Please settle an argument between 9 yo and 10 yo re electricity

[CrinklyLion]

Paging Kim to the magic-smoke-emitting courtesy phone....

SmallFriend
"Electrons actually move at a snail's pace and the electricity jumps between the electrons and that is how it moves along a wire"

EldestCub:
"Well the electrons are part  of the electricity so how can the electricity be moving between the electrons it would have to be energy moving between the electrons and because the energy is jumping in between the electricity the electrons are part of the electricity hence the name electrons"

This over dinner debate is getting heated!

[Peter]

Try them with this!

The convention is that the movement of electrons is the "current" but that the current is regarded as moving in the opposite direction to the electrons.

Then leave the room.

[Euan Uzami]

Paging Kim to the magic-smoke-emitting courtesy phone....

SmallFriend
"Electrons actually move at a snail's pace and the electricity jumps between the electrons and that is how it moves along a wire"

EldestCub:
"Well the electrons are part  of the electricity so how can the electricity be moving between the electrons it would have to be energy moving between the electrons and because the energy is jumping in between the electricity the electrons are part of the electricity hence the name electrons"

This over dinner debate is getting heated!

AIUI, is electrons themselves DO indeed move at a snail's pace, but the influence of them is what moves round at the speed of light.
The electricity doesn't jump between the electrons as such, it's the energy that jumps between them, and it's this movement of energy that creates electricity.
I'm not sure if that's the correct technical description but it may help to resolve the confusion.
I have apparently got a degree in Physics with Electronics so I should really know better than that, but that's my best guess I'm afraid.

[HTFB]

They're both right, sort of.

The conducting electrons do drift along the wire as the current flows. For a big current (the starter motor for a car, say) this is a speed of millimetres per second at most.

But all the time, whether there's a current flowing or not, the conducting electrons are also vibrating about bouncing off each other. This is just because their temperature is not at absolute zero, so they're carrying some thermal energy. At room temperature this thermal vibration happens at about 100 kilometres per second. So about 100 million, or a billion, times faster than the drift.

All that vibration just goes into the electrons exerting pressure onto each other, which is mostly why the drift is so slow. (In a vacuum tube, like a CRT television, the current flow can be very fast indeed, because in a vacuum there aren't other electrons and nuclei getting in the way).

But because they're bumping about so fast, even a gentle shove of extra energy can be passed very quickly along the wire from one electron to another. None of them move very far but the pulse of energy goes right along.

A confident teacher can set up a pretty good demonstration of this in a corridor with the class acting as the free electrons.

[Kim]

They're both almost right...

In a metal (semiconductors can be weird) the electricity *is* the flow of the electrons.  The electrons are sitting there (well, vibrating, or spinning around, or being quantum, or whatever model of the atom you feel like using) being Important Parts Of Atoms anyway even when there's nothing electrical going on, but if you squeeze an extra electron onto the atom one end of a conductor, the force shoves an existing electron over to the next atom along, and the one next to that, and so on until one pops out the other end.  Like marbles in a tube, or people trying to cram onto benches the one on the other end pops out immediately, even though the one you just squeezed in will take ages to get to the other end.  Because electrons all carry energy, energy is transferred to the other end of the conductor.

In an insulator, the electrons are all tied up with Holding The Atoms Together in such a way that they can't easily shove along to the next atom.  Like a bench with arm-rests.  So it doesn't conduct electricity.  (Or, sometimes, only conducts electricity when you melt it.)

And yes, the electrons making your light work are moving at roughly snail speed (okay, they actually keep turning round and going back the other way 50 times a second so they don't actually get to go very far, but they're still moving at snail speed).  Luckily the movement gets to the other end of the wire at the speed of light, so you don't have to wait half an hour for the room to stop being dark.

It's like when you've got a bike with a Chain Of Greater Than Average Length:  Most of the time, the chain links are all just sitting there doing nothing, but when you pedal, one link pulls another, and the one after that and so on and the wheel turns immediately.  But if you want to find the special link that lets you take the chain apart, you have to sit and turn the cranks several times before the one you're looking for pops out of the end of the tube (usually just when you get bored and stop paying attention, so you have to go all the way round again to get at it).

The magic word is "drift velocity", but that'll probably bring up some A-level physics.

[rower40]

Think of a tube made up of tennis ball holders, joined up end-to-end, full of white tennis balls.  Push one green tennis ball in at one end, and a white tennis ball drops out at the other, pretty-much immediately.  Then push another green one in, and so on.  It takes quite a long time for the first green one to come out.

So the electricity flows quickly, but it takes ages for the individual electrons to get all the way from one end to the other.

Arrgh.  Beaten to it by Kim.  Again.

[Meegat]

Surely if we are digging for metaphors it is compulsory to link to the 'think of it as spinning a bicycle wheel with your hand' explanation?

http://amasci.com/miscon/eleca.html#electron

Best not to try that with studded tyres...

[CrinklyLion]

Hurrah.  Apparently YACF speaks with an authority that mere mothers may not muster.  There have been solemn nods, cries of "So we're both wrong!", firm nodding "OK"s of understanding, and an agreement that they were both sort of right about some bits and maybe not explaining dreadfully well what precisely they were trying to say.  The battle for supremacy has moved arena to Cars on the Wii.

[rogerzilla]

Both explanations are wrong.  The electrons do move at a snail's pace (you can show this with a simple experiment using coloured ions).  However, the speed of the current is simply how long it takes for the electrons to start moving at the other end when you turn on the supply.  As a simple mechanical analogy, if you push one end of a stick, however slowly and even with a pretty long stick, there is no apparent delay before the other end of the stick starts to move.

The speed of electric current in a copper wire is about half the speed of light, I think.

[rogerzilla]

Oh, and if you want to boggle their minds, get them to google Millikan's oil-drop experiment.  It's possibly the biggest "OMG" experiment in A-level physics*.


*assuming it isn't too hard, and therefore reserved for Masters students these days

[CrinklyLion]

Both explanations are wrong.

Well, of course they are.  What else would one expect when you have two world-class pedants arguing, with one staunchly defending what they "read in my book" and the other quoting verbatim from QI?  And, of course, neither actually listening to what the other is saying....

They'll fit right in around here in a few years time, won't they 

[Kim]

The speed of electric current in a copper wire is about half the speed of light, I think.

I vaguely recall measuring it as such at A-level, with a kilometre of coax and a very finicky oscilloscope.  There's something deeply satisfying about doing that sort of experiment yourself.

I don't think we covered (we certainly didn't attempt) the oil drop experiment at A-level.  I remember it coming up in a first year tutorial at uni, though.

[clarion]

Pretty certain we did Millioilcan's experiment at A level.  Great stuff.

I thought that the speed of electricity in a cable was much nearer c.  Drift speed for ac should be roughly 0, and for dc could be measured in furlongs per fortnight.

[Gruff]

There have been solemn nods, cries of "So we're both wrong!", firm nodding "OK"s of understanding, and an agreement that they were both sort of right about some bits and maybe not explaining dreadfully well what precisely they were trying to say.

It sounds like you have two budding engineers there. Ages 9 and 10 too, perfect time for an Arduino starter kit! (Actually, any age is perfect for an Arduino starter kit)

http://proto-pic.co.uk/sparkfun-arduino-inventors-kit/

[Kim]

I thought that the speed of electricity in a cable was much nearer c.

I've just had a google.  It appears to depend on the construction of the cable.

[rower40]

[Donning Railway Signalling Engineer's hat]
The speed of electricity in a cable is wholly dependent on whether it's been stolen and sold off at the local scrappie.  Whereas the resale value of fibre-optic is practically zero, irrespective of the speed of the photons therein, so we're re-wiring our signalling systems to use that rather than copper wherever possible.

[Kim]

Doesn't the fibre go brittle and break from repeatedly pulling the weight of the semaphore arms thobut?

[mcshroom]

Doesn't the fibre go brittle and break from repeatedly pulling the weight of the semaphore arms thobut?

We still have semaphore arms on our line

[rower40]

The really clever bit is if the fibre optic cable is used to pull the semaphore arm, AND to repeat back to the signalbox the colour of the light shown to the driver.  Or at least to show to the signalman that the signal lamp is still lit.

If any YACFers are in Cornwall on June 13th, 14th, 26th, 27th, then please come to Bodmin General station, where I will be performing the signalman duties at the end of the platform.  I can demonstrate much 1950's/1960's technology, with a smattering of more modern stuff (such as LEDs).  These were installed because very few signalmen are willing to shin up the signal posts to keep the lamps topped up with oil.

[Cudzoziemiec]

[Donning Railway Signalling Engineer's hat]
The speed of electricity in a cable is wholly dependent on whether it's been stolen and sold off at the local scrappie.  Whereas the resale value of fibre-optic is practically zero, irrespective of the speed of the photons therein, so we're re-wiring our signalling systems to use that rather than copper wherever possible.

Nevertheless, I remember an occasion when half of Poland* was left without so much as a single internet between it, due to a vitally important length of fibre-optic cable having been dug up and nicked. So it does have some resale value to someone - possibly it was sold on to Railtrack, or whatever they were called at the time? 

*Probably rather less than half in truly truthful factual fact. It included the place where I was working but I think we still had a connection at home on the other side of the same city.

[mrcharly-YHT]

So it does have some resale value to someone -

fibre optic is worth sod all.

The idiots nick it because they think it is comms cable containing copper and therefore valuable. 

[pcolbeck]

So it does have some resale value to someone -

fibre optic is worth sod all.

The idiots nick it because they think it is comms cable containing copper and therefore valuable.

Correct. With fibre optic comms its the SFPs (little match box sized things that terminate the cable and then plug into the switches or routers) that the cables plug into that are worth a fortune.

[Cudzoziemiec]

So probably it was the SFPs they nicked. It had the effect of cutting us off, anyway!

[pcolbeck]

The SFPs are inside the exchanges etc so no its usually just the cable thy nick. They cut it at one manhole and then cut it at another and drag it out using a pickup or similar. SH fibre cable is worthless.

[David Martin]

In SOuth Africa they had a department responisble for buying back the stolen copper cable. It was cheaper than getting new cable and there was a thriving market. Nicking the whole substation was a tour de force though.