# Speed of Current

1. Jan 13, 2008

### Sparky_

Greetings,

I would think this question belongs in a "Electro-magnetics" section, if it is not relevant in this section please move it accordingly.

How can I calculate the velocity of the electrons or the current within a circuit?

Meaning, how can I show how resistance affects the speed of the current?

How can I show how various conductors affect the speed of individual electrons or current?

Likewise for various dielectrics?

Thanks
Sparky_

2. Jan 13, 2008

### Ben Niehoff

Last edited: Jan 13, 2008
3. Jan 13, 2008

### Troels

You might want to note that the speed of the current is not the same as the speed of the electrons.

The drift velocity of the electrons mentioned by Ben Niehoff is *snails pace*. It would take something like a year or so for a single electron to cover the distance from the plug to the household applicant, assuming it runs on DC. In an AC circuit the average drift velocity of the electrons are zero.

But the electric field that drives the electrons propagates through the wire at near speed of light, so *all* conduction electrons in the wire begins to move at nearly the same instant, and thus the speed of the current would be the speed of light in the appropriate medium.

Also, it doesn't really make sense to speak of electron speed or current speed in dielectrics, as a dielectric by definition is a insulator. Still, the electric field propagates with the speed of light in the given dielectric.

4. Jan 14, 2008

### AhmedEzz

why don't the electrons themselves travel , aren't they free to travel or are they only free to "roam" about their atoms ?

what does resistance do in the atomic level?

i hope sparky doesn't consider this as high jacking his thread...?!

5. Jan 14, 2008

### Troels

THe electrons are free to move, but their motion is by far dominated by the random thermal fluctuations, but these average to zero, so it is net displacement of an electron when you apply the electric field that is refered to as the drift velocity

As for resistance, the electrons colide with the massive nuclei as the move through the wire, transfering their kinetic energy to thermal energy as they go. Picture it, if you like as you driving your car with constant acceleration, but being forced to stop at red light at every 50 meters