Grounding in electrical circuits

[alpine2beach]

I feel as if the difference between grounding in a circuit and grounding in mains power is not very clear. So my questions are, what does a ground do in circuits and is it necessary?

 

[mdjensen22]

http://en.wikipedia.org/wiki/Ground_(electricity)#Circuit_ground_versus_earth

 

[alpine2beach]

http://en.wikipedia.org/wiki/Ground_(electricity)#Circuit_ground_versus_earth

I think some data is omitted. It says it is a neutral reference point, but what does that mean? It also refers to signal grounding, which it explains in context of grounding in a circuit. Could you please explain this?

 

[Studiot]

https://www.physicsforums.com/showthread.php?t=382007&highlight=earth

 

[mdjensen22]

Voltage is a differential quantity. To measure the voltage of a single point, a reference point must be selected to measure against. This common reference point is called "ground" and considered to have zero voltage. This signal ground may not actually be connected to a power ground. A system where the system ground is not actually connected to another circuit or to Earth (though there may still be AC coupling) is often referred to as a floating ground.

Ground is a reference - in some cases (AC Mains), ground is actually the GROUND. Neutral is often connected to ground outside your house to provide a consistent voltage reference.

In other cases, it is called ground when it is really just circuit common (or 0V reference). For example, on battery power circuits, the (-) terminal of the batteries which makes up the supply negative is often called ground.Edit:
Thank you Studiot.

 

[alpine2beach]

Thank you Studiot and mdjensen22! But this answer also raised a few more questions from me. What purpose does a circuit common/0V reference provide? Also, if I were to hook up the positive terminal of a battery to a ground (such as one used in AC mains), would there be a current?

 

[Studiot]

if I were to hook up the positive terminal of a battery to a ground (such as one used in AC mains), would there be a current?

Why would you expect more than the momentary equalisation current you get whenever you touch two pieces of metal?

Search the forums, this has been debated as naseam

What purpose does a circuit common/0V reference provide?

Suppose you have a microphone running off a 1.5 volt battery feeding a preamplifier, running off a 15 volt supply, driving a power amplifier running off a 60 volt supply.

You would designate (in this case) the negative terminal of each supply as the 'common' and call the common value zero.
This way you could measure voltage from common to any point in any of the parts and get a consistent set of readings.
This is using the common as a reference.

Sometimes we designate the high side of the supply as reference.

 

[alpine2beach]

Suppose you have a microphone running off a 1.5 volt battery feeding a preamplifier, running off a 15 volt supply, driving a power amplifier running off a 60 volt supply.

You would designate (in this case) the negative terminal of each supply as the 'common' and call the common value zero.
This way you could measure voltage from common to any point in any of the parts and get a consistent set of readings.
This is using the common as a reference.

Sometimes we designate the high side of the supply as reference.

So, what you're saying, is that the ground common is actually just the negative terminal of the battery/power supply?

 

[Studiot]

No I didn't mention ground and I said use the negative in this case.

 

[alpine2beach]

No I didn't mention ground and I said use the negative in this case.

I see...sorry, I was misreading your response. You were supplying an answer to the question about what the common is. It is the point where you can measure voltage relative to other components of the circuit.

Anyways, thank you guys so much!

 

[Studiot]

The common may or may not be grounded, depending upon circumstances and the application. Many mike/amp applications are not grounded but have 'earth lifts' incorporated to avoid Earth loop hum pickup.

Say you have some sort of transducer providing a signal to a signal processor which outputs to a data logger. You also decide to monitor the signal at the transducer, the processor and the logger with both an oscilloscope and a voltmeter (3 of each). Sorry I can't draw a diagram tonight, but convince yourself that you can choose one terminal for each of these pieces of equipment and connect them together - this is obviously the common terminal. The other terminal on each instrument will be connected differently as appropriate.

go well

 

[jambaugh]

From a theoretical point of view this has to do with the fact that voltage is relative. Explicitly it means nothing to say that one point in a circuit is at a given voltage. One can only say there is a voltage difference between two points. But by arbitrarily setting one point in the circuit to be "the ground" you are implicitly finding the potential difference between it and any other point when you say that point is at V voltage.

It is like saying a given particle is at position (x,y,z). This is implicitly saying it is displaced by the amounts x, y, and z from an origin point. Without the origin a coordinate is meaningless. The ground acts as an "origin point" so we can use a language of absolute voltage for what is really a relative difference.

Now from a practical standpoint in a practical electrical device one can use the chassis or even the Earth itself as part of our circuit. All the grounds in a circuit are implicitly connected. (You only need to run one wire to your tail light since it uses the metal structure of the car as the second wire in the circuit... although this is not done as frequently in modern vehicles for various reasons.)

 

[Studiot]

Good morning, Jambaugh.

I think Earth's or grounds are more complicated than your picture. They certainly do not need to be physically connected.

How do you explain virtual Earth's or artificial Earth's?
They certainly have current flow into/out of them.

But they satisfy the definition I provided in the thread I linked to, in the first case forced by an op amp in the second forced by the circuit configuration.

An Earth is a body whose potential does not alter, regardless of the current flows into or out of it, within the design limits of the system.

As far as I am aware the term Earth is synonymous with ground - One is basically UK usage, the latter US.

go well

 

[I_am_learning]

An Earth is a body whose potential does not alter, regardless of the current flows into or out of it, within the design limits of the system.

Potential with respect to what?

 

[jim hardy]

Simplify your thinking. I use the humble flashight, an old one with metal case, to teach the concept.

"Ground" is an unfortunate name that usually means circuit common.

To grasp the concept go back to Kirchoff's current law which in its simplest form is just:
"Electricity always gets back to where it came from".
Hold an old fashioned D cell flashlight battery in your hand with the positive button poinitng up.
Every electron that comes out the bottom(negative) will make its way back to the top (positive). That's a physical manifestation of Kirchoff's current law.

If you stack a second D cell atop the first one. You now have a two cell battery.
If you place that two cell battery in an old fashioned metal flashlight, you now have an electric circuit that will work just fine with no connection to Earth ground.


If you set the flashlight on the ground it is now connected to Earth ground through the dirt or grass or whatever it is laying on. It still works fine. There will be no current to Earth ground because the electrons coming out of your battery want only to get back to the other end of that battery.

You can now pick up the flashlihgt and set it atop a Van deGraff generator and charge it up to a million volts. It'll still work fine for the same reason, electrons coming out one end of the battery only want to get back to its other end. Kirchoff says so.

If you decide to pick a point on your flashlight and call it "Circuit Common" that is you choice. Circuit Common is simply a convenient point to hook one lead of your voltmeter when doing circuit analysis.
Defining circuit common is COMPLETELY up to the person doing the circuit analysis.
A prudent analyst usually bases his choice of a point on convenience.
In an airplane it's usually the aluminum skin, in a not-fiberglass car it's vehicle body, in a computer it's the PC board track where the power supplies all tie together.
It's often called "zero volt reference" or "Power Supply Return", but all too often it's mis-named "Ground" and that's unfortunate. It may or may not be tied to Earth ground.

In picking a "Circuit Common" for your two cell flashlight it would seem natural to choose the metal body. That's the easiest place to put your voltmeter lead. And it's where your hand touches the circuit.

If indeed you chose the metal body for your circuit common and measured voltages,
you'd get:
at junction of the two batteries(which is where top of lower cell and bottom of upper cell touch), positive 1.5 volts;
and at top of upper cell positive 3 volts.

If you decided to be weird and chose the junction of the two D-cells as your circuit common, and somehow got a voltmeter lead connected there, you would read:
to bottom of lower cell (which is metal body of flashlight), negative 1.5 volts
and to top of upper cell, positive 1.5 volts



House wiring is like a two cell flashlight.
The transformer on your power pole has a center tapped secondary.
The center tap is exactly same as the junction of two D-cells we just discussed, it's where two windings inside the transformer join..
That center tap is tied solidly to Earth ground by the big copper wire running down the side of your power pole. (Dont ever cut it...)
That center tap is tied solidly to the bare cable coming into your house around which the other two are wrapped. So the bare cable serves double duty - it carries current back to the transformer and it holds up the other two (hot) wires.

The bare wire is also tied to all the white wires in your house wiring, to the longer slot in all your outlets, to the threaded part of your light sockets, and to all the little third prong grounds in your outlets.

In house wiring that bare wire is regarded as "Circuit Common" because it is power supply return. In house wiring "Circuit Common" JUST HAPPENS TO BE same as Earth ground. It is called "Neutral" and is always a white wire.

Unfortunately the terms have got confused over the years. Car chassis is called "Ground" even though it's insulated from ground by the tires.

work that in your head a while- you need to separate the concepts.
old jim

 

[Evil Bunny]

Thanks Jim. Nice write up... One question:

In house wiring that bare wire is regarded as "Circuit Common" because it is power supply return.

Isn't the white neutral the power supply return?

In this case the transformer is the power supply and everything (120V anyway) is "returning" on the center-tapped neutral, correct?

EDIT: I just realized you're talking about the large bare wire between transformer and service entrance. Not to be confused with the bare (sometimes green) wire in the branch circuits of your house. My apologies...

I was going to be a wise guy and ask if the L1 and L2 leads of the transformer were 180 degrees out of phase with each other, but I decided against it ;-)

 

[Studiot]

Well goodness, jim, that was a long post.

But you still haven't explained what is special about an earth. In what way is an Earth different from other points in the circuit? If it is not special why do we bother?

Nor have you addressed my comment on virtual or artificial Earth's.

 

[Evil Bunny]

I feel as if the difference between grounding in a circuit and grounding in mains power is not very clear. So my questions are, what does a ground do in circuits and is it necessary?

I think this has already been said, but I'll say it again another way...

"grounding in a circuit" is basically a common reference point to take a voltage measurement at another point. "the ground" is the same as saying "common reference point". When you analyze that circuit and you are reading a voltage, you are reading that voltage with respect to the common "grounding" point. Convenience.

Grounding in electrical distribution systems literally means attaching one side of the circuit to the ground... The Earth we stand on...

Now your question as to WHY we attach one side of the distribution network to the earth... that is a very complicated and complex question.

One very important reason is to dissipate static discharges (lightning), and unintentional high voltage spikes, into the ground and divert this current away from the inside of your house and direct it into the earth.

 

[jim hardy]

well i just lost a half hour's typing due to that %#@^%&%(@U(*QRT!~~*** login gauntlet.

Why does it time out while you're composing a reply?

Why does it dump your text if you mispell your password? I have fat fingers and often hit two keys at onse.

might try again later tonite if nobody else fields these questions.

old jim

ps thanks EB

 

[jim hardy]

Thanks Jim. Nice write up... One question:



Isn't the white neutral the power supply return?

In this case the transformer is the power supply and everything (120V anyway) is "returning" on the center-tapped neutral, correct?

EDIT: I just realized you're talking about the large bare wire between transformer and service entrance. Not to be confused with the bare (sometimes green) wire in the branch circuits of your house. My apologies...

I was going to be a wise guy and ask if the L1 and L2 leads of the transformer were 180 degrees out of phase with each other, but I decided against it ;-)

Thanks EB

Indeed the white neutral is power supply return. Neutrals from all over the house are collected in breaker panel and connected to the centertap of the transformer by the big bare cable, thence to Earth via that copper wire down side of power pole. There should be a second connection to earth, via a copper plated steel rod driven into the ground near the meter box.

As you pointed out the white neutrals are the power supply return. Inside the house, if any current gets into a green safety ground wire it's because some insulation somewhere has ceased to insulate and current is leaking into where it shouldn't.
Ground Fault Circuit breakers, called GFCI's, sense that condition and trip when around 1/100 of an amp is returning by some path other than the white wire. 1/100 amp is about half the accepted threshold for lethal current, won't harm you but is a real healthy "bite" .
So- the green safety wire is there to provide a path of least resistance for leaked current . Better it flow through the green wire than you, should you be holding onto the leaky appliance.

Also you are right on about phase on L1 and L2. Which of course leads to your next point - neutral current is the diference not the sum of load currents on the two transformer windings... have at it my friend!

______

""...But you still haven't explained what is special about an earth. In what way is an Earth different from other points in the circuit? If it is not special why do we bother?
...Nor have you addressed my comment on virtual or artificial Earth's. ""

Good question, Studiot - i hope i understand it !

We should be careful in our choice of names for things, they should lead the unaccustomed mind directly to the correct understanding.
That's why i avoid the term "ground"
and much prefer to use "circuit common" or "power supply return" for points in the circuit, and "Earth" for a wire that goes into the earth..

I've worked on a very well thought out system that had a 1 inch square copper bus bar across bottom of cabinet for circuit common/ power supply return. It was labelled "Zero Volt Bus" which painted a good mental picture of what its function was.
All internal voltages were measured with respect to that zero volt bus bar.
Zero volt bus bar was mounted on insulators and connected to Earth ground, the earthed metal cabinet, by a 47 ohm 200 watt resistor.
The name "Zero Volt Bus" and the insulators made it real clear that circuit common was not Earth or ground.

Now back to your question "...
what is special about an earth..."
Using my definition of an earth, , ,
If you are standing barefoot on moist ground and touch a point in a circuit that is earthed, you will not get a shock.
If instead you touch circuit common you will experience whatever voltage exists between circuit common and earth, which may be quite a shock..
If circuit common is earthed you will experience zero volts and will not get a shock.

You experience this in cold dry winter weather when exiting your automobile - that shock you get if your hand is stilll touching metal when your foot first hits the ground . The car's power supply return (chassis) forms one plate of a capacitor, the Earth below it is the other plate, and you feel it when that capacitance discharges through you. Similarly you'll notice a little wire sticking up at highway toll booths. Its purpose is to discharge that capacitance so the toll booth attendant doesn't get a shock from every motorist as they hand him their quarters.

That to me is what's special about an earth. It conducts charge into the earth

Using instead what i think is your definition of earth, which i would instead name "zero volt reference" for the circuit,
any "specialness" exists ONLY IN THE MIND OF THE CIRCUIT ANALYST.
It is a convenient place to put a voltmeter lead and nothing more .
It's usually a big track on printed circuit boards so as to minimize voltage drop along it.
A prudent analyst takes all his readings with respect to one point just to keep his thinking straight. Power supply return is usually a good place to use, though some high end audio gear has separate signal and power commons. That helps keep down power supply hum.

".. virtual Earth's.."

I more often see "virtual ground" but I'm stateside. And i again think it should be called "virtual common" for opamps and "virtual neutral" for power systems to distinguish it from a point that is solidly earthed..
In both cases your "virtual (insert noun) is a point in the circuit that is pushed by active devices toward same voltage as circuit common or neutral.
Speaking generally, electronics uses opamps and power uses transformers to do the pushing. They push charge around to achieve that virtual zero potential diference.

Wow I'm way too wordy. Wish i could draw pictures.

Hope this makes sense. Reason i spent so much time is - over a lifelong career in industry i'd say around five percent of the engineers i knew had a real clear understanding of grounding.
if this helps one individual, well, pass it on.

Best boooks i know of on the subject are:
an IEEE publication "Green Book", it covers power system grounding (EDIT oops i mean earthing;)

and "Grounding and Shielding Techniques in Instrumentation" ( can't recall author)
it covers industrial electronics and basic physics of shielding.

old jim

 

[jim hardy]

"Why do we bother?"

Industry learned a hard lesson some decades ago.
A large factory was built with an electrical system that completely floated - no connection at all to earth.
On dry days electric motors all over the plant would fail.
Turned out that windage in the rotating machinery made static electricity , it built up high enough voltage to pierce insulation at the motor windings.

So power systems always have some connection to earth. It can be a solid one like in house wiring, or a high reststance that just bleeds off static electricity, but it's there.

In my power plant, on the station 125 volt battery, we used ordinary light bulbs. They provided ample drainage for static electricity and if they flickered it told you something peculiar was going on.

over and out. Y'all must be tired of me by now, g'night...

old jim

 

[I_am_learning]

Y'all must be tired of me by now, g'night...

Not at all. It is a privilege to hear things from an experienced person like you.
Great post.

 

[Studiot]

well i just lost a half hour's typing due to that %#@^%&%(@U(*QRT!~~*** login gauntlet.

Two solutions:

1) Compose offline in Word/Wordpad/Whatever and paste

2) Ctrl+A > Ctrl+C immediately before clicking on the submit button. Then if you loose the connection you can paste it into (1) above or just hold it on the clipboard until you log back in.

go well

 

[Studiot]

Potential with respect to what?

This is a good question.

I will try to expand on my definition with the aid of the attached diagrams.

First

Why can't we choose any old point in the circuit, use it as a reference and connect it to Earth (rather than at F as shown)?

Well let us examine some candidates in Fig1.

Say, Point K

Well this is where the signal comes into the circuit.
What would the output of our circuit be if we earthed this point?

Well what about point J?

J will have some DC bias from the circuit. What effect will connecting J to Earth have?

OK so B has no connection to the DC biasing of the circuit.

But a significant current flows through B and the load loudspeaker thus the potential at B with respect to the rest of the circuit (ie any other point in the circuit) varies.

In fact points K,J, L, A and B are all in the signal path and therefore subject to a varying potential with respect to other points in the circuit.

So hopefully we can all agree none are good choices either as a reference base or as a point of the circuit to connect to earth.

OK so what about the negative (zero) power rail?

Well G, C, D, E and F are all directly connected and in simple theory at the same potential so what if we called this zero reference and worked from there?

The line G, C, D, E and F form a common connection, which was part of the original question. The line could be extended to accept further connections.

Reality, however, intervenes as soon as our circuit passes current because significant currents flow in the output circuit around TR1 and the loudspeaker, introducing a significant voltage difference between C and D which varies with time. A similar voltage appears between D and E due to TR2. Furthermore, power currents flowing in the rectifier/filter circuit introduce significant voltages between E and F.
When I say significant, they are not significant in relation to the output, but if any point between C and F are used as a reference for the input to A1 these voltages will be in series with the input and may be a substantial fraction (or even equal to) the signal voltage at this point.



Moving on to Fig 2 which shows two pieces of equipment, one mains powered, connected by signal leads.
If the return lead is not earthed the signal generator chassis may attain significant voltage (up to 100 volts) by electrostatic coupling with respect to the other equipment.
This effect is shown as the dashed capacitor .
The chassis of the second equipment is pulled up to this voltage by the return connection between the two pieces of equipment, labelled AB.
Now suppose we wish to make a circuit change to the circuit under test so we switch off its power supply and apply an earthed soldering iron.
If the chassis is not earthed, the charge flow to Earth from the induced voltage on the chassis can easily be enough to destroy semiconductor components.

Fig 3 shows the classic ground loop where two mains powered apparatus are earthed at their respective mains plugs which is also connected to the signal return between the equipment.
Shown dotted is the large area of a ground loop that picks up voltages (especially hum) from any magnetic fields that thread the loop.

This is all a good basis for discussion as to why we don’t want the Earth potential to vary and in relation to what this unwanted variation can arise. Part 2 will include safety Earth's.

 

[alpine2beach]

Thank you Jim and Studinot, you have provided a lot of insightful infomation. Anyways, I have one last question?

Ok, so let's say I have a small circuit with a 555 timer in it. One of the connections has to go to ground. How would I make a ground? Would any piece of metal do?

 

[Studiot]

One of the connections has to go to ground.

Why?

 

[alpine2beach]

Nevermind. I understand that sometimes the ground symbol in schematics actually means the "common".

 

[I_am_learning]

In fact points K,J, L, A and B are all in the signal path and therefore subject to a varying potential with respect to other points in the circuit.

So, you mean to say, voltage at K varies w.r.t J (or L or A or G or C or any another point), so its unsuitable for reference or earthing. Ditto of J, L, A etc.

Well G, C, D, E and F are all directly connected and in simple theory at the same potential so what if we called this zero reference and worked from there?

Well, G, C, D, E and F have same (constant / non-changing ) potential w.r.t themselves.
The potential of G (or C/D/E/F) is not constant w.r.t to point K or J etc.
So, their potential isn't non-changing (constant) w.r.t. to all other points in circuit.
So, how can they be selected for reference then?

I find that point k and G are perfectly symmetric except point G has lots of friends (same potential points).For this matter, I could mark several points along the line from K to the op-amp (such as k1 k2 k3 k4 k5 etc) and since their potential would be same w.r.t to themselves, then by your logic, it could be rendered suitable for earthing or reference.

There can be no way point G is suitable and point k isn't for grounding. Your grounding definition is in difficulty.

I can understand from circuit layout point of view, from the physical size of connections involved and other factors GCDEF may be suitable for grounding.

Reality, however, intervenes as soon as our circuit passes current because significant currents flow in the output circuit around TR1 and the loudspeaker, introducing a significant voltage difference between C and D which varies with time. A similar voltage appears between D and E due to TR2. Furthermore, power currents flowing in the rectifier/filter circuit introduce significant voltages between E and F.
When I say significant, they are not significant in relation to the output, but if any point between C and F are used as a reference for the input to A1 these voltages will be in series with the input and may be a substantial fraction (or even equal to) the signal voltage at this point.
Moving on to Fig 2 which shows two pieces of equipment, one mains powered, connected by signal leads.
If the return lead is not earthed the signal generator chassis may attain significant voltage (up to 100 volts) by electrostatic coupling with respect to the other equipment.
This effect is shown as the dashed capacitor .
The chassis of the second equipment is pulled up to this voltage by the return connection between the two pieces of equipment, labelled AB.
Now suppose we wish to make a circuit change to the circuit under test so we switch off its power supply and apply an earthed soldering iron.
If the chassis is not earthed, the charge flow to Earth from the induced voltage on the chassis can easily be enough to destroy semiconductor components.

Fig 3 shows the classic ground loop where two mains powered apparatus are earthed at their respective mains plugs which is also connected to the signal return between the equipment.
Shown dotted is the large area of a ground loop that picks up voltages (especially hum) from any magnetic fields that thread the loop.

This is all a good basis for discussion as to why we don’t want the Earth potential to vary and in relation to what this unwanted variation can arise. Part 2 will include safety Earth's.

Thanks for providing these really good knowledge.

 

[jmsa7]

The car's power supply return (chassis) forms one plate of a capacitor, the Earth below it is the other plate, and you feel it when that capacitance discharges through you.
old jim

SO that means that my body is in between the "plates" like a dielectric. Do capacitors discharge through the dielectric?

 

[Evil Bunny]

If you are standing barefoot on moist ground and touch a point in a circuit that is earthed, you will not get a shock.
If instead you touch circuit common you will experience whatever voltage exists between circuit common and earth, which may be quite a shock..
If circuit common is earthed you will experience zero volts and will not get a shock.

You experience this in cold dry winter weather when exiting your automobile - that shock you get if your hand is stilll touching metal when your foot first hits the ground . The car's power supply return (chassis) forms one plate of a capacitor, the Earth below it is the other plate, and you feel it when that capacitance discharges through you.

So these electrons that traveled from the circuit common (or the car's return chassis), through you, and into the ground...

Were they "stolen" from the pool of electrons that were cruising around in the original circuit?

I suppose they could also have been "inserted" into the pool of electrons that were cruising around in the original circuit, depending on polarity...

But the point of my question... Is the "net total" of electrons in the original circuit changing? When these transfers take place between the Earth and a closed circuit, are we manipulating the electron numbers that were in the original loop?

 

[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.