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How is possible for current to flow so fast when charge flows so slow?

We know electrons travel very slowly while charge travels at ~the speed of light.

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3 Answers 3

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It seems you are contrasting the speed of propagation of current with the speed of the individual charge carriers.

These two things are clearly separate. There are many examples. Consider sound.

A fire cracker goes off at the other end of a football field from you. You hear the sound a few 100 ms later. The air molecules that were by the firecracker didn't end up by you. They didn't travel far at all. However, they pushed on their neighbors, which pushed on their neighbors, etc, all the way to your ears. This pushing can propagate a lot faster than individual molecules can move.

Think of a long hollow cardboard tube filled with small balls just a little smaller than the inside diameter of the tube. All the balls are touching each other. You push on one ball on one end and move it 1 mm. The ball at the other end then moves 1 mm. However, none of the balls themselves moved more than 1 mm and they did that as slowly as you pushed, yet the propagation of the push was instantaneous on your human scale.

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Thank you. It's pretty clear now. –  Lay González Feb 24 '13 at 23:19
    
this answer is good..but i have question after this..so what is electricity. Or specifically what is electric energy. In analogy, if balls are pushed against each other than how a bulb make heat out of it. –  Muhammad Umer Oct 14 '13 at 13:47
    
and when talking about resistance is it about resistance to how many electrons can pass or how much they can transfer to each other. To me as of now everything is out-of-control :D –  Muhammad Umer Oct 14 '13 at 13:49
    
@Muha: In a resistive material, the electrons don't move completely freely. They bump around between molecules of the material and loose energy that way. This means a higher voltage is required to cause the same current flow. In the balls analogy, resistance is friction of the balls against the inside of the tube. –  Olin Lathrop Oct 14 '13 at 19:13

The moving charged particles within a conductor move constantly in random directions, like the particles of a gas. In order for there to be a net flow of charge, the particles must also move together with an average drift rate.That's what drift velocity is called.The drift velocity deals with the average velocity that a particle, such as an electron, attains due to an electric field. In general, an electron will move in a conductor randomly. Free electrons in a conductor vibrate randomly, but without the presence of an electric field there is no net velocity. When a DC voltage is applied the electrons will increase in speed proportional to the strength of the electric field. These speeds are on the order of millimeters per hour. AC voltages cause no net movement; the electrons oscillate back and forth in response to the alternating electric field Electrons are the charge carriers in metals and they follow a random path, bouncing from atom to atom, but generally drifting in the opposite direction of the electric field. The speed at which they drift can be calculated from the equation: I=nAvQ where I is the electric current n is number of charged particles per unit volume (or charge carrier density) A is the cross-sectional area of the conductor v is the drift velocity, and Q is the charge on each particle. Similarly when we tak about the speed of electricity we generally talk about the flow of signal not about the velocity of electrons.

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One needs to distinguish between two things when it comes to electricity, electric currents and voltages.

1) The electric current is flow of electrons in metal wires, (or in fluids like electrolytes). The electrons are moving in the wire at the drift velocity

$v=\frac{I}{enA}$

where: $I$ is the electric current; $e$ is the elctric charge on the electron; $n$ is the electron number density in the metal material of the wire; $A$ the cross section area of the wire.

Depending on the values of $I$, $n$ and $A$, the speed $v$ has a typical value of several cms$^{-1}$!

2) However, the cause of the motion of the electrons is the electric field, that you set in the wire when you switch on the light say, that travells along the wire at the speed of light. As the field travells along the wire so fast, it sets the electrons along the way into motion all along the wire. So it appears as if the electrons are moving very fast, when in fact they don't. I hope this clarifies your point you were trying to make?

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@Lay Gonzalez Please read my answer for extra information to that given by Olin Lathrop. –  JKL Feb 24 '13 at 23:26
    
Thanks for making explicit the concept "drift". The formula is also good to know. Unfortunately I can't upvote yet. –  Lay González Feb 25 '13 at 1:40
    
Now I can upvote, there you go. –  Lay González Feb 25 '13 at 21:03
    
@Lay Gonzalez Thank you very much, but most important of all, it is good that you have some deeper understanding of what is going on inside the wire. –  JKL Feb 25 '13 at 21:30

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