electron velocity

[ericgibbs]

hi,

Most of us 'know' the velocity/speed of radio waves and electrical effects.

A tutor once asked me, what is the actual drift velocity of 'individual' electrons, as current carriers in a conductor.

I took the trouble to reference the answer this, its an unexpected result.

I expect the more experienced members will know.

Any Newbie want to give it a shot?

EricG

 

[luisgerman]

hi,

.. what is the actual drift velocity of 'individual' electrons, as current carriers in a conductor.

Eric:

I was about to upload a .doc file, but it exceeds the permissible weight
Please e-mail me so I can send it to you

Kind Regards:

Luis German

 

[Hero999]

I don't know exactly what you're asking, but electricity generally flows through wires at a lower speed than the speed of light. Perhaps you should do some research into transmission lines.

https://www.google.com/search?clien...gation+delay&sourceid=opera&ie=utf-8&oe=utf-8

 

[luisgerman]

I don't know exactly what you're asking, ............

This is what he´s asking about:

(excerpts from the document I tried to upload)

The drift speed of electric charges
The mobile charged particles within a conductor move constantly in random directions. In order for a net flow of charge to exist, the particles must also move together with an average drift rate. Electrons are the charge carriers in metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the electric field. The speed at which they drift can be calculated from the equation:
.......................
Over a wide range of conditions, the flow of the charges quickly achieves a steady state value and remains constant. The "average speed" at which the "free" charges are moving in the wire is called the drift velocity vd.
..........................
The current in a wire can be expressed as function of the number of charge carriers/volume, the magnitude of the charge carriers, the drift velocity of the charge carriers, and the cross-sectional area of the wire.
...........................
Electric currents in solid matter are typically very slow flows. For example, in a copper wire of cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") at about a tenth of the speed of light.

The above are just excerpts which don´t include the math, discussion ...etc

 

[ericgibbs]

hi luisgerman,

Many thanks for you informed response.

Like you I have an understanding of the actual flow rate.

Once an novice engineer realises what is actually happening at the electron flow level, it brings a greater understanding of the meaning of 'resistance' and 'impedance also the causes of heating in a conductor.

Ohms law is ideal for solving circuit equations but does not explain the cause.

Regards
EricG
PS: I will PM asap.
Is it possible to zip your *.doc and try to re-post it, I am sure other members would appreciate it.

 

[ericgibbs]

hi hero,
Thanks for the 'transmission line' link, but as you can see from Luis reply,
that was not what I had in mind when I posted the OP.

Like yourself, many engineers think that the electron current carriers 'move'
along the conducter just below the speed of light. As you can see its actually
'flows' at mm/Sec.

Thanks for the interest.

Regards
EricG

 

[Nigel Goodwin]

hi hero,
Thanks for the 'transmission line' link, but as you can see from Luis reply,
that was not what I had in mind when I posted the OP.

Like yourself, many engineers think that the electron current carriers 'move'
along the conducter just below the speed of light. As you can see its actually
'flows' at mm/Sec.

It doesn't make any practical difference though - you stick a signal down a piece of wire and it travels at near enough the speed of light to the other end - I don't care what the electrons are doing!.

 

[luisgerman]

hi luisgerman,

Is it possible to zip your *.doc and try to re-post it, I am sure other members would appreciate it.

Sure I haven´t noticed that a zip can be 9.54Mb (I read 9.54K by mistake; I´ve got to change my glasses)

Regards:

 

[Sceadwian]

You might not care Nigel and it doesn't make a practical difference but it does make a difference from a scientific standpoint. Simply teaching only the way things practically work is basically trying keeping people dumb about a fundamental way things work. Not saying the whole world has to be brainwashed into only doing things from one standpoint but saying it's not important is short sighted.

 

[ericgibbs]

hi nigel,
I placed the OP on this section as its the 'theoretical' section, to stimulate a discussion on the topic.

I see that you have also posted to the thread, OP 'voltage'

You can see the that a poster to 'voltage' has limited knowledge of whats actually happening in a metallic conductor carrying a current. If he had some insight into electron flow, he would know the answer to his question.[ the guy using the car analogy]

Regarding 'I don't care what the electrons are doing' in your post, how do explain to your students the causes of 'noise' generation in a conductor carrying current.?

How do you explain the function of semiconductors ?

Most students still think a semi-conductor is a conductor that dosn't conduct very well!!
[the answer one gets to technical questions, when interviewing students for technical posts can be quite 'scary']

Regards
Eric
PS: thanks luis for zip.

 

[earjun]

Wait before we close the discussion now the drift velocity is very small in practical situations for example the drift velocityof electrons through a copper wire of cross section 2millimetresquare if 1 cc of copper contains 8.5*10 to the power 22 electrons is 0.0036mm/s which is very small but the moment we switch on things start working which are connected by copper wires covering long distance can someone explain this?

 

[luisgerman]

............ which is very small but the moment we switch on things start working which are connected by copper wires covering long distance can someone explain this?

One thing is the drift velocity,an another the speed of transmission of an
electrical signal down a wire.Electrical signals are electromagnetic waves which propagate at very high speed outside the surface of the conductor (moving at the speed of light, as can be deduced from Maxwell's Equations).

The waves of electromagnetic energy propagate rapidly through the space between the wires, moving from a source to a distant load, even though the electrons in the wires only move back and forth over a tiny distance.

Although the velocity of the flowing charges is quite low, the associated electromagnetic energy travels at the speed of light, the velocity factor being the ratio of the signal velocity versus the speed of light.

When a Voltage is applied across the ends of a wire, an Electric Field is created inside the wire.

In a vacuum the electric field would cause a charge to accelerate. In a wire, collisions of the conduction, charges with impurities, imperfections, and vibrations of the atomic lattice causes the motion of the conduction charges to be slowed down. This represents a loss of energy which is dissipated as heat.

The "average speed" at which the "free" charges are moving in the wire is called the drift velocity vd.

The current in a wire can be expressed as function of the number of charge carriers/volume, the magnitude of the charge carriers, the drift velocity of the charge carriers, and the cross-sectional area of the wire.

(arranged excerpts from zip above)

 

[earjun]

THANX Luis I couldnt open the zip file .so current is not flow of electrons????????

 

[luisgerman]

THANX Luis I couldnt open the zip file .so current is not flow of electrons????????

Current is a measure of the rate at which charge is flowing past some point in a wire.

Electrical current is a coarse, average quantity that tells what is happening in an entire wire.

So, as stated above the current in a wire can be expressed as function of the number of charge carriers/volume, the magnitude of the charge carriers, the drift velocity of the charge carriers, and the cross-sectional area of the wire, not just the flow of electrons.

 

[Oznog]

Well in some ways it's similar to a wave travelling across the water. The wave travels at a fairly high speed but the water itself goes nowhere. Like a stick in the water will stay in the same place as the wave rushes past.

Of course there are major differences- electrons do actually move in the wire.

 

[Sceadwian]

The water doesn't go anywhere until it gets close to shore at least, what little water motion actually occurs in large bodies of water is basically the same as heat generation in a conductor. There's actually very little difference.

 

[elMickotanko]

In a vacuum the electric field would cause a charge to accelerate. In a wire, collisions of the conduction, charges with impurities, imperfections, and vibrations of the atomic lattice causes the motion of the conduction charges to be slowed down. This represents a loss of energy which is dissipated as heat.

Is it not the other way round? I mean its heat that causes the vibrations of the atomic lattice, not the vibrations causing heat?
Its been a while since my devices class so i cant remember much.

 

[ZIGGY_DAN]

Is it not the other way round? I mean its heat that causes the vibrations of the atomic lattice, not the vibrations causing heat?
Its been a while since my devices class so i cant remember much.

If two particles are next to each other, and they are given energy in the form of motion, then they collide, they change their relative amount of kinetic energy(motion) and transfer some of it as heat. Energy can neither be created nor destroyed.

 

[luisgerman]

Is it not the other way round? I mean its heat that causes the vibrations of the atomic lattice, not the vibrations causing heat?
Its been a while since my devices class so i cant remember much.

elMickotanko:

Acute observation, yet not precise elMickotanko

(but indeed this kind of queries are what make things to be cleared up, and concepts to become solid and improved; thanks)

Heat transfer occurs by atomic lattice vibrations in solids. These vibrations can be broken down into the superposition of normal modes normal vibration. Through quantum mechanics and the wave-particle duality, these normal modes can be treated as particles. These particles are called phonons, which are in the class of particles called bosons. Lattice vibrations, and therefore phonons, travel at the speed of sound through a solid.

In thermodynamics and solid state physics, the Debye model is a method developed by Peter Debye in 1912 for estimating the phonon contribution to the specific heat (heat capacity) in a solid. It treats the vibrations of the atomic lattice (heat) as phonons in a box, in contrast to Einstein model, which treats the solid as many individual, non-interacting quantum harmonic oscillators. The Debye model correctly predicts the low temperature dependence of the heat capacity. Just like the Einstein model, it also recovers the Dulong-Petit law at high temperatures. But due to simplifying assumptions, its accuracy suffers at intermediate temperatures.

Following Planck’s idea on “energy quanta” originally applied to black-body radiation (1900), or according to the quantum mechanics founded by Heisenberg and Schr¨odinger (1925-26), physicists usually go forward as follows.

(1) Consider the lattice vibration as an infinite-dimensional system of harmonic oscillators.
(2) Decompose the system into independent simple harmonic oscillators, and calculate the distribution of vibration frequencies.
(3) Apply statistical mechanics to determine the macroscopic equilibrium state (the Gibbs state) of the quantized lattice vibration, and compute the internal energy U = U(T) (per unit cell) where T is the absolute temperature.

Then the specific heat is given by

C(T) =∂U/∂T

The surface relaxation effect and the local clamping effect are shown to be responsible for the specific heat of a small particle.

What you might refer to, and there you´re right it's about the amplitude of lattice vibration as a function of the temperature which is also calculated by the Debye model of a solid (Thermal Lattice Vibration).

For example, the wafer temperature during ion implantation is normally below 400 K, the lattice vibrations can influence the trajectories of the implanted ions. Especially the probability for scattering an ion out of a channel (de-channeling) is increased by increasing the wafer temperature. Due to the fact that the knowledge of the atomic lattice behavior is required for the simulation it is necessary to apply a temperature dependent lattice vibration model.

 

[elMickotanko]

elMickotanko:

Acute observation...

...What you might refer to, and there you´re right

Sorry, I paraphrased a bit. Put my on 'spin' on it hehe. I kind of understand some of it, but a lot of it is over my head because i dont have much knowledge of quantum mechanics/physics. I had a couple of classes on semiconductor devices which was really interesting but its hard to get my head around it all (and they were simplified to fit a broad branch of physics into a 1 semester class for engineers)! I understand that energy cant be created or destroyed so fair enough when two particles collide some energy is given of as heat.
I just meant that my understanding was that the lattice vibrations are caused by thermal energy (heat).