Why does EVERYONE claim voltage is electrical pressure?

[sophiecentaur]

"sizing the output piping (resistance) can control the flow rate"
Unfortunately this is where that particular analogy can break down. The resistance of the pipe isn't really analogous to electrical resistance because the real work done or energy transferred is not within the pipe - especially when the flow is reasonably smooth.
Even James Joule found it very hard to measure any heating effect due to water flowing in a pipe or even when it lands after a fall of several hundred metres because water has such a huge specific heat capacity compared with the GPE it has due to its height.
In contrast, electrical resistor gets noticeably hot with the energy, supplied by the battery, etc.
The main energy transfer in a water system has to be when the moving water actually does some mechanical work - involving a turbine or ram. Best to treat the pipes as ideal wires and include explicit 'machines' as the energy transducers.

 

[Studiot]

So, Per Oni, how do you explain anything at all about electricity to a School Pupil who has no idea of the concept of a field, be it electric, magnetic or both?

Exactly so.

I was actually horrified to find that the local secondary school which supposedly follows the National Curriculum introduces Forces by talking about Friction.
Friction must be the most difficult force to deal with and is still not fully understood let alone intuitive.

Keep it Simple.

 

[sophiecentaur]

It's hard to reconcile Newton's First Law with all our experiences of moving things if you don't consider friction.
Do you expect kids to come into School with Degree Level knowledge and appreciation of things? How can you "keep it simple" if you don't include friction?

 

[Studiot]

I did say introduces force. I was talking 12 year olds as you were.

My original physics teacher defined force for me as

A push or a pull.

Then he got a spring balance and held a competition in class to see who could pull it out the furthest.

That generated real interest in the nature of force.

 

[elect_eng]

I guess I understand how electrical potential is sort of related to water pressure, but it is a poor way of thinking about it...it seems so half assed that it is just seems wrong...

Sorry if I seem kinda rude, but this just frustrates me.

If it frustrates you, then just ignore it. You seem to understand the physics well, so you don't need an analogy to help, and it even can confuse you. So, just forget about it and realize that it helps some people that do not have a firm grasp on the physics yet.

Ironically, I learned about electricity before fluid dynamics. So, I used the ideas of ohms law to understand water flow in pipes. And, I used my understanding of Maxwell's equations for electromagnetics to understand the vector equations (Navier Stokes) of fluid dynamics.

At the same time, I can understand the issue of not liking an analogy. When I try to equate electrical circuit equations (coils, capacitors and resistors) to mechanical systems with springs, masses and dampers, I get more confused. The mechanical system seems very simple to me and I have no need for the analogy. It just gets in the way, so I ignore it.

There are entire books devoted to these types of mechanical and electrical analogies. The are all based on mathematical parallels in different types of systems.

 

[Per Oni]

So, Per Oni, how do you explain anything at all about electricity to a School Pupil who has no idea of the concept of a field, be it electric, magnetic or both?

To a School Pupil I would explain exactly as you do. In fact your analogy with gravity is quite close to the real thing. However most of us here are not School Pupils and perhaps want to look a bit further.

This thread / question is actually a very difficult one to resolve satisfactorily.

Has it been resolved? From wiki:

DC Power flow in a concentric cable
Application of Poynting's Theorem to a concentric cable carrying DC current leads to the correct power transfer equation P = VI, where V is the potential difference between the cable and ground, I is the current carried by the cable. This power flows through the surrounding dielectric, and not through the cable itself.[7]

However, it is also known that power cannot be radiated without accelerated charges, i.e. time varying currents. Since we are considering DC (time invariant) currents here, radiation is not possible. This has led to speculation that Poynting Vector may not represent the power flow in certain systems.

Also if you got a copy of “Lectures on physics” by Feynman look at page 27-8. There he works out the field theory and then calls it “obviously nuts”.

So what is today’s accepted theory? Plumbing or field ?

 

[sophiecentaur]

"Has it been resolved?"
The resolution to which I was referring was about the issue of analogies and "what's really happening?". Anyone who really thinks we will have the answer to that particular question has got the wrong idea about Science. Individuals, in their time, may think they have cracked it but it always turns out that they haven't. That doesn't invalidate the quest for knowledge, though and nor does it totally invalidate the apprpriate use of analogy to aid understanding. (Note the word 'appropriate')

 

[rcgldr]

I use the analogy of height above ground rather than pressure.

I also think that's the best way to explain a potential, such as voltage. I would also include the fact that gravitational potential reasonably close to the surface of a planet is related to the gravitational force per unit mass times height (meters). For the earth, the gravitational force per unit mass is 9.80665 Newtons / kilogram, and for the moon, it would be 1.622 Newtons / kilogram. To get the same gravitational potential, an object would have to be 6.046 times as high on the moon as it would on the earth.

Then for voltage, I would use the example of two very large (compared to the distance between the plates) charged plates producing a net force per unit of positive charge = E (Newtons / coulomb) between the plates with the direction of the force towards the negative plate (since it's a unit of positive charge), and that the voltage would be equal to E times distance (meters) from the negatively charged plate (towards the positively charged plate), reacing a maximum at the positively charged plate. If asked, I would point out that if the plates are sufficiently large, then the force is zero at any relatively small distance outside of the plates.

I would then point out that in this case since voltage = E times distance (meter), then the unit for E could also be defined as volt / meter in addition to Newtons / coulomb.

 

[russ_watters]

"sizing the output piping (resistance) can control the flow rate"
Unfortunately this is where that particular analogy can break down. The resistance of the pipe isn't really analogous to electrical resistance because the real work done or energy transferred is not within the pipe - especially when the flow is reasonably smooth.

Actually, the analogy does not break down there. The concept works the same way in both cases, it is just applied differently in many real life examples. The reality is that a wire - any wire - has a resistance and sometimes this resistance matters. On the flip side, a pipe can be sized large enough that the resistance inside the pipe is negligible. This, in fact, often happens in variable flow systems.

Even James Joule found it very hard to measure any heating effect due to water flowing in a pipe or even when it lands after a fall of several hundred metres because water has such a huge specific heat capacity compared with the GPE it has due to its height.
In contrast, electrical resistor gets noticeably hot with the energy, supplied by the battery, etc.

So what? That's just a difference in the way the heat is sensed - a joule is still a joule. In my line of work, I'm often searching for and eliminating that energy. It may not show up as a big temperature difference in a pipe, but it still costs the owner a poorly designed system lot of money.

The main energy transfer in a water system has to be when the moving water actually does some mechanical work - involving a turbine or ram. Best to treat the pipes as ideal wires and include explicit 'machines' as the energy transducers.

As said above, it is often the case that nearly all of the pressure drop in a system is across a partially open valve. It is very useful to compare this to a variable resistor for amperage and wattage (flow rate and power) calculations. It helps understand, for example, why as resistance goes up, power goes down - which is a subject that is often problematic.

 

[turbo]

Thanks, Russ. I often used the analogies to explain the need for service work on nice old tube amps to musicians who had no appreciation for EM theory. Some of them thought that that if they bought a '65 Super Reverb or a '57 Deluxe, they would sound fantastic regardless of how the amp was tuned and regardless of how the components were serviced. I had one guy tell me not to replace the caps in the power supply of an early '60's Fender amp, and after he rejected the very simple explanation of why this might be necessary, I urged him to take his amp to a music store for service. He had been told that I was the "go-to" guy for amp-tuning and tone, but the people making the recommendations apparently forgot to mention that failing electronic components need to be replaced, and the rest of the circuits need to be tuned to reflect the replacements.

 

[Gokul43201]

Potential difference does not "drive" the current. Coulomb force and/or energy is what drives the current. To get potential difference, or voltage, charges must be separated. But that requires movement of charges. That is current. In a nutshell, you need current to get voltage. You need voltage to get current. It's chickens & eggs. Neither one "drives" the other.

Ummm...this is wrong. Coulomb force (on a charge) and/or energy can be directly calculated from the potential difference.

The mean Coulomb force, F, on an electron in a wire of length L with potential difference V across its ends is simply, F = eV/L. And the difference in potential energy is just PE = eV. If the force and/or energy can be thought of as the things that drive the current, the potential difference most certainly can too!

And there are no chickens and eggs here. You are confusing the charges that create the potential with the charges that respond to it. Those are two different eggs there.

 

[elect_eng]

There are entire books devoted to these types of mechanical and electrical analogies. The are all based on mathematical parallels in different types of systems.

Here is an example of a book on the subject of system analogies.

http://www.pmillett.com/Books/intrlson_1943_Dynamical_Analogies.pdf

This PDF for it is now in the public domain, as the book is out of print for a sufficient amount of time.

Dynamical Analogies, by Harry F. Olson, Published by D. Van Nostrand Co. in 1943

The full text can be downloaded for free at the following site.

http://www.pmillett.com/technical_books_online.htm

 

[sophiecentaur]

Actually, the analogy does not break down there. The concept works the same way in both cases, it is just applied differently in many real life examples. The reality is that a wire - any wire - has a resistance and sometimes this resistance matters. On the flip side, a pipe can be sized large enough that the resistance inside the pipe is negligible. This, in fact, often happens in variable flow systems. So what? That's just a difference in the way the heat is sensed - a joule is still a joule. In my line of work, I'm often searching for and eliminating that energy. It may not show up as a big temperature difference in a pipe, but it still costs the owner a poorly designed system lot of money. As said above, it is often the case that nearly all of the pressure drop in a system is across a partially open valve. It is very useful to compare this to a variable resistor for amperage and wattage (flow rate and power) calculations. It helps understand, for example, why as resistance goes up, power goes down - which is a subject that is often problematic.

If you are considering pressure and flow inside pipes of varying diameter as being analogous to the potential on wire conductors of varying diameters how do you reconcile the Bernouli Effect where the pressure Increases back up again when the flow goes from a thin pipe to a thick pipe? You never 'get volts back' when you go round a resistor network. And when you use the analogy, do you stress that you are talking in terms of high speed circulation through capillary pipes and a heating effect?
There's a huge caveat there, I think you have to agree.
A thin pipe is a really poor analogy to a piece of resistive wire. Although I've read and heard many attempts to use the analogy, they miss the whole point of Energy Transfer. It is only at the transitions between pipe widths that there are significant pressure changes and I have yet to see any analogy / model in which there is any mention of this or which considers the energy involved.

Also, I would make the point that most peoples' understanding of what goes on in plumbing and water circulation systems doesn't put them in anything like a good position to use it as an aid to understanding something that, in many ways, is actually more straightforward. I have given up in many discussions with plumbers when I've heard their idiosyncratic views of the Physics involved with what they're doing. (I'd trust them implicitly not to cause leaks and blockages and mostly to get the Central Heating to work properly - but that involved doing things by rote and is quite another matter)

 

[Studiot]

Yes there are lots of possible objections to the pipe analogy.

One is that pipes have several flow regimes.
Partial bore.
Full bore laminar.
Full bore turbulent.

Each obey a totally different set of equations, not reflected anywhere in electric circuits.

However the most important objection I raised earlier, that no one seems to have picked up on is.

If I connect one pipe to a reservoir I can get water out of the other end period.

If I connect one wire to one terminal of a battery how much electricity can I get out of it?

 

[sophiecentaur]

Do you mean I don't need to screw lightbulbs into empty sockets, to prevent the electricity from leaking out all over the house?

(With acknowledgments to James Thurber
More of him at http://www.brainyquote.com/quotes/authors/j/james_thurber.html)

 

[Studiot]

Yup someone has got it at last.

Big Smile

 

[russ_watters]

If you are considering pressure and flow inside pipes of varying diameter as being analogous to the potential on wire conductors of varying diameters how do you reconcile the Bernouli Effect where the pressure Increases back up again when the flow goes from a thin pipe to a thick pipe?

Total pressure is constant, but in any case I probably just wouldn't take the analogy that far.

And when you use the analogy, do you stress that you are talking in terms of high speed circulation through capillary pipes and a heating effect?

I use the analogy only when a situation calls for it - it is the person who asks the question who sets up the scenario.

There's a huge caveat there, I think you have to agree.

Of course there are caveats. It isn't perfect: it's an analogy.

 

[sophiecentaur]

"Total pressure is constant"
Total Pressure = ?
First it's high, then it's low then its high again. What's constant about it?
I think we'll have to differ on this one - perhaps because of our different audiences. I couldn't afford for GCSE and A level students to get it that wrong.

 

[rcgldr]

Personally, I think using pressure as an analogy for voltage is mis-leading. Voltage is a potential. At best, a pressure gradient in a particular direction would somewhat correspond to a field intensity, a directed force per unit area, as an analogy to a directed force per unit mass or unit charge. It wouldn't take into account how potential is also affected by the relative distance in the direction of force between two points.

The alternate analogy of comparing height times gravitational force per unit mass is a much better analogy, since this is simply comparing gravitational potential with electrical potential.

 

[russ_watters]

"Total pressure is constant"
Total Pressure = ?
First it's high, then it's low then its high again. What's constant about it?
I think we'll have to differ on this one - perhaps because of our different audiences. I couldn't afford for GCSE and A level students to get it that wrong.

Bernoulli's principle states that total pressure is constant along a streamline. Often people make oversimplifications in their description and drop the word "static" when saying:

...an increase in the speed of the fluid occurs simultaneously with a decrease in [static] pressure or a decrease in the fluid's potential energy.

http://en.wikipedia.org/wiki/Bernoulli's_principle
...which often leads to an incorrect understanding. If you look at the equation, you see:

static pressure + velocity pressure + gravitational pressure = constant [total pressure]

You are referring to static pressure.

 

[Studiot]

Russ, you are skillfully avoiding my comments.

 

[Studiot]

Wow someone I take to be a beginner has asked exactly the question I anticipated.

https://www.physicsforums.com/showthread.php?t=381947 : post#3

 

[sophiecentaur]

"Bernoulli's principle states that total pressure is constant along a streamline. Often people make oversimplifications in their description and drop the word "static" when saying: etc
"
So is the 'well known' pipes demonstration not really happening? And are these links seriously in error?
http://home.earthlink.net/~mmc1919/venturi.html
http://www.ceet.niu.edu/faculty/kostic/bernoulli.html

Funny, 'cos I've seen it work often. The pressures are continuously high then low then high, whilst the water is flowing through a constriction - unlike the voltages down a potential divider which are always monotonic. I don't understand your distinction between dynamic and static pressure. When water is flowing, the dynamic pressure is present and when water isn't flowing you simply have static pressure which is independent of pipe diameter.
Have you not ever seen this effect? Could you explain where your interpretation of Bernoulli fits in? Have I missed something? Perhaps it's just a matter of the values of velocity, pipe diameter and static head. But, if you are using high static pressures, where is the work being done by your water? What does you analogy show apart from a set of gradually reducing pressures? And why bother with a very limited analogy when a much better one exists which is just as easy to show and describe? Can you not see the difference and how much better the energy-based analogy is?
You might as well show students how a hot Central Heating water pipe gets colder as it goes around the circuit. That wouldn't be a very good analogy for electrical resistance either but it would also give you a graph which 'went the same way'.

 

[Studiot]

I did comment back along that it is possible to construct an electric circuit analog from a hydraulic circuit all at the same pressure. You can demonstrate many circuit elements with tanks, wide bore pipes or spillways, water wheels as batteries. You can model series and parallel circuits, demonstrate Kirchoff, energy equation E = IV with I modeled by the mass flow (=volume flow rate for an incompressible fluid like water) and V modeled by height or head.

 

[Dale]

However the most important objection I raised earlier, that no one seems to have picked up on is.

If I connect one pipe to a reservoir I can get water out of the other end period.

If I connect one wire to one terminal of a battery how much electricity can I get out of it?

This is an incorrect objection. If you put a low resistance connection to ground you will get current whether you are talking about water in pipes or electricity in wires. If instead you terminate your conductor with an infinite resistance then you will not get flow whether you are talking about water in pipes or electricity in wires until your cap breaks.

I am not saying that the analogy has no limits or is always appropriate, but it is often useful including in this situation.

 

[Studiot]

If you put a low resistance connection to ground you will get current whether you are talking about water in pipes or electricity in wires

Only in so far as if you connect any two bodies you may get a temporary flow of charge equalistion.

Take a 9 volt battery and tell me, hand on heart, that I can obtain a sustained current flow from it by connecting one terminal only to ground by any resistance you care to name.

Oh, and by the way you also need to acknowledge that 'ground' is a very special concept that can apparently defy normal circuit laws as it works in a different way from any other 'component'.

 

[sophiecentaur]

I did comment back along that it is possible to construct an electric circuit analog from a hydraulic circuit all at the same pressure. You can demonstrate many circuit elements with tanks, wide bore pipes or spillways, water wheels as batteries. You can model series and parallel circuits, demonstrate Kirchoff, energy equation E = IV with I modeled by the mass flow (=volume flow rate for an incompressible fluid like water) and V modeled by height or head.

Great stuff - as long as no one starts talking about pressure as if it's voltage. It's energy that is the equivalent to volts. Then you have an excellent demo / analogy.

I can't understand why there is still a diehard attitude to the flawed analogy when you have an almost perfect one available with almost the same equipment.

 

[Studiot]

It's energy that is the equivalent to volts.

No

The analog is
Volts (V) = Height or head (H)
Charge (q) = mass m
Current I = dq/dt = rate of mass flow dm/dt
Power = VI = ghdm/dt
Energy = VIdt = Vq = ghm

Edit
Regretfully I rushed headlong into an old beartrap. Ouch.
The product VI is of course power.
Energy is the time integral/sum of power.

 

[russ_watters]

Russ, you are skillfully avoiding my comments.

[edit: wrong person]

I don't see your arguments as useful. You're basically arguing a tautology and trying to find a flaw where none exists: if a person who uses the analogy doesn't use it beyond its domain of usefullness then it is useful. Finding an example of where it doesn't work doesn't prove anything if people never use the analogy in that situation.

 

[russ_watters]

"Bernoulli's principle states that total pressure is constant along a streamline. Often people make oversimplifications in their description and drop the word "static" when saying: etc
"
So is the 'well known' pipes demonstration not really happening? And are these links seriously in error?
http://home.earthlink.net/~mmc1919/venturi.html
http://www.ceet.niu.edu/faculty/kostic/bernoulli.html

Funny, 'cos I've seen it work often...


Have I missed something?

Those are fine demonstration of Bernoulli's principle. Your second link clearly says what I was saying:

The terms on the left-hand-side of the above equation represent the pressure head (h), velocity head (hv ), and elevation head (z), respectively. The sum of these terms is known as the total head (h*). According to the Bernoulli’s theorem of fluid flow through a pipe, the total head h* at any cross section is constant (based on the assumptions given above).

...with the caveat that "head" is synonomous with "pressure". So "total head" is "total pressure".

However, they also drop the word "static" in a few places...

 

[Studiot]

What did I avoid

I asked what to say to a beginner who asks

"If flow of electricity in wires is like flow of fluid in pipes why can I get fluid out of a single pipe, when I can't get electricity out of a single wire"

And yes a beginner just asked that very question.

 

[Studiot]

with the caveat that "head" is synonomous with "pressure".

That is a dimensionally unsound statement sir.

 

[russ_watters]

I asked what to say to a beginner who asks

"If flow of electricity in wires is like flow of fluid in pipes why can I get fluid out of a single pipe, when I can't get electricity out of a single wire"

And yes a beginner just asked that very question.

Sorry, responded to the wrong person in that post...

...
I don't see your arguments as useful. You're basically arguing a tautology and trying to find a flaw where none exists: if a person who uses the analogy doesn't use it beyond its domain of usefullness then it is useful. Finding an example of where it doesn't work doesn't prove anything if people never use the analogy in that situation.

 

[russ_watters]

That is a dimensionally unsound statement sir.

Maybe, but since I live on earth, it still works and therefore is often used synonomously (and it is in that link). I suspect you already knew that though. Either way:

Also called pressure head. Hydraulics.
a.the vertical distance between two points in a liquid, as water, or some other fluid
b.the pressure differential resulting from this separation, expressed in terms of the vertical distance between the points.
c.the pressure of a fluid expressed in terms of the height of a column of liquid yielding an equivalent pressure.

http://dictionary.reference.com/browse/head

This is starting to get annoying.

 

[Dale]

Take a 9 volt battery and tell me, hand on heart, that I can obtain a sustained current flow from it by connecting one terminal only to ground by any resistance you care to name.

Your scenario is self-contradictory, if you connect one terminal only then the resistance is infininte. If you have an infinite resistance then you don't get current flow in either case. Again, there are failings of the analogy, but this is not one.

Btw, in the plumbing analogy a 9 volt battery is not equivalent to a tank, it is equivalent to a pump.