Most Current Travels 4 Inches per Second

Q=3D 6.2 x 10 ^ 18 electrons in a coulomb

copper density of conduction electrons =3D 8.5 x 10 ^22 / cc =3D d

wire segment .1 cm x .1 cm x 10 cm (4 inch wire length of 17 gauge wire)

average speed of each electron in the current =3D 10 cm/sec =3D 4 inches/ sec

volume of wire segment .1 cc =3D b

n =3D number of electrons in wire segment =3D bd

f =3D fraction of an amp flowing =3D n/ Q =3D bd/Q

f =3D (.1cc x 8.5 x 10 ^22 electrons / cc) / 6.2 x 10 ^ 18 electrons

or if 14 amps are flowing, the speed is 100 times slower =3D .04 inches per second

... 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

OK, What's the rms velocity of an electron in a wire. (assume room temperature.)

George H.

Geo. asked: "What's the rms velocity of an electron in a wire?"

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

The speed of a signal in a wire (transmission line) is 1/(c(sqrt(LC)) where L and C are calculated here:

C is commonly 67 pf / meter

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.

Someone needs to make a list of this stuff.

Bret Cahill

OK, go ahead! ;-)

Cheers! Rich

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

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.

le

I see. If all electrons are considered, not just those in the conduction band, then the kinetic statistical model cited is too simple.

Even the wave in a stadium follows the same principles. The wave itself move much faster than a person being moshed would. ;-)

Cheers! Rich

so

That's not something that is intuitively obvious from childhood experiences like most Newtonian physics.

The train coupling example is somewhere in between. It can get people to think when they ordinarily wouldn't think.

Bret Cahill

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ople

Not quite, electrons in solids are a bit different from electrons in free space.

Do you 'do' electronics or do you just like to web search?

George H.

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.

A slinky can also work - you can do longitudinal or transverse waves.

Did you ever see those videos of the astronauts playing with the slinky in orbit? That was way kewl!

Cheers! Rich

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. :^)

George H.

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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.

Bret Cahill