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WHY does CURRENT attain STEADY state?

  1. Jun 28, 2010 #1
    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.
     
  2. jcsd
  3. Jun 28, 2010 #2
    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.
     
  4. Jun 28, 2010 #3

    K^2

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    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.
     
  5. Jun 28, 2010 #4
    Thanks guys!....@K^2: Is the pressure differential is less across the larger area section because the flow is steady or the other way?
     
  6. Jun 28, 2010 #5
    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 travelling down a solid rod when you strike the end. No matter how hard you hit it, the speed is the same.
     
  7. Jun 28, 2010 #6
    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.
     
  8. Jun 28, 2010 #7
    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.
     
  9. Jun 28, 2010 #8
    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.
     
  10. Jun 28, 2010 #9
    "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.
     
  11. Jun 28, 2010 #10
    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...
     
  12. Jun 28, 2010 #11
    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?
     
  13. Jun 28, 2010 #12

    Dale

    Staff: Mentor

    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.
     
  14. Jun 28, 2010 #13
    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.
     
  15. Jun 28, 2010 #14

    Dale

    Staff: Mentor

    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.
     
  16. Jun 28, 2010 #15
    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.

    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?

    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.
     
  17. Jun 29, 2010 #16
    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 Schrodinger Equation what Q is to Coulombs law.
     
  18. Jun 29, 2010 #17

    K^2

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    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.
     
  19. Jun 29, 2010 #18
    Relena gave the correct answer.

    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.
     
  20. Jun 29, 2010 #19
    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?
     
  21. Jun 29, 2010 #20
    Electricity flows round a loop if you wave a magnet over it.:tongue:

    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 dependant 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.
     
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