Electrons in a wire commonly travel much more slowly than most people expect. A simple calculation will show that in many common wires in houses, radios, and computers, the average speed of current flow is less than one meter per second. That slow current carries signals at more than 20% of the speed of light in a vacuum. Details at 11...
... and in AC circuits, the electrons just go back and forth and never make net progress down the wire!
The teaching analogy I like best for the slow drift velocity but high information speed is " marbles in a pipe". I made such a model to demonstrate to my (then) young nephew, using a foot of 7/8 PVC pipe, filled with same-colored marbles.
Use a long rubber band from one end of the pipe to the other to hold the marbles in place when you turn it vertical. Prepare this ahead of time, so the subject(s) don't know what you've done, and present it like magic trick: You gently push a (same-colored) marble into the bottom of the pipe (past the rubber band) and it appears to instantly pop out of the top! (Shoots up in the air a bit, when it overcomes the tension of the rubber band.)
Do this a few times, then push a different-colored marble in the bottom. Hmm, an original-colored marble pops out the top. Keep pushing in new marbles, until finally the odd one pops out.
Explain that this is like electricity, where one electron going into a wire pushes all the others along so that a different electron "instantly" pops out the other end. Since all electrons are interchangeable, it only *seems* that they move with great speed through the wire.
(Follow up: Years later, my nephew is now preparing to become an auto mechanic... so maybe this great teaching tool didn't stick. But at the time, he sure seemed to enjoy all the marbles bouncing around the room!)
Best regards,
Bob Masta DAQARTA v6.01 Data AcQuisition And Real-Time Analysis
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Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI Science with your sound card!
Geo. asked: "What's the rms velocity of an electron in a wire?"
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The rms (root mean square) thermal speed of electrons is given by Vrms =3D (3kT/m)^(1/2) where k is Boltzmann's constant, T is the temperature, and m is the electron mass... Vrms =3D 1.15 x 10^5 m/s at 20 degrees C. or rather.... 100,000 m/s.
The speed of light in a vacuum (c) is 300,000,000 meters per second or
3000 times as fast an that electron.
The speed of a signal in a wire (transmission line) is 1/(c(sqrt(LC)) where L and C are calculated here:
correction the speed of a signal on a wire : v= 1/sqrt(LC) the speed of light in an insulator v = 1/sqrt(permeability times permittivity) see the similarity?
Kind of like a train changing acceleration [jerk] when the velocity is only 5 mph. The slack in the couplings is only about an inch or so so the clatter up or down the train moves at 400 mph.
This phenomenon is easy to quantify intellectually but is still emotionally surprising, at least more than what might be expected.
Waves are almost all like that. Normal wave behaviour requires particle velocities much less than the wave propagation velocity. A compression wave moving faster than the speed of sound is called a 'shock wave'.
Cheers
Phil Hobbs
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Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
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email: hobbs (atsign) electrooptical (period) net
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Well, nearly. That's only for a free electron gas, not for the highly constrained electrons in a wire. The valence electrons are the only free ones (other electrons in lower shells are stuck), and they have an 'effective mass' that includes the electron's interaction with all the surrounding matter.
Both. Doing electronics ranges from inventing to pushing buttons. Search engines are a tools for reviewing basic electronics or glimpsing someone's research. This week's searches have been to learn about OLED, organic light emitting diodes. Also, a crystal ball has been done as a paper design. Imagine a 3D display as a sphere of glass mixed with samarium and europium phosphors, lit by 3 colors of laser beams. There is a problem with multiple points being illuminated that are not wanted. More ideas are needed.
OK, most of my electronics is stealing someone else's circuit and then trying to find parts that match my application. I'm much more of a knob turner, buttons are so digital. :^)
Maybe some waves are easier to understand than train clatter.
I'm still trying to get them to do the Brazil nut effect.
Toss a rock through sand coming off a conveyor belt and the rock will penetrate. Toss a rock down onto a beach and the rock won't get very far. The "beach" lasts longer on the bottom of the can than the top so the rock moves upwards. This is why there is a maximum effect at a certain acceleration/displacement.
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