Drift velocity and propagation velocity

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elimenohpee

Joined Oct 26, 2008
47
I'm having a hard time understanding the difference between drift velocity of electrons and the propagation velocity of the field or signal through a conductor.

Electrical current is the flow of charge per unit time past a given reference point, but when you study the actual drift velocity of the electrons in a given conductor, it is on the magnitude of millimeters per second. I'm having a hard time wrapping my head around the fact that the actual electrons drift so slowly, however propagation of the signal is on the order of 70% of the speed of light. Intuitively, it would make sense to think that the electrons are whizzing around the conductors at the same speed as the propagation of the signal.

Can anyone provide an intuitive way to think about this?
 

vk6zgo

Joined Jul 21, 2012
677
There are a couple of analogies which have been used in the past:

(1) A group of people are queueing for a bus.
If the rear person pushes the next person,the "push" travels through the
queue very much faster than any of the people move.

(2) A tube is filled with tennis balls.
If we now press the rear ball,a ball falls out the other end.
In that time the ball we pushed has only moved one ball diameter,but the "push" has moved from one end of the tube to the other.

Actually,in an open wire transmission line,signals move at considerably more than 70% of the speed of light.
 

MrChips

Joined Oct 2, 2009
31,079
I like analogy (2) better than (1).

The thing to remember is that while electrons move at the drift velocity, it is the "information" that travels at close to the speed of light.
 

nsaspook

Joined Aug 27, 2009
13,552
I'm having a hard time understanding the difference between drift velocity of electrons and the propagation velocity of the field or signal through a conductor.

Electrical current is the flow of charge per unit time past a given reference point, but when you study the actual drift velocity of the electrons in a given conductor, it is on the magnitude of millimeters per second. I'm having a hard time wrapping my head around the fact that the actual electrons drift so slowly, however propagation of the signal is on the order of 70% of the speed of light. Intuitively, it would make sense to think that the electrons are whizzing around the conductors at the same speed as the propagation of the signal.

Can anyone provide an intuitive way to think about this?
Electrons move because of the electrical fields moving near them. Don't think about the electrons in this context and concentrate on the fields that carry the energy and signal information. So forget intuition or analogies and use the facts.

http://amasci.com/miscon/ener1.html
 

vk6zgo

Joined Jul 21, 2012
677
Electrons move because of the electrical fields moving near them. Don't think about the electrons in this context and concentrate on the fields that carry the energy and signal information. So forget intuition or analogies and use the facts.

http://amasci.com/miscon/ener1.html
You say "forget analogies",& yet the discussion you linked to uses a similar analogy:-

"The EM energy moves from electron to electron (it's a bit similar to the waves which occur when a hammer strikes the end of a very long rod.)"
The OP knows that electromagnetic energy travels at close to the speed of light,he is just looking for a way to "get his head around it".

Analogies have a long & honourable history in Electronics.

For instance,EEs still use the term "Tank circuit" when referring to a Parallel Resonant circuit,(although this term has virtually died out among other Electronics folk).
 

nsaspook

Joined Aug 27, 2009
13,552


Analogies have a long & honourable history in Electronics.

IMO they cause more harm than good...
They way to understand and get your head around it is to study electrodynamics. An electromagnetic field (dynamic entity moving according to relativistic field equations) is as real as any physical object with unique properties that don't really mesh into a clean analogy with a physical object. Understand that "current and voltage" are not analogy’s but are simplifications of complex equations about the flow of electrical energy in space and it's interaction with matter. You don't need to understand complex calculus to begin to see how beautiful it really is when the analogy veil is lifted.

The electronic–hydraulic analogy (derisively referred to as the drain-pipe theory by Oliver Heaviside) is the most widely used analogy for "electron fluid" in a metal conductor.
http://en.wikipedia.org/wiki/Hydraulic_analogy#Limits_to_the_analogy

The hydraulic analogy can give a mistaken sense of understanding that will be exposed once a detailed description of electrical circuit theory is required.
http://www.youtube.com/watch?v=iMDTcMD6pOw&feature=related
 
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vk6zgo

Joined Jul 21, 2012
677
IMO they cause more harm than good...
They way to understand and get your head around it is to study electrodynamics. An electromagnetic field (dynamic entity moving according to relativistic field equations) is as real as any physical object with unique properties that don't really mesh into a clean analogy with a physical object. Understand that "current and voltage" are not analogy’s but are simplifications of complex equations about the flow of electrical energy in space and it's interaction with matter. You don't need to understand complex calculus to begin to see how beautiful it really is when the analogy veil is lifted.
I don't really care a damn how "beautiful" it is!:D

I'm happy to understand what it does,& get on with it.

The analogy never interfered with my visualisation of how it really worked.
Some people do have problems at a very early stage of their studies,& to tell them to "go study electrodynamics" is more likely to discourage them than anything else.



http://en.wikipedia.org/wiki/Hydraulic_analogy#Limits_to_the_analogy

http://www.youtube.com/watch?v=iMDTcMD6pOw&feature=related
I'm not sure what Dr Feynman showing off has to do with the point of discussion.:D
 

nsaspook

Joined Aug 27, 2009
13,552
I'm not sure what Dr Feynman showing off has to do with the point of discussion.:D
Dr Feynman
No ball bearings and springs
as analogies.

I think people underestimate the ability of a person just starting out in "electronics" to understand the basic nature of it being the manipulation of fundamental forces. If you're explaining it to a person on the street who will never have to study anything beyond how to wire a light switch then it's ok but I'm sick and tired of having to retrain and educate otherwise great technicians with two years of school behind them on basic field theory because they draw a blank on a simple question about RF energy flow in a process machine. Analogies work best when we also understand the dangers of first impression and inform the students of the limitations and how they fall apart easily. This seems to be missing in courses today as I see well trained people straining to stretch the hydraulic analogy to crazy lengths in technology based discussions about electronics.

I love the artistry in electronics. If you can't hear the music of it, you don't really understand it or maybe you're just tone deaf to the rhythm. ;)
 
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