Relativity came to be verified.

The trouble with light is that it isn't matter (whatever matter is!). It's ripples in space time itself. If you wonder how ripples in space time can be the same as a photon particle - join the club!
Since the universe is curved in on itself, so is the trajectory of light. the curvature is somehow caused by the presence of mass within the universe.

This is all standard stuff that must have been covered elsewhere in the forum.

 

[Dale]

This thread is rapidly degenerating to pure speculation. I would like to remind all participants on the rules about overly speculative posts.

Light is not ripples in spacetime, light travels along geodesics, gravitational nodes and poles are undefined terms, there are no known particles with charge but not mass, etc.

 

[Naty1]

tan99

When charges are flowing (current) through a combination of conductors of different resistances in series, then why don't the charges flow faster(more current) in the material with lower resistance and slower(less current) in high resistance material?

The simple answer to your question is that IF electrons in any portion of a series circuit were to try to move faster, where would they go?? They'd immediately, at almost the speed of light due to their electric field, run into other slower ones in the next material and be repelled...


If you were to rethink your question in a parallel circuit instead you'd get other insights not discussed here. In that case the current flow among materials with different IS different...can you explain why?? and is also in a "steady state" which depends on the potential not usually the conductors.

Also try Wikipedia:
http://en.wikipedia.org/wiki/Electric_current

http://en.wikipedia.org/wiki/Electrical_resistance

 

[Naty1]

tan99:

By the way, the energy that is transferred as heat, etc. to say, a light bulb is carried by the electrons, right? correct me if I'm wrong.

that's partially correct. But you don't want to think that the increased kinetic energy of conduction electrons makes them move any faster..in fact as a light bulb filament heats up current flow slows. All particles experience increased kinetic energy (heat).

It is now known that Joule heating is caused by interactions between the moving particles that form the current (usually, but not always, electrons) and the atomic ions that make up the body of the conductor. Charged particles in an electric circuit are accelerated by an electric field but give up some of their kinetic energy each time they collide with an ion. The increase in the kinetic or vibrational energy of the ions manifests itself as heat and a rise in the temperature of the conductor. Hence energy is transferred from the electrical power supply to the conductor and any materials with which it is in thermal contact.

http://en.wikipedia.org/wiki/Joule_heating

 

[brainstorm]

Gravitation is mass-related not charge-related. A photon of light carries no charge and has zero rest-mass although it does possesses energy which equates to mass.
It IS affected by gravitation - that's how the General Theory of Relativity came to be verified.

The trouble with light is that it isn't matter (whatever matter is!). It's ripples in space time itself. If you wonder how ripples in space time can be the same as a photon particle - join the club!
Since the universe is curved in on itself, so is the trajectory of light. the curvature is somehow caused by the presence of mass within the universe.

This is all standard stuff that must have been covered elsewhere in the forum.

I realize this is speculative, but since the notion of charge without mass was raised, I responded by speculating about what massless energy is (i.e. light/em-radiation) and then considering how it could have a charge. Since it doesn't have a charge, I reflected on how the concept of charge could be interpreted in a broader sense. The conclusion I reached was that charge is the ability for distinct "nodes" (these could be particles, objects, points, etc.) to channel field-force between them unidirectionally. So, for example, electric current results from a negative "node" supplying free electrons in the direction of a positive "node," which lacks and therefore received them. Presumably, the cause is that systems are seeking balance (i.e. equilibrium).

How could matter be seen as the "positive node" of light where light-emission is the negative (i.e. sending) node? If gravity is indeed the definitive form-giver to the contours of spacetime, and light can only travel according to those contours, then could it not be said that spacetime is a conduit for light/energy? How does the positive node in an electric field/current get its positive charge? By lacking enough electrons to balance the positive charge of the protons, right? Similarly, how does a black hole shrink to a volume smaller than its schwartzchild radius? It collapses after expending its energy-generating potential, right? So couldn't you say, theoretically, that a black hole is like a positive node for light, since it attracts and receives it without re-emitting it?

If black holes are the positive-nodes/receptors of light, then could it also not be said that all non-black-hole forms of matter exert gravitation in such a way that produces resistance for light, in the same sense that electric current encounters resistance, slows down, and translates its energy into other forms? Here I'm lumping together various interactions/behaviors of light/energy vis-a-vis matter/gravitation such as absorption/re-emission, chemical effects. KE generation, and directional shifts. Generally, couldn't all deviations of light from its direct path to a black hole be viewed as stops along a particular conduit of spacetime?

I realize this is very theoretical and speculative, so please let me know if it should be reposted as a new thread. The issue raised about charge without mass was an interesting and provocative one and I'm just responding by thinking about the issue. This is not an attempt to hijack the thread in any sense.

 

[Dale]

I realize this is very theoretical and speculative, so please let me know if it should be reposted as a new thread.

It should be posted to a different forum where such speculation is encouraged. PF is about helping students and others learn aboud mainstream science. There is nothing in your post that contributes to that goal.

 

[brainstorm]

It should be posted to a different forum where such speculation is encouraged. PF is about helping students and others learn aboud mainstream science. There is nothing in your post that contributes to that goal.

1) When someone else posts speculative discussion about the idea of charged energy without mass, why do I get blamed for speculating by responding to it?

2) Don't you think it helps anyone, including students, think about what the meaning of "charge" or "mass" is in a more general sense? Also, don't you think that this kind of fundamental level of understanding is what ultimately drives radical scientific developments? Speculation about quirky-sounding ideas can propel both understanding of established concepts as well as provoke pushing the envelope of existing science.

Since I already know that you are a person who won't stop until you dominate someone with your police-ethic, don't bother because I'm already intimidated from continue this line of discussion. It's a waste, though, because the original idea of charged energy without mass was interesting, and the discussion of how light may exhibit charge-like properties in its responsiveness to gravity is not speculative but comparative-analytical.

 

[Dale]

Since I already know that you are a person who won't stop until you dominate someone with your police-ethic, don't bother because I'm already intimidated from continue this line of discussion.

I put off responding to this because I wanted to give it some thought first. I understand from your comments that my outspoken insistence on the forum rules bothers you, and in the interest of having a friendly community I thought it might be helpful if I explained my motivations for doing so.

PhysicsForums was not my first online physics community; the place I found first was SciForums. SciForums encouraged speculation as you suggest, and as a result nearly every single thread degenerated into an ugly shouting match between a vast array of crackpots with different worldviews and a few mainstream apologists. Since most of the worldviews were completely uninformed by experiment or logic the discussions were completely pointless. My personal experience is that speculation on an internet forum does not accomplish the goals of "pushing the envelope of existing science" nor promoting the "understanding of established concepts".

In particular, the poor students who wandered in came out far worse off than had they never found the site. It is often impossible for a student to distinguish between fact and fiction in such an environment. The founders of PhysicsForums wanted something different from the many other sites, they wanted a place that would actually be helpful to students. That is why they set up the homework forums as well as the rule against speculation. Those rules are not a hindrance to PF's mission, but the core mechanism for accomplishing it and making PF a unique place.

The reason that I have my "police-ethic" is that I understand from personal experience the practical result of not having these rules, and I don't want to see PF degenerate into that kind of a forum. If you like PF, then I would suggest that, although you may not realize it, these rules are a large part of the reason why you like it. Also, the rules are completely voluntary, you don't have to post here, so if you choose to do so you should be happy to live by the rules you voluntarily agreed to.

If someone really wants to speculate then there are other places online to do so, but I strongly feel that it is important for the integrity of PF and its value to the net that they not do so here.

I hope that helps you understand my point of view and my reasons for doing what I do. I am sorry if I have offended you, but perhaps you can see why it is important to me.

 

[kmarinas86]

When charges are flowing (current) through a combination of conductors of different resistances in series, then why don't the charges flow faster(more current) in the material with lower resistance and slower(less current) in high resistance material? Why is the current same(steady) in the whole path?
Like, in fluids, flow is faster when area of cross-section is less and vice-versa.

Think of the "electron gas" concept. The energy stored due to compression of a gas is pressure * volume. Higher voltage means the amount of electrical potential energy stored per charge is greater, which is much like having a fluid at higher pressure. Higher voltage of static charges means electrons are closer to each other. This is because like charges (repelling charges) have higher potential energy when they are closer together than when they are farther apart.

When current is produced, it is the result of turning the electrical potential energy to the kinetic energy of moving charges. Therefore, there is a limited amount of work that can be done on charges in a given closed circuit.

Now because electrons in a metal behave like a fluid, charge flow moving into a thin wire requires that individual charges speed up, and this is because area * velocity = volume flow rate. Volume flow rate is your "current".

For a wire of given length, half the cross-sectional area means that a given current passing through it at a given voltage will pass half as many charges at the same time. That means that to generate the same current, the individual charges need to move twice as fast through that length. If individual charges are forced to move twice as fast, then their specific kinetic energy, and therefore the drag per charge in the wire, must increase by the square. The amount of drag that results depends on the material that the charge is moving through.

We know that heat losses equal resistance * current^2. Well it turns out that passing the same current through a wire with half of the cross-sectional area will double the heat losses, and this corresponds to the fact that we have half the charge which has four times the specific kinetic energy. This assumes that the wire has the same electrical resistivity, a term which refers to the inverse conductivity. A wire having the same dimensions as another but with twice the resistivity has twice the resistance.

So in short, equilibrium is attained by a force balance between the pressure of the electron voltage and the drag forces between electrons and the conducting medium.