Grounding in electrical circuits

[sophiecentaur]

Why use the idea of electrons moving around a circuit, as if they will actually get anywhere. With AC they merely oscillate back and forth by a fraction of a mm and, with DC, you would be dead of electric shock before any electron had moved from the metal you touched through the surface of your skin. The actual movement of electrons, whilst being a 'fact' is irrelevant to nearly all electrical circuit problems (with the exception of what goes on in a CRT).

 

[Evil Bunny]

Why? Because they are what is moving... The distance any single electron traveled is irrelevant. The main point of the question remains, regardless of how far any individual electron moved.

 

[sophiecentaur]

It is interesting that those who are actually involved with electronics, circuit design, power engineering - y ou name it, do not discuss problems in terms of electrons except, sometimes, at the component level. Why do you think that is? Do you seriously think that it is due to ignorance?

 

[Evil Bunny]

No Sophie. I'm not implying ignorance. I think most people "who are actually involved with electronics, circuit design, power engineering" would know all about electron drift (edit: http://en.wikipedia.org/wiki/Drift_velocity" ) and still understand the point of the question.

But in case I'm wrong, let me rephrase it...

Is the "net total" of charge in the original circuit changing? When these transfers take place between the Earth and a closed circuit, are we manipulating the charge numbers that were in the original loop?

There... I substituted the word "charge" for the word "electrons".

Is that a better way to ask it?

Again... The point remains.

 

[jim hardy]

indeed there's a lot of tradition.

Engineering textbooks always describe "Conventional Current", which would be movement of positive charge.

I guess that's because the behavior of electric current was observed and described with equations before they knew what was moving.
Ampere's Law was 1826, Thompson discovered electron 1897-ish.

In the power plant i worked thirty years with both engineers and technicians so became fluent at explaining stuff in both terminologies. My technicians, who all spoke electron flow, loved to poke fun at "Engineer's Current".

I taught a course in how to use common mode voltage to troubleshoot instrumentation.
Then we formed teams and did practical exercises looking for shorted components using Kirchoff's laws and schematics.
My technicians beat the engineers hands down.

One really needs to be bi-lingual about electron vs conventional current if he's going to make it in industry.



old jim

 

[jim hardy]

""When these transfers take place between the Earth and a closed circuit, are we manipulating the charge numbers that were in the original loop?
""

sounds to me like you've got it.

 

[Studiot]

Is the "net total" of charge in the original circuit changing? When these transfers take place between the Earth and a closed circuit, are we manipulating the charge numbers that were in the original loop?

Yes, of couse, it may well change.

This brings us to the point that an Earth is a circuit component whose potential does not alter when this happens.

 

[sophiecentaur]

indeed there's a lot of tradition.

Engineering textbooks always describe "Conventional Current", which would be movement of positive charge.

I guess that's because the behavior of electric current was observed and described with equations before they knew what was moving.
Ampere's Law was 1826, Thompson discovered electron 1897-ish.

In the power plant i worked thirty years with both engineers and technicians so became fluent at explaining stuff in both terminologies. My technicians, who all spoke electron flow, loved to poke fun at "Engineer's Current".

I taught a course in how to use common mode voltage to troubleshoot instrumentation.
Then we formed teams and did practical exercises looking for shorted components using Kirchoff's laws and schematics.
My technicians beat the engineers hands down.

One really needs to be bi-lingual about electron vs conventional current if he's going to make it in industry.



old jim

Agreed. And using the word 'current' allows one to move smoothly from electron movement to hole movement to electroplating to what happens around an antenna.
Someone in eduction, some while ago, decided to 'explain' electricity to the kids in terms of electrons and, at the same time, to leap straight in with 'particles' when explaining many macroscopic phenomena. In many ways, a great disservice to students.

 

[Evil Bunny]

Okay... Thank you for the answers. But now I have another question.

So let's just say that lightning deposited an extra zillion electrons into our ungrounded closed loop circuit...

Aside from there now being a potential between the circuit and the Earth (where there was no potential before)... what effect does this have on the original circuit?

Everything was satisfied with x number of electrons zooming around originally... but now there are a zillion more electrons zooming around.

Does this mean that our current just changed?

 

[Studiot]

Pay attention

Aside from there now being a potential between the circuit and the Earth (where there was no potential before)... what effect does this have on the original circuit?

This brings us to the point that an Earth is a circuit component whose potential does not alter when this happens

 

[Evil Bunny]

This circuit was ungrounded.

unearthed, you might call it.

Pay attention.

 

[Studiot]

I was paying attention.

You stated that there was no potential difference between the circuit and the Earth before the strike.

After the strike the ground potential does not change so there is still no pd between the circuit and the ground.

 

[sophiecentaur]

Okay... Thank you for the answers. But now I have another question.

So let's just say that lightning deposited an extra zillion electrons into our ungrounded closed loop circuit...

Aside from there now being a potential between the circuit and the Earth (where there was no potential before)... what effect does this have on the original circuit?

Everything was satisfied with x number of electrons zooming around originally... but now there are a zillion more electrons zooming around.

Does this mean that our current just changed?

"Zooming"?
The use of that single word justifies all my objections to the popular 'electron' model of electricity. It shows the tip of an enormous misconceptional iceberg.

 

[Evil Bunny]

You stated that there was no potential difference between the circuit and the Earth before the strike.

After the strike the ground potential does not change so there is still no pd between the circuit and the ground.

Studiot... It is being suggested here that static can "build up" on these circuits if they are ungrounded. I'm trying to figure out how that can be.

We have electrons zooming around (yes, zooming) in a closed circuit... Through motors turning, wind, lightning... whatever... it is being suggested that a potential difference is being developed between the Earth and the ungrounded circuit, making it dangerous.

Where is this extra charge (these extra zillion electrons) sitting?

Are they hanging around outside the wire watching the electrons zooming around inside the wires like spectators at a nascar race? Are they joining in the race with the other electrons (oops, I mean becoming part of the orignial closed loop circuit)?

 

[Studiot]

You are misunderstanding the nature of protective earthing.

It has been stated that the theory of electrons does not enhance circuit theory and I endorse that.
Such theory is useful for chemical bonding, ionic action, drude theory, x-ray diffraction and so on, but not for common or garden engineering circuit theory.

It is fundamental that a conductor in a changing magnetic field experiences a potential difference between two points in the conductor. This potential difference is real, can be measured, and is used to generate electricity.

In particular no electrons are transferred from anywhere to generate this pd.

Existing charges are simply rearranged within the conductor .

Now back to earthing.

One reason conductors are earthed because they may experience exactly the same effect due to stray fields. Such conductors include metal water pipes, metal balustrades, and other building metalwork. This is the reason I say that there is more earthed metal that is not part of any circuit, than there is that is part of an electric circuit.

 

[sophiecentaur]

We have electrons zooming around (yes, zooming) in a closed circuit... Through motors turning, wind, lightning... whatever... it is being suggested that a potential difference is being developed between the Earth and the ungrounded circuit, making it dangerous.

What speed qualifies for the term "zooming", I wonder? You do realize that they go just a little faster, in a wire, than a snail does on a wet rock?

 

[Evil Bunny]

In particular no electrons are transferred from anywhere to generate this pd.

Existing charges are simply rearranged within the conductor .

Let's say you have two identical pieces of copper floating in mid-air.

Now let's use something like a Van de Graaf generator to charge one of them up.

Is there anything physically different about the copper pieces now? Does one not have more electrons than the other one?

When you walk across your carpet and then touch your door knob and you get a shock, did electrons get transferred?

Oh... And the snail analogy is a little weak because while the net movement in one direction due to an applied electric field might be that slow, those electrons are kind of "rattling around" inside there, bumping into each other and stuff :-)

 

[Studiot]

Is there anything physically different about the copper pieces now? Does one not have more electrons than the other one?

I have already said yes but so what?

Clouds are a better example since they do float in the air and can possesses an electron excess or deficit.

 

[jim hardy]

""Everything was satisfied with x number of electrons zooming around originally... but now there are a zillion more electrons zooming around.

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.

--------

""""After the strike the ground potential does not change so there is still no pd between the circuit and the ground. """

I don't quite agree with that one -

because now there exists an electric field between the surface of your "circuit" and earth,
and there's energy in that field where there was none before.
that energy ought to be in accord with energy of a charged capacitor, (1/2)C X V^2 where C is capacitance between your 'circuit' and earth.. and V is the voltage.

so it shows up as plain old volts to earth.
Charge on a capacitor Q = C X V ,
so the voltage is the capacitance divided by the charge.

Capacitance to Earth is probably in picofarads for anything reasonably small,
let us guess ten picofarads (USWAG methodology, Unscientific Wild-*** Guess)

to charge 10 picofarads to 1kilovolt
takes charge of Q = C X V
Q = 10^-11 X 10^3 = a meager 10^-8 coulombs
a current of 0.00000001 amp for 1 second , or 1 amp for 10^-8 second
in more practical terms - if you tried to measure it with an analog voltmeter
it might briefly deflect the needle just barely enough for you to see.

BTW that would be about 6E10 electrons (oops make that units of charge)

Sophie has a point , the electrons don't move very fast.
If they were positive units of charge they still wouldn't move very fast.
But they interact with each other at speed of light, just like waves traveling down a Slinky toy.
equations describing waves in a Slinky toy are different than those describing the Slinky when it's not stretched and is a solid mass.

Sophie is encouraging us to raise the level of discourse to a higher plane
which is the right thing to do.
after we become comfortable with the very basics.

old jim

 

[sophiecentaur]

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? Surely the gas laws are what you would use. And don't try to kid anyone that the RMS velocity of electrons is ever considered in the simple schoolboy model of electrons 'rushing' around a circuit.

 

[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.