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velocity of free electron?

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paris

Wed Jan 16 2013, 09:23PM

Registered Member #3042 Joined: Wed Jul 28 2010, 12:36AM
Location:
Posts: 121

info gathered.....

velocity in a DC field = mm's per hour
velocity in AC field = net zero
velocity in absense of field= net zero
velocity in silicon = 14000mm sec
velocity of em wave = 300 000 km sec

free electron must also be considered a neg ion

So what would the electron velocity be in fullwave rectified AC , the rectifier is made of si and theres AC 1 side and DC the other????

Patrick

Thu Jan 17 2013, 01:42AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

paris wrote ...

velocity in a DC field = mm's per hour

im not sure, given my experience with DeSeversky lifters, that your above comment is right.
If you increase the DC field to a very high value, electron motion becomes significant. Even in an atmosphere, much less a vacuum tube.

ive been told if you "paint a single electron" and were able to watch it through a narrow copper wire at several amps, it would take surprisingly long to move to the other end, i forget the specific numbers at the moment.

not sure if thats what you meant or not.

I need more cough syrup, very sick, reason annd logic are blurring ATM.

AndrewM

Thu Jan 17 2013, 03:00AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Are you talking about the velocity of a "free electron", as in one travelling through free space as in a lifter?

Or are you talking about electron drift velocity, as in a wire or conductor?

To the former you'd have to know the electron energy in electron-volts (i.e. if it passed through a 1kV field end-to-end it'd have 1000eV), convert eV to Joules (its a constant look it up) and then use E=.5*mv^2 to find the velocity.

To the later convert amps to Coulombs per second (again a constant), divide by the charge on an electron in Coulombs (constant again), and then divide by the cross sectional area of the conductor to get velocity.

EDIT: I think I forgot something in the latter example, you'll need to know charge carrier density, which I used to know how to calculate but... I forgot and am feeling lazy. Look that up and then mix it in somehow and you should have it. :)

klugesmith

Thu Jan 17 2013, 03:26AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Paris:

Your question needs more detail, to make any sense at all.
Electron drift velocity (whether average, peak, or instantaneous) is a function of CURRENT.
"Fullwave rectified AC" is not enough to know the CURRENT waveform, unless you specify a load (R? C? L?) on the DC side of your rectifier.

Once you know the current, the material, and the cross-sectional area of conductor, you can determine the "drift velocity".
Google it!

A key parameter is the number of charge carriers per unit volume of conductor material (free electrons per cm^3).

Or, if you like, the density of mobile charge (coulombs per cm^3) in the material.
That's a pretty large number, in metals.
If you multiply it by the wire volume swept out in 1 second (drift velocity x cross sectional area) you get the number of coulombs per second (a.k.a. amperes). Just like water moving in a hose.

The carrier density in semiconductor devices varies over many orders of magnitude, all much less than in metals. So the drift velocities are much higher for a given current density (amperes per cm^2).

...

Thu Jan 17 2013, 05:34AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The previous replies cover it pretty well, but one key point that is worth mentioning is that you need to define your 'velocity' more precisely to get a meaningful answer.

In vacuum things work as you would expect, the electron is accelerated by an applied electric field. Also note that an 'em wave' is a photon not an electron, and should not be included in your list.

In a solid things are much more complicated, because an electron cannot travel very far before it bumps into an atom, which changes the atoms direction, and thus the velocity in the direction of propagation. In this situation the actual speed of the electron is not a useful thing to know, and every electron will be moving at a different speed, so we use the term 'drift velocity' which is a sort of average speed of all of the electrons in the material. This can be calculated from the *current* (not voltage, as is the case in vacuum), and the density of electrons in the material, or the *voltage* across the device and a property called 'mobility' which is essentially the resistance of the material expressed in funny units.

For something like copper with a relatively low current density this can be in the mm per hour range. The number you specify for silicon looks like it is specified for the active region of a transistor (or perhaps you confused it for the mobility?, which is about 140000mm^2/v*s), which is not a fair comparison to your 'velocity in a DC field' because it is only so high due to the very high current density that one sees inside of a transistor. If you used a piece of bulk silicon the same size as a piece of copper wire the drift velocity for the same current would be about the same as that in copper, although due to the difference in mobility/resistivity the velocity for a DC field would be much higher in copper (due to the higher current flowing compared to lowly doped silicon).

hope that sends you down the right track...

paris

Thu Jan 17 2013, 08:05PM

Registered Member #3042 Joined: Wed Jul 28 2010, 12:36AM
Location:
Posts: 121

thanks guys ,
Dohhh! I didnt even leave a link to what I was going on about. I was reading up on drift velocity , mobility and then the thought of the silicon rectifier threw me.

I was not specific about the copper wire being the conductor.
I cant see how the silicon can have the figure 140000cm sec because of collision etc I need more reading in that dept

If the DC is not smooth then there is still the wave length of the freq , eg 300 000k/60 =5000km

net zero vel in AC = check ,
drift vel in Cu wire ,mms per hour = check

velocity/mobility in silicon ,14000cm sec = OUCH!! ....I need reading there

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