For you electronically adept types

[280plus]

Can't think of a better place to toss this one out. 

I have noticed many similarities between the way water flows through pipes and electrons flow through wires. So the other day I was watching a thing on whirlpools and got to a wonderin'. Would it be possible to make an electron whirlpool? AND How cool would that be? 

[makattak]

Electron Whirlpools


It was an important conceptual step to realize that an
impinging magnetic field B could be viewed as creating
tiny whirlpools, so-called vortices, in this lake of
charge—one for each flux quantum f05h/e of the magnetic
field [Fig. 12(d)]. The notion of a whirlpool is quite
appropriate, since such vortices have indeed a quantummechanical
‘‘swirl’’—a phase twist—to them. Inside the
vortex, electronic charge is displaced, dropping to zero
in the center and recovering to the average surrounding
charge density at the edge of the vortex. The extent of a
vortex is roughly the size of the area which contains one
quantum of magnetic flux (area3B5f0).

[Nick1911]

Magnetrons basically do that.  The circular motion imparted on the electron cloud creates reverberations in the cavities along the rim - the cavities are sized to resonate at 2.4Ghz.  And that's what creates the RF in a microwave oven.     Cool stuff!

[RoadKingLarry]

Magnetrons basically do that.  The circular motion imparted on the electron cloud creates reverberations in the cavities along the rim - the cavities are sized to resonate at 2.4Ghz.  And that's what creates the RF in a microwave oven.     Cool stuff!

Just to be clear, the magnetrons for microwave cookers are tuned to 2.4Ghz, different frequencies for different RADARs. But more or less, yeah.

[RevDisk]

So the other day I was watching a thing on whirlpools and got to a wonderin'. Would it be possible to make an electron whirlpool? AND How cool would that be? 

I just heated my Thai food with one.   

[birdman]

The actual movement of an electron in a wire is quite limited.  In fact, for AC they really don't move much at all, and in just about any DC applications, they move an almost Imperceptable amount.
Leave your laptop on all day and -maybe- an electron that started in the battery will make it to the CPU.
(Google: drift velocity)

[280plus]

Hah! Very cool. I was just talking to the kid about this. He was just promoted to "Senior Aerospace Engineer" at GD so I like to toss stuff like that at him too. HE says no but I kinda knew it might be possible. I also tossed the "drift velocity" at him in terms of the "hole theory" that days the electrons don't move forward (we'll say) but the holes between them actually move backwards. He wasn't down with that either. lol See, you can still have fun with the kids even when they's all growed up.

[280plus]

I just heated my Thai food with one.   

Which is, in fact, very cool! Or warm! Whichever you prefer

[230RN]

Here ya go:



Well, a spiral anyhow. 

[AJ Dual]

The actual movement of an electron in a wire is quite limited.  In fact, for AC they really don't move much at all, and in just about any DC applications, they move an almost Imperceptable amount.
Leave your laptop on all day and -maybe- an electron that started in the battery will make it to the CPU.
(Google: drift velocity)

I never knew about that. 

So... like in a USB cell phone charger at 1 amp, with a wire that's maybe 1mm thick, the electrons are actually moving, at about .00001 meter a second or so. And voltage doesn't play into it, just the amperage.

Damn physics, you crazy.

 

[280plus]

See, now that is very interesting because the definition of an "amp" is one coulomb of electrons passing any given point (IIRC) (Apparently I do) : http://www.britannica.com/science/coulomb

But that may not necessarily be true? 

 

[230RN]

^ That what I was thinking, especially in terms of things like electroplating and current through a vacuum tube (valve) or cathode-ray tube (CRT).

<Section on points of confusion deleted.>


Noting birdman's qualification of "in a wire," I reckon I'd better read up on his suggestion to do some googling.

Terry, 230RN

[280plus]

Yes, tubes definitely crossed my mind.

[birdman]

See, now that is very interesting because the definition of an "amp" is one coulomb of electrons passing any given point (IIRC) (Apparently I do) : http://www.britannica.com/science/coulomb

But that may not necessarily be true? 

 

It's absolutely true..,there are just an assload of electrons in play.
Copper has about 8x10^23 electrons per cubic cm.
One coulomb is ~6x10^18 electrons
So if you have current density in amps/cm
1 A/cm2 = 1 coulomb/s moving through that square cm
So let's say the "wire" is 1cm long
1 amp through this stubby wire means ~6x10^18 electrons moved into one face, and the same moved out the other side, and all the ones in the middle just moved a little bit
(And they all rolled over and one fell out...)
So the speed is 6x10^18 / 8x10^23 cm/s per amp/cm2
Or ~7.5x10^-6 cm/s

Now, most wires typically carry several amps per square mm, so the velocity in those is 2-300x as high
Which means ~1.5x10^-3 cm/s
So after an hour...the electrons only move about 2 inches.  And that is at a few amps/mm2 (a bunch)

^ That what I was thinking, especially in terms of things like electroplating and current through a vacuum tube (valve) or cathode-ray tube (CRT).

<Section on points of confusion deleted.>


Noting birdman's qualification of "in a wire," I reckon I'd better read up on his suggestion to do some googling.

Terry, 230RN

Math works here too.
This is why electroplating takes so much current...effectively, 1 amp for 1 second is 1/100,000 of a mole of electrons...and most electroplating processes are 1 electron = 1 atom deposited.

So for copper that means 1 amp for 1 second = 1/100,000 of a mole = ~640 -micrograms- of copper
(For 1 mil thick copper that's ~3 square mm)
So think about the last time you electroplated copper...a board 10cm square on both sides with 1 mil (25um) copper takes about 6700 amp-seconds, or ~2A for an hour...seems about right, right?

But the speed of the electrons is still pretty slow...a 1 molar electrolyte solution has ~6x10^20 electrons per cm3, so in the above circuit board example, the current per unit area is 0.01A/cm2, or 6x10^16 electrons/cm2/s...so the electrons are moving in the so,union at ~1/10,000 cm/s

For CRTs, again, let's do the math.
The beam current is <100 micro amps, or about 10^15 electrons per second.
Now, the fun part.  Let's assume a 1.5 megapixel display at 60hz refresh, with a 0.3mm dot pitch
That's 10^8 pixels/s, each of which gets ~10^7 electrons

slight digression:
The raster dot has to move about (10^8 * 0.3mm) 30 million mm/s (neato, the dot moves laterally at 30,000m/s, or 0.01% the speed of light).

Ok, back
Since the electron velocity has to be sufficient to put 10^7 electrons into the 0.3mm wide pixel in 10^-8 seconds, we can assume the electron ps have to move faster than 0.3mm in 10^-8 seconds (for a 45deg beam angle...most are less than this, meaning slightly slower electrons, but for this calc, that's fine)
So here the electrons are moving at 0.3*10^8 mm/s, or 30,000m/s!!

so here the electrons are moving 10 billion times faster than in the electrolyte (even though the current per unit area is 100uA/0.3mm Diameter, 0.14 A/cm2 is 10x as high as the electrolyte, and 10x smaller than the wire).
BUT the electron density is ~10^7 electrons in this 0.3mm diameter, 0.3mm long packet, or an electron density of 10^7 electrons in 1/50,000 of a cm3, or 5x10^11 electrons per cubic cm...which is a -billion- times smaller than in the electrolyte solution!
So now, the 10 billion times faster makes sense...the average electron speed goes up with current density, and down with electron density--so one billionth the electron density = one billion times faster, and 10x the current per unit area = 10x faster than that = 10 billion.


Basically...what we just discovered is the origin of resistance in a way--in the CRT, thousands of volts are needed to move that current density across a few 10's of cm...in the electrolyte, a few volts, and in the copper, a few millivolts (note, its kinda apples and oranges here, but the general trend is what I'm talking about).
For metals, the more free mobile electrons Ina given volume, the slower the electrons need to move for a given current = the less energy of each lost when the collide with an atom = the less electromotive force (voltage) needed to get them moving again = the lower the resistance.
(There are many other effects...like it's actually atom dependent on how much the electron is attracted to the atom and how much EMF it takes to make it mobile again...which is why silver is more conductive than gold, but again, I digress)

One easy way to examine this:
Measure the resistance of a glass of water...
Add a little salt and dissolve...
Measure again...
Keep adding salt (waiting for it to dissolve) and measuring
Th resistance will keep decreasing because you are adding more and more mobile charge carriers = slower movement = less voltage required to keep them moving = lower resistance (voltage required to move a given charge at a given velocity :) )

[280plus]

"And they all rolled over and one fell out". Sweet! lol

[birdman]

Side note, the reason the electron density of the beam in tubes is small is electrons repel each other...so if you put too many in one place...the beam pushes itself apart (Google "space charge limits")--so for CRTs, to get smaller pixels and/or faster scanning, you need to get the electrons going faster so you can get enough down in one place without increasing the density too much...so you increase the voltage.

To put in perspective, I have worked on some electron...well, let's call them "tubes" that had current densities of 100's of amps / sqcm...and these required hundreds of thousands of volts for even a few cm gap, yielding electron densities only 10-20x more like than the CRT in my above example...because even when that "tube" is 50x100cm...that level of space charge needs high power magnets to keep the beam from repelling itself out the sides.


In electrolyte solutions and metals (and plasmas)...for every electron there is a positive ion, and thus the average charge is zero and the electron density can be really high.  


*note on the CRT example...the electrons actually move 100s of times faster than this to prevent smear and even further reduce space-charge, because in my first example, that electron density would mean a HUGE amount of beam spread.  Most of the time, CRTs have electron velocities of ~10% the speed of light or even higher...which means their electron densities are 10-100x smaller than my first example.  Just so you know...
:)

[230RN]

^ Thank  you.  "Basically...what we just discovered is the origin of resistance in a way--in the CRT, thousands of volts are needed to move that current density across a few 10's of cm...in the electrolyte, a few volts, and in the copper, a few millivolts (note, its kinda apples and oranges here, but the general trend is what I'm talking about)."

That's when I started to "grasp*" it, when I compared the small voltage drop in a small length of the wire required to keep them moving plus the large number of electrons involved in a cross section of the wire....compared to the high voltages in that tiny electron beam in a "tube" device... i.e., as in a CRT, measured in kilovolts....

I'm still having trouble with them leaping across a spark gap, though.  As in lightning, f'r instance.  Lemme cogitate on that awhile...

Of course, that's not "in a wire," but in a conductor with very small (comparatively) number of electrons per unit cross sectional area.... that is, air.... hmmmm.

Or in low-pressure neon in a neon bulb operating on DC.

Terry, 230RN

* That's the part I deleted in my above post.... just playing with the concepts in my high-resistance head.

[birdman]

^ Thank  you.  "Basically...what we just discovered is the origin of resistance in a way--in the CRT, thousands of volts are needed to move that current density across a few 10's of cm...in the electrolyte, a few volts, and in the copper, a few millivolts (note, its kinda apples and oranges here, but the general trend is what I'm talking about)."

That's when I started to "grasp*" it, when I compared the small voltage drop in a small length of the wire required to keep them moving plus the large number of electrons involved in a cross section of the wire....compared to the high voltages in that tiny electron beam in a "tube" device... i.e., as in a CRT, measured in kilovolts....

I'm still having trouble with them leaping across a spark gap, though.  As in lightning, f'r instance.  Lemme cogitate on that awhile...

Of course, that's not "in a wire," but in a conductor with very small (comparatively) number of electrons per unit cross sectional area.... that is, air.... hmmmm.

Or in low-pressure neon in a neon bulb operating on DC.

Terry, 230RN

* That's the part I deleted in my above post.... just playing with the concepts in my high-resistance head.

Plasma (arc, lightning, neon tube) different than CRT...no space charge (well, it does matter...but getting into that depth of plasma physics is way beyond this forum).  But, if you google the physical dimensions of a lightning bolt, and it's peak currents...and how long it lasts...you will likely find that it's somewhere in-between in terms of electron density and electron velocity...as with TIG arcs, plasma torches, etc...with a corresponding increase in V/m over conductors, an decrease vs vacuum e-beams.

Also note, the "resistance" idea for vacuum beams isn't quite proper...because they will propogate forever technically if in a vacuum...but if you kinda sorta think of "limiting" the beam to a given spread width, like a virtual "wire" you will see that they have to go faster and faster to go farther with the same spread limit...so the resistance analogy is somewhat descriptive (even though, unlike a wire the voltage to spread to distance is highly non-linear as the electrons accelerate away from each other at a density-dependent force...AND at a certain point, relativity matters and you don't go faster...but the relativistic mass increase means they accurate away from each other less (increase in virtual mass, but the Coulomb repulsion stays the same, so it still kinda works...but is even more non-linear)

But hey, start with the simple analogy and work from there...

[birdman]

Btw, I HATED my plasma physics classes.

[280plus]

(well, it does matter...but getting into that depth of plasma physics is way beyond this forum). 

My wife said something similar to me when IO asked to explain what it is she does at work. She was right.

Interesting on electrons repelling each other. You would think that would be intuitive but it never occurred to me before.

[brimic]

I'm just going to sit at the back of this lecture hall, nod at appropriate times, and see how much of the water coming from the firehose I can capture with my thimble.   

[HeroHog]

I'll be on Facebook or playing "Angry Birds"...

[230RN]

The thimble-firehose analogy applies to me, too, but it gives a knowledge core to work on.  Thanks !

Terry

[280plus]

My son is a riot. He's all dead serious about this and I'm like just yanking his chain. Apparently, per him, we can't create an electron whirlpool but photons are another thing. Again, in the unbeknownst to me category, microwaves are made of photons. Who knew?

But now, having read the latest comments, I have a new line of questioning. Why are these birds so angry?

[Firethorn]

Again, in the unbeknownst to me category, microwaves are made of photons. Who knew?

*raises hand*

Technically speaking everything from x-rays to infrared are all photons of varying energy level.

But now, having read the latest comments, I have a new line of questioning. Why are these birds so angry?

Never played the game?  The pigs stole their eggs.