Grounding in electrical circuits

[jim hardy]

""If a body of air is moving at a few mm per second, is it relevant, for discussing pressure and volume, to consider the velocity distribution of the molecules?""

? why not? that's where they started in my high school PSSC physics course, 1963.
it worked okay then.

What is you objection to concept of charge in motion?
That's an honest question not an argumentative one.

old jim

 

[sophiecentaur]

It depends at what level you're operating. If you're talking macroscopic then where does another layer get you?
I'm all for talking in terms of net charge movement but individual electrons have enormous RMS speeds and it is very misleading to attribute them with the behaviour described in popular elementary books. I am sure that many users of this forum know better but the wrong message can easily be picked up from the use of an incomplete picture.
From remarks you can find in many posts, there are clearly some really dodgy ideas about what goes on in electric circuits.imho there are very good reasons for treating 'electricity' as a macroscopic thing. For a start, there is no maths for doing it the other way.

 

[SailorDude22]

Let me get this straight, SophieCentaur. Reading what you are saying, is what I've been taught about current, namely that it is the freeing of electrons in the outer valence shells of atoms (like Cu) to other outer valence shells caused by a polarity difference between a negative and positive body, factually incorrect? I'm only a technician working for the Navy on gear older than I am, but even if an individual electron is not moving that fast or that far, it seems clear to reason that the aggregate effect would be a rapid movement of electrons through a closed circuit. Afterall, is not one ampere the movement of 6.241 × 10^18 electrons moving through any given point at a given moment in an electronic circuit?

 

[sophiecentaur]

Of course the net movement of electrons is a fact for current flowing in a metal. But that is not the only way that a 'current' can flow in general, for a start. Holes and positive ions can flow, too.
The popular model of electrons moving around a circuit of wire is shown in pictures and in animations as little dots progressing steadily and rather quickly round pipe-like wires. This model is no nearer to any 'reality' than a conventional, macroscopic, non-quantised current flow. I think it is disingenuous to suggest that plumbing would be explained 'better' by bringing the molecular nature of water into the explanation or that gas flow would be better explained using gas molecules. The quantities are all manifest as large scale and diving down into the microscopic can hardly help.
I think that a problem has arisen because the net electron flow is in the opposite direction to the conventional flow direction. "They got it wrong" and other pointless objections are frequently heard from people who want a quick and easy way into electricity. Anyone who can't accept and use the 'triangle' formulae for current, resistance, power and volts can only progress as far as concepts like "you need a complete circuit for something to work". Being 'anti maths' is really not the way forward for any Science. It's a bit like wanting to be able to speak a foreign language without learning any vocabulary or basic grammar conventions.

As you say, 6X10^18 electrons flow past a point per second when 1A flows. This needs to be looked at in the context of a total number of dissociated electrons in your wire - which will be in the order of 10^22 (say). This is a bit analogous to a small stream entering one end of a lake and another small stream leaving it at the other end. Yes, you could say that there is a 'flow' of water through the lake but would it be measurable, how would it manifest itself? There is, in fact, an extremely slow mixing of the input water with water at the top of the lake with some of the water molecules entering the lake taking possibly hundreds of years to exit the other end. So the bulk of the water is flowing but individual molecules are just lost in the mass of other molecules. It's the same for the tiny proportion of electrons that actually enter and leave a conductor when a current flows through it. So why demand a 'bicycle chain' model?

 

[jim hardy]

lost two posts --- later

 

[SailorDude22]

As you say, 6X10^18 electrons flow past a point per second when 1A flows. This needs to be looked at in the context of a total number of dissociated electrons in your wire - which will be in the order of 10^22 (say). This is a bit analogous to a small stream entering one end of a lake and another small stream leaving it at the other end. Yes, you could say that there is a 'flow' of water through the lake but would it be measurable, how would it manifest itself? There is, in fact, an extremely slow mixing of the input water with water at the top of the lake with some of the water molecules entering the lake taking possibly hundreds of years to exit the other end. So the bulk of the water is flowing but individual molecules are just lost in the mass of other molecules. It's the same for the tiny proportion of electrons that actually enter and leave a conductor when a current flows through it. So why demand a 'bicycle chain' model?

This analogy makes a lot more sense given your argument. How exactly do you arrive at how many potentially freed electrons there can be in a given length and gauge of wire? Using Cu again, would you just find the aggregate amount of atoms per square millimeter and from that count just the two valence electron the each have in their 4s shell?

For semiconductor material, would you use a similar method to find the total amount of P-type atoms capable of accepting an electron?

 

[dlgoff]

This analogy makes a lot more sense given your argument. How exactly do you arrive at how many potentially freed electrons there can be in a given length and gauge of wire? Using Cu again, would you just find the aggregate amount of atoms per square millimeter and from that count just the two valence electron the each have in their 4s shell?

For semiconductor material, would you use a similar method to find the total amount of P-type atoms capable of accepting an electron?

I thought I would jump in here and give you a http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html" .

 

[sophiecentaur]

This analogy makes a lot more sense given your argument. How exactly do you arrive at how many potentially freed electrons there can be in a given length and gauge of wire? Using Cu again, would you just find the aggregate amount of atoms per square millimeter and from that count just the two valence electron the each have in their 4s shell?

For semiconductor material, would you use a similar method to find the total amount of P-type atoms capable of accepting an electron?

I grabbed a number near the Avogadro number and reckoned on one or two delocalised electrons per atom. Whatever actual number may be, it makes the point about the 'anonymity' of individual electrons as the charge 'moves' through the metal.

I'd go along with your idea of charge carriers in a semiconductor being related to the number of impurity atoms it's been doped with. The distribution of binding energies would be different compared with a metal. This relates to the resistivity.

 

[jim hardy]

"Anyone who can't accept and use the 'triangle' formulae for current, resistance, power and volts can only progress as far as concepts like "you need a complete circuit for something to work". Being 'anti maths' is really not the way forward for any Science. It's a bit like wanting to be able to speak a foreign language without learning any vocabulary or basic grammar conventions."

i don't know whether by "triangle formulae " you mean ohm's law or the del symbol from vector calculus.

to me anything that helps ordinary folks understand and fix their car's electrical system and keep the wheels of industry in good order is a plus for mankind. Ohm's law and "water in pipes" analogies do that.

there will be a few gifted folks who'll delve into the higher math .

Remember - "new math" teaching method of placing 'why' before 'what' didnt work out well.

We should provide help appropriate to the ability of the people we're trying to help
and encourage their growth
else we're just showing off.


old jim

 

[Studiot]

"new math" teaching method of placing 'why' before 'what' didnt work out well.

Gosh I must remember that little aphorism - it applies so well to other disciplines as well as maths.

 

[sophiecentaur]

@Jim

That's a fair enough view but it doesn't really constitute Physics imho. You don't need Physics to fix your car electrics, usually, but there are a number of problems I have solved which have benefited a lot from something more than the 'complete circuit' idea.
This is supposed to be a Physics Forum - not a car fixit helpline. I think it only reasonable that people who present problems, based on Physics, should expect answers which are at least a bit demanding.

Is it showing off to tell people that they need to do more than first year Science and they will need some reasonable Maths if they are going to get a better understanding? Is it showing off to tell people that they will not be able to run a Marathon without some serious training? Or that they will need to learn to swim before they jump into the deep end of the pool?

Inverted snobbery never got anyone anywhere.

I agree that the New Math approach didn't work - but that isn't really a parallel. The 'why' is essential when you want to do more than arithmetic. There are very few valid arm waving 'explanations' to help someone perform a Fourier Transform or Correlation.

 

[sophiecentaur]

Gosh I must remember that little aphorism - it applies so well to other disciplines as well as maths.

"just sodding learn it. Then we can discuss it."
That gets my vote every time!

 

[jim hardy]

"""I think it only reasonable that people who present problems, based on Physics, should expect answers which are at least a bit demanding."""

Fair enough.



old jim

 

[Evil Bunny]

Does this mean that our current just changed?""

no, it just means the electrons are(or should we say charge is) are packed a little closer together inside the wire.
current will flow in accordance with Ohm's law just as it did before the strike.

Thank you Jim... This makes sense to me now. I can see now how we can have a complete circuit and still have a potential to ground. I never really thought of it that way until you made this statement.

I appreciate your way of explaining things... It is quite helpful to some of us.

On a side note... I think it is unwise to dismiss the importance of a complete circuit, especially when the discussion revolves around grounding. It is surprising to me how many people think that electricity wants to flow into the earth. This thread hits on the heart of the matter where "circuit electricity" blends with "static electricity". While some may think this is a trivial thing, it is widely misunderstood in my experience.

Thank you to all who have contributed to this. I hope this thread has helped people to have a better understanding of the concept. It has definitely helped me...

As Studiot would say... Go well

 

[sophiecentaur]

"a complete circuit" is not 'dismissable'. It's just the first step in getting to grips with electronics - and no more.

 

[jim hardy]

Thank you Jim... This makes sense to me now. I can see now how we can have a complete circuit and still have a potential to ground. I never really thought of it that way until you made this statement.

I appreciate your way of explaining things... It is quite helpful to some of us.

....

Thank you to all who have contributed to this. I hope this thread has helped people to have a better understanding of the concept. It has definitely helped me...

As Studiot would say... Go well

Thank you EB for your kind, kind words.
I believe we need simple models to get our thinking in order.part of a good education is an awareness of just how meager our knowledge really is.

i think so long as we keep in mind that every step in our learning process invites another , we'll be okay. It's dangerous to think we have arrived at some summit of knowledge. At least for me, it always results in some humbling event.
I once read in an erudite journal a statement to this effect:
"Electrical energy does not flow along wires, it flows in the electric and magnetic field surrounding them."
Now THERE was a corker, for the electrons-in-motion (oops charge-barely-in-motion) model that i grew up with doesn't look outside the wire at all.

But it helps people resolve apparent logical conflicts like you described.

So - keep on refining your mental models of how things work.
Maybe you'll explain to me about that energy being in the field along the wire...

old jim