How electron movement produces current,instead of having a slow drift speed

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Solution 1

Am I correct that you can rephrase your question to 'electrons move so slow, how come that when I flip the light switch the light comes on basically instantly?'?

It's true that the electrons travel very slowly. But these electrons don't have to travel across the wire to power your light bulb.

In electromagnetism, we have the continuity equation $\nabla J = 0$. It says that current can't 'heap up' somewhere in the wire. So when you flip the switch, all electrons in the wire start moving simultaneously.

It's analogous to a bicycle - when you start pedalling, the entire chain starts moving rather than the links closest to the pedals.

Solution 2

The information about beginning of the flow of current is transmitted through the propagation of electromagnetic waves and not with drift velocity of electrons. Hence, any electric appliance turns on almost instantly, when the switch is closed.

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Harry Potter

Updated on August 01, 2022

Comments

  • Harry Potter
    Harry Potter over 1 year

    Just need a clarification here, how the current is produced due to the movement of electrons, in an external circuit,having a very slow drift speed.

    Normally in a battery there is high potential terminal and low potential. Using these two terminals the external circuit is closed. Now within the battery the direction of the current flow and the electron flow is opposite to that of the external circuit. If I consider that positive current is flowing from the positive terminal to the negative terminal of the battery through the external circuit then we can say that positive terminal is at higher potential then the negative terminal of the battery.

    Now when we are closing the switch of an external circuit, in that case the electrons are moving from negative terminal to positive terminal of the battery, through the external circuit. But we also know that the drift speed is very slow, of the electron. But when we switch on some of the electrical devices, within a fraction of second the device starts working. If drift sped of electron is low, so how the device is working so fast ,(near about the speed of light,I guess), as we know that current flows due to the flow of the electrons.So how it is possible, in spite of electrons are having such a low drift speed ?

    Please help me guys !!!!!

    • Ruben
      Ruben over 9 years
      While the electrons themselves have a low drift speed, they start moving everywhere simultaneously the moment a potential difference is established.
    • Harry Potter
      Harry Potter over 9 years
      Normally what I've understood,the wire is having lot of electron(from their atomic structure context). So negative terminal of the battery is having electrons which are having high energy(in terms of negative charge). But rest of the electrons are not having the high energy. So just need to clarify whether these high energetic electrons are repelling the rest of the electrons in the atoms through the entire wire. So that their drift speed might be slow, but they are moving fast due to the electric field produced by the negative terminal of the battery. Am I correct ?
    • Ruben
      Ruben over 9 years
      The electric field is indeed instantaneous, but all electrons have the same charge $q=1.6*10^{-19}\text{ C}$, and also the same energy. The electrons move because of a potential difference established by the battery.
    • Harry Potter
      Harry Potter over 9 years
      Now throughout the wire,there exist a big number of electron, and also in the electrical device.But the electrons at the negative terminal of the battery is having high energy.AFAIK any electrical device is a energy transformation system. So when the switch is on, at that time the high energized electrons are not reached to that electrical device. But the device is producing output, so from where the energy is coming ? So can it be said like this that energy is propagating from one electron to another ? The electrons which are confined within the electrical device, how they are getting energy?
    • Ruben
      Ruben over 9 years
      The device is powered by the current produced by the moving charge. The electrons at the negative terminal have higher potential energy, given by $U=qV$. This does not mean that they 'magically' have more energy in the sense that they will make the (e.g. light bulb) shine more brightly. This is because they have lost that potential energy by the time they get to the electrical device. The power of the electrical device comes from the voltage times the current $P=VI$, both of which are established immediately once the circuit is closed.
    • Ashish Ahuja
      Ashish Ahuja over 2 years
      cross-post on electronics SE electronics.stackexchange.com/q/100008/97373
  • Harry Potter
    Harry Potter over 9 years
    Normally what I've understood,the wire is having lot of electron(from their atomic structure context). So negative terminal of the battery is having electrons which are having high energy(in terms of negative charge). But rest of the electrons are not having the high energy. So just need to clarify whether these high energetic electrons are repelling the rest of the electrons in the atoms through the entire wire. So that their drift speed might be slow, but they are moving fast due to the electric field produced by the negative terminal of the battery. Am I correct ?
  • Harry Potter
    Harry Potter over 9 years
    Now throughout the wire,there exist a big number of electron, and also in the electrical device.But the electrons at the negative terminal of the battery is having high energy.AFAIK any electrical device is a energy transformation system. So when the switch is on, at that time the high energized electrons are not reached to that electrical device. But the device is producing output, so from where the energy is coming?So can it be said like this that energy is propagating from one electron to another?The electrons which are confined within the electrical device,how they are getting energy?
  • Kvothe
    Kvothe over 9 years
    They're not moving fast; they're pushing and pulling ('repelling' as you say) the electrons in front of them. It's comparable to the chain on a bicycle.
  • Harry Potter
    Harry Potter over 9 years
    OK,Now throughout the wire,there exist a big number of electron, and also in the electrical device.But the electrons at the negative terminal of the battery is having high energy.AFAIK any electrical device is a energy transformation system. So when the switch is on, at that time the high energized electrons are not reached to that electrical device. But the device is producing output, so from where the energy is coming?So can it be said like this that energy is propagating from one electron to another?The electrons which are confined within the electrical device,how they are getting energy?
  • Kvothe
    Kvothe over 9 years
    When you ride a bicycle you turn the pedals, the gear starts spinning and this exerts a force on the chain links currently hooked in the gear. The instant you do this, the opposite gear starts spinning as well. Where is that energy coming from and how is it being transferred to the other gear?
  • Steve Jessop
    Steve Jessop over 9 years
    "can it be said like this that energy is propagating from one electron to another" -- yes, and it does so faster than the drift velocity of the electrons.
  • MDMoore313
    MDMoore313 over 9 years
    Good analogy...
  • luk32
    luk32 over 9 years
    I think another good example is a Newton's cradle. If you start pushing a ball on one end, the others start to move instantaneously. Regardless of the speed you apply at one end, the other will move at the same speed at once. It is the information that they "need to move" which is traveling fast (about speed of light), not the particles themselves.
  • Steve Jessop
    Steve Jessop over 9 years
    @luk32: right, and in the case of Newton's cradle I guess the ball at the far end starts to move not quite instantaneously, but after a delay roughly the speed of sound in steel along the row. Not roughly the speed of the incoming ball along the row :-)
  • Martin Petrei
    Martin Petrei over 9 years
    The electric field is a force field, that establish almost instantaneously. This field is responsible of electron's movement. This includes all electrons, in the wire, in the device... Then the energy comes from the work applied by the electric field on the electrons.
  • luk32
    luk32 over 9 years
    @SteveJessop Why sound, not light? My intuition is that "touch" is transferred by electromagnetic forces primarily. I know there are van der Waals forces also, but is think they are related to exchange of photons too. (Of course this still is in order magnitudes away from the "observable movement" so the original point holds just as firmly).
  • Kvothe
    Kvothe over 9 years
    @SteveJessop, because the speed of sound (in a material) is the speed at which vibrations (sound!) travel through the material.
  • Steve Jessop
    Steve Jessop over 9 years
    @luk32: What Kvothe says. The speed of sound in the material is the speed at which a physical shock wave travels through it. In other words, if you bash one end it's the time required before the other end moves. It's true that all of this is to do with the electromagnetic force -- I'm afraid I cannot explain why the speed of sound in a conductive material can be less than the speed of propagation of an electrical potential difference...
  • luk32
    luk32 over 9 years
    @SteveJessop But vibrations are internal to the system, I am talking about situation there all balls touch each other and you push one. From this perspective you have no vibrations, and the system moves as a whole. I am not sure if the notion of "touch" should be related to internal vibration, maybe i am not sure.
  • Steve Jessop
    Steve Jessop over 9 years
    @luk32: I repeat, if you bash one end it's the time required before the other end moves :-) It doesn't make a whole lot of difference whether the motion is a vibration (that is to say, is closely followed by a motion in the opposite direction) or not, although I dare say it makes a difference under some circumstances. In super slow-motion and with sufficient magnification you could see a compression wave travel the length of the row of balls on impact.
  • Steve Jessop
    Steve Jessop over 9 years
    (could see in principle, I mean. I think actually the distortion would be minuscule for a low-speed impact).
  • luk32
    luk32 over 9 years
    @SteveJessop I think the main difference would come from the difference of "bash" and "already touching". I do not work with mechanics enough to be sure. I can ask around though. I can get a proper scientific answer if this is of any interest. Also the "bash" can travel at speed of sound, BUT the whole system (at least the intermediate balls) is stationary in regard to the "outside world" frame of reference if you just bash one ball against the other, that is something different when you move the whole system.
  • Steve Jessop
    Steve Jessop over 9 years
    @luk32: but if you have a steel rail, it is "already touching" all the way along. Nevertheless, sound and other shock waves travel in it at approximately 6000m/s, nothing close to the speed of light. You seem to be saying that an impulse will travel faster than that through a row of balls. I'd be interested to know what the connection is though between the speed of longitudinal and transverse waves, but I think this is all basically off topic since it's just an analogy for the real question.
  • luk32
    luk32 over 9 years
    @SteveJessop Sound yes, of course, I think the dispute, or rather my trouble is whether the nature of phenomenon we are talking about is moving whole system, or just introducing a phononic excitement to it. Maybe it is the same. I actually think this is a matter for another question ATM. It is interesting to me, and still not much relevant for the one here.
  • Auden Young
    Auden Young about 7 years
    Hello Frank and welcome to Physics Stack Exchange! Generally, if there is already an answer, only answer if you have a new point of view.