electron velocity

[luisgerman]

Sorry, I paraphrased a bit. Put my on 'spin' on it hehe. .

That´s quite all right elMickotanko

Note to the forum users: please excuse me if I´m extending this topic to a feasible not adequate stage, but it´s just for the sake of responsibilty with the inquiry which I believe demands a response.

The statement is from a geometric standpoint; more specifically lattice vibrations in crystalline solids, from a geometric view point, in particular concerned with a mathematically sound computation of the specific heat, a typical thermodynamic quantity in solid state physics.

Maybe a distinction between heat and temperature must be recalled:

In physics, heat, is defined as energy in transit. Generally, heat is a form of energy transfer associated with the different motions of atoms, molecules and other particles that comprise matter when it is hot and when it is cold.

Heat is the transfer of energy caused by the temperature difference.

The temperature of a system is related to the average energy of microscopic motions in the system. For a solid, these microscopic motions are principally the vibrations of the constituent atoms about their sites in the solid.

Temperature increases as the energy of this motion increases. The motion may be the translational motion of the particle, or the internal energy of the particle due to molecular vibration or the excitation of an electron energy level.

Think of an isolated model:

Regardless of the temperature(except for the "theoretical" 0 Kelvin),lattice vibrations <A periodic oscillation of the atoms in a crystal lattice about their equilibrium positions> occur

Solely,the inter-atomic forces allow the vibrations of atoms which involve small excursions from the equilibrium positions.

There are two components to thermal energy. One component is the internal potential energy of the system - the energy the system contains at any moment due to the relative placement within the system of all its constituent parts. The second component is the internal kinetic energy of the system - the energy the system contains at any moment due to the relative motion within the system of all its constituent parts.

So there´s always a particular potential and kinetic energy associated with it, for any given temperature (not 0 K)

In physics, a phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid

Phonons play a major role in many of the physical properties of solids, including a material's thermal and electrical conductivities

A crystal lattice at zero temperature lies in its ground state, and contains no phonons. According to thermodynamics, when the lattice is held at a non-zero temperature its energy is not constant, but fluctuates randomly about some mean value. These energy fluctuations are caused by random lattice vibrations, which can be viewed as a gas of phonons. (Note: the random motion of the atoms in the lattice is what we usually think of as heat.) Because these phonons are generated by the temperature of the lattice, they are sometimes referred to as thermal phonons.

In insulating solids, phonons are also the primary mechanism by which heat conduction takes place.

In a regular lattice with harmonic forces between atoms, the normal modes of vibrations are lattice waves.

 

[thushy]

Let m be a total number of current carriers (eg electrons)

then m = nAL where n=number of carriers per unit volume, A=cross sectional area of the conductor and L=length of the conductor.

thus the total charge in the conductor Q = nALe where e=1.602E-19(elementary charge)

Vd(Drift velocity) = L/t and i=Q/t

Solving these equations leads to Vd = i/(nAe) = J/ne (J=current density)

Obviously this result only holds true for conductor with constant A and DC.

 

[Cabwood]

Mostly you don't design electronic circuits with the individual charges in mind. You don't plumb your home with any consideration for water molecules. Some (e.g. semiconductor, radio, microwave engineers) might need to consider fields and charge movement, but for the most part voltage and current changes in one part of a circuit change the circumstances of the whole and at roughly the speed of light. For all intents and purposes, instantly.

So it's not appropriate to lump physics and electronic design/analysis together. They are separate disciplines with some overlap.

For newcomers to electronic design, my opinion is that it doesn't help to understand charge flows in the majority of cases, because it conflicts with the application of principles like Ohm's and Kirchhoff's laws. Therefore I discourage it.

Ohm's law and Kirchhoff's laws are all applied holistically. One doesn't think "current flows in here, get's "modified" somehow, and flows out, then it goes through here, but over there current hasn't yet reached", and so on. The whole "charge movement" scenario only encourages that kind of thinking, which can't help the beginner.

I believe it is best to only consider charge movements and speed of propagation only after the fragile and rocky initial stages of learning circuit design and analysis is complete. If you must learn both, they are best treated as separate disciplines, and not combined - yet.

 

[mneary]

You don't plumb your home with any consideration for water molecules.

Actually the distinction here is instructive. When you turn the tap on, your flow rate doesn't depend on the drift velocity. But if you're waiting for hot water, it's something you need to know.

 

[Cabwood]

That's a wonderful analogy - waiting for hot water. May I use it?

 

[ericgibbs]

hi cabwood,

Not wishing to pull the 'plug' on this water analogy.

When I started this thread running, it was to dispel the notion that the individual electrons forming an electric current in a conductor, travel at the speed of light thru the conductor.

Looking back thru the postings, some members, thought this to be so.

We all take for granted the electrical effect, as far as we are concerned, is instantaneous.

My tutor, way back, explained the reason for a conductor having resistance, is due to the 'impeded' movement thru conductor of individual electrons.

This also makes the student aware of one the reasons conductors can get hot if carrying current.

I agree 'early' on in a 'electronic' students learning curve, he/she should learn and understand the fundamentals.

Regards
Eric

 

[mneary]

It sounded brilliant to me at the time, but now I see the analogy is flawed.

Perhaps the fact that water comes out immediately could be more analogous.

Feel free to correct it and use it.

 

[RODALCO]

The simple way too look at the speed of transfer of electrons in a medium, and easy to understand is:

You have a horizontal pipe fully loaded with marbles, which are all touching each other, you put in one extra marble on one side, an other marble gets pushed out immidiately at the other side, hence an instant transfer of information.

The speed is almost equal as the speed of light 300,000 km/sec.

In case the marbles are not tightly packed you have a type of propagation delay which happens in certain timing circuits. One marble gets pushed in the pipe, next one bumps the next one and so on. The little spaces between the marbles cause small delays in the transfer of information and the marble at the end doesn't fall out immidiately.

And i have not lost my marbles yet, believe you me

 

[luisgerman]

..... You don't plumb your home with any consideration for water molecules......

Understanding the principles involved I think it´s the purpose of
this kind of threads

One might plumb a home without any consideration for water molecules, but an appropriate conceptual comprehension of the main principles involved is what makes the difference between a plumber and a hydraulics engineer

 

[Keruskerfuerst]

Speed of electrons in a wire: 1cm/s.

 

[Sceadwian]

1cm a second? Where'd you come up with that random number?

 

[Cabwood]

1cm/s. Right, that's that all cleared up then. I don't suppose you happen also to know the speed of water molecules in a pipe?

 

[Nigel Goodwin]

I'm just really pleased I moved this silly thread to the **** Chat forum!

 

[luisgerman]

I'm just really pleased I moved this silly thread to the **** Chat forum!

After reading some of your posts I wouldn´t expect anything different from you.

The Super Moderator calling this thread a silly thread

Next think I expect from you is to throw me out of this forum

I´ll be pleased if you do so

(My last post as long as you are the moderator)

 

[Hero999]

This thread is silly

:cry: I don't like what you just said. I'm leaving! :cry:

 

[Sceadwian]

As long as it's actually in the ****-chat forum now I don't feel so bad about leaving one sentance pointless replys.

 

[NeoMach]

Current measured in amps, 1amp = 1Coulum/1sec. 1 coulum = 6.25X10^18Electrons.

 

[crashsite]

Wait before we close the discussion now the drift velocity 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?

The usual analogy is a hose that's already full of electrons....er...water. When new water comes in one end, old water immediately comes out the other due to the interactions of the water molecules (but, not instantly. There is a propagation delay for the effect to travel through the hose). The actual water travels slowly but pressure changes propagate rapidly.

 

[dknguyen]

Speed of electrons in a wire: 1cm/s.

THat number sounds familiar to me. I think we came up across it in my second electromagnetics class. Except I think it was more like 2-3cm/s. I forget how we calculated it.

 

[neon]

according to einstein's laws e[v+phase ]=hc where e=electron charge =1.6x10-19 coulombs phase=work function in volts =6.6x10-34 joule/sec c=velocity of light=3x10+10 so now do your math.ephase=hc/lanbda=12000 armstrong units of course thiese are photons emissions.