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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 !!!!!

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  • $\begingroup$ While the electrons themselves have a low drift speed, they start moving everywhere simultaneously the moment a potential difference is established. $\endgroup$
    – Ruben
    Feb 17, 2014 at 9:33
  • $\begingroup$ 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 ? $\endgroup$
    – Harry Potter
    Feb 17, 2014 at 9:45
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    $\begingroup$ 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. $\endgroup$
    – Ruben
    Feb 17, 2014 at 11:35
  • $\begingroup$ 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? $\endgroup$
    – Harry Potter
    Feb 17, 2014 at 12:56
  • $\begingroup$ 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. $\endgroup$
    – Ruben
    Feb 17, 2014 at 22:53

5 Answers 5

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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.

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  • $\begingroup$ 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 ? $\endgroup$
    – Harry Potter
    Feb 17, 2014 at 12:40
  • $\begingroup$ 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. $\endgroup$
    – Kvothe
    Feb 17, 2014 at 13:02
  • $\begingroup$ 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? $\endgroup$
    – Harry Potter
    Feb 17, 2014 at 13:03
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    $\begingroup$ 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? $\endgroup$
    – Kvothe
    Feb 17, 2014 at 13:17
  • $\begingroup$ "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. $\endgroup$
    – Steve Jessop
    Feb 17, 2014 at 13:45
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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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Although the electron velocity is very low, which is propagated almost instantaneously is the electric field. This causes the effect that all the electrons in the wire to start moving simultaneously (almost).

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  • $\begingroup$ 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? $\endgroup$
    – Harry Potter
    Feb 17, 2014 at 12:57
  • $\begingroup$ 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. $\endgroup$
    – Martin Petrei
    Feb 17, 2014 at 18:49
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Electromagnetic force is not propagated by electrons, it is propagated by photons. By definition these travel at the speed of light (in the material). Impedance and capacitance play a part in how quickly the system responds to you turning it on / connection a battery, but are generally very small in a plain wire.

The electrons are moved by electromagnetism (in this case specifically by electrical potential difference), and the electromagnetic field is caused by the movement of electrons. The rate at which they move and are caused to move has nothing to do with the speed of propagation of the electromagnetic field that moves them.

You might as well say, "I was walking home at 4mph when I called my friend and invited him to meet me at my house. He drove there at 30mph. Given that we move at those speeds, how can the telephone message travel faster?". The analogy is not perfect since walking does not inherently cause phone calls, and the energy to move the vehicle didn't come via the phone call. The similarity is that the signal was caused (indirectly) by your movement home, and it caused your friend to move. Despite this causation the signal was not propagated by either of those motions, and neither was the energy.

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This is to clarify Martin Petrei's answer. Electrons have an electric field and this field is already established between every electron in the conductor by field-to-field coupling. It is just like a long line of rail cars where their couplers are already compressed for pushing or are already in tension for pulling. You push or pull on one electron and this field-to-field coupling pushes or pulls every electron such that there is a near simultaneous action at the other end of the conductor.

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  • $\begingroup$ 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. $\endgroup$
    – auden
    Oct 11, 2016 at 0:45

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