Why does EVERYONE claim voltage is electrical pressure?

[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.

 

[Studiot]

Russ, I respectfully suspect you are the one missing the point.

I agree no model is perfect.

This thread was started by someone pointing out some of the flaws in a teaching model used for beginers.

A respected teacher at that level has stated that s(he) finds it at best unhelpful in teaching at that level.

Several others have leapt to the defence of the model regardless of whether it actually helps beginners.

[edit]

I agree that some aspects of electrical circuit behaviour can be modeled by considering pressure drops in pipes. I have never said otherwise.

But there are other models, which may model some things better,
I'm sorry to say I am witnessing what seem to me as a newcomer to be attacks on those proposing other models.

[/edit]

Is this rational debate?

 

[Studiot]

Your scenario is self-contradictory, if you connect one terminal only then the resistance is infininte.

Certainly stirred up a hornets nest here.

Why is what I said self contradictory?
It is perfectly consistent. The connection has as low a resistance as you care to name.

But

The other terminal is not connected.

This is perfectly consistent with the one pipe / one wire scenario.

And my main objection to the pipe analogy. A piped system does not require to be a circuit to be useful. An electrical one does.

And the concept of a circuit is very fundamental and very important to introduce to a beginner, which I understand this thead is all about.

And yes I agree that in the pressure model a battery is equivalent to a pump. It may also be equivalent to a pump in the height model, but there are other mechanisms that do not develop pressure difference, that can also be used to model batteries in the height model.

 

[Dale]

Why is what I said self contradictory?
It is perfectly consistent. The connection has as low a resistance as you care to name.

But

The other terminal is not connected.

This is perfectly consistent with the one pipe / one wire scenario.

OK, I understand what you are talking about, but the analogy is good in this scenario too:
Pump outlet to pipe to lake with no pipe from lake to pump inlet -> no flow
Cathode to wire to ground with no wire from ground to anode -> no current

Your objection on this point is simply wrong. If you want to criticize an analogy you need to actually use the analogy correctly and show where it still fails even when used correctly.

 

[Studiot]

DaleSpam I object.

If you simply criticize my statements we can have a rational discussion.

But you criticize me as well.

This is a pity because your application of your analogy is fallacious.

Connecting only one port of a pump is not analagous to connecting only one terminal of a battery.

By itself a pump is not a faithful analog of a battery as a battery is an electrical energy source between its terminals.

A pump on the other hand is just a pump between its ports. The analog is only made real by adding the reservoir. So a true analog would be the pump plus reservoir. The ports corresponding to the battery terminals are then the pump outlet and the reservoir inlet.

And of course if you connect a single pipe to this outlet and turn on the tap you will get wet.

Totally unlike a battery where you have to make a connection to both terminals to obtain electricity.

 

[Phrak]

An ideal battery supplies constant voltage. An ideal centrifugal pump supplies a constant pressure. An ideal positive displacement pump, on the other hand supplies a constant volume of fluid. It is equivalent to a consatnt current source. With this difference recognized the analogy is good. But a reservoir has no equivalent element of correspondence within the ideal battery.

 

[Phrak]

And of course if you connect a single pipe to this outlet and turn on the tap you will get wet.

Totally unlike a battery where you have to make a connection to both terminals to obtain electricity.

You have improperly connected the battery. Air is a medium where the water is free to flow. Air is not a medium where current is free to flow.

In contradition to the analogy you have not embedded your battery in a equivalent conducting medium where charge is free to flow.

If you wish to criticize an analogy you might put some effort into understanding it.

 

[Studiot]

You have improperly connected the battery. Air is a medium where the water is free to flow. Air is not a medium where current is free to flow.

Would you like to expand on this statement?

 

[sophiecentaur]

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.

Yes of course you're right : It's 'Joules per Coulomb'
But it's much further from Newtons or Pascals etc than Energy and that's the gripe.

 

[sophiecentaur]

Would you like to expand on this statement?

(Post 77)
If we were fish, we might be having a different argument about the analogies. We would need to have end caps on all our cut wires - just as plumbers need to cap off the ends of water pipes. We tend to take for granted that an air gap is just a very high impedance circuit element.

Here's an interesting quirk of language which I have found confuses some foreign students.
An OPEN switch is one which doesn't permit a current to flow and a CLOSED one does.
OTOH
A CLOSED tap prevents flow and an OPEN tap allows it.
These confusions run deep.

 

[Dale]

If you simply criticize my statements we can have a rational discussion.

But you criticize me as well.

Please quote the specific statement I made where you believe I criticized you instead of your statements. That was certainly not my intent, but I understand that this is a difficult medium to communicate sometimes and it is certainly possible to unintentionally criticize the person instead of the statement.

 

[rcgldr]

The analogy 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

Wasn't this originally just about voltage, not all the other analogies? For voltage, you left out the field intensity term. Voltage = E(h) x h, where E(h) is the field intensity at h. Gravitational potential = g(h) x h.

Analogy using an infinitely large disk with a finite negative charge or finite amount of gravity per unit area, and "h" as distance from that disk:

Intensity= -E (Newtons / coulomb) => gravitational intensity = -g (Newtons / kilogram)
Force = -Eq => -gm
volt = joule / coulomb
potential = V (volts) = Eh => gh = V (joule / kilogram)
potential energy = U (joules) = Eqh => gmh = U (joules)

Power for a moving fixed amount of charge or mass:
power = Eq (dh/dt) = q (dV/dt) => gm (dh/dt) = m (dV/dt)

Power for a contintous flow of charge or mass bewteen two points in space:
I = dq/dt
ṁ = dm/dt
power = E (dq/dt) (dh/dt) = (dq/dt) (dV/dt) = I (dV/dt) => g (dm/dt) (dh/dt) = (dm/dt) (dV/dt) = ṁ (dV/dt)

 

[Dale]

Connecting only one port of a pump is not analagous to connecting only one terminal of a battery.

By itself a pump is not a faithful analog of a battery as a battery is an electrical energy source between its terminals.

A pump on the other hand is just a pump between its ports. The analog is only made real by adding the reservoir. So a true analog would be the pump plus reservoir. The ports corresponding to the battery terminals are then the pump outlet and the reservoir inlet.

And of course if you connect a single pipe to this outlet and turn on the tap you will get wet.

Totally unlike a battery where you have to make a connection to both terminals to obtain electricity.

I try to avoid the word reservoir because it can be taken to mean a man-made lake or a tank. If you mean a lake, either man-made or natural, then the top of the lake is equivalent to ground, a place where the water/charge is all at the same potential and where lots of flow/current can easily go in or out without changing the potential.

A disconnected wire is analogous to a pipe with an end cap, as sophiecentaur has pointed out. They are both places where flow/current cannot go regardless of pressure/voltage and both represent an infinite resistance to ground.

A pump is in fact analogous to a battery, both are the source of the pressure/voltage for the flow/current but neither are a source of the water/charge itself. They both have two ports, with a pressure/voltage difference between them and the flow/current in one port being equal to the flow/current out of the other port.

A pump with the inlet connected to a lake would be equivalent to a battery with the anode connected to ground. If you wish to disconnect the battery from ground then you must also disconnect the pump from the lake to maintain the analogy.

 

[krishnaraj]

you cannot say that pressure is force.both are different.pressure is something you give continousally which cause some drastic change..but force can become pressure

 

[sophiecentaur]

I hope you all realize that no one can be 'right' about this.
But, clearly, some of you have definitely got a few things wrong about the real Physics involved.
I just hope that people will use analogies with care and avoid their 'students' going away with drastically wrong ideas. I have just been on two courses, well accredited by a major recreational organisation and the tutors, although knowing more than enough to give us all the necessary information for the purposes of the course, had far too limited a grasp of the Physics involved to pass on any sense of 'why' things were happening. Other attendees were left worse than when they started.

 

[Studiot]

Hello Dale this was the comment I felt attacked me personally.

If you want to criticize an analogy you need to actually use the analogy correctly and show where it still fails even when used correctly.

I accept your susbsequent statement that this was not meant in a personal way, but if you look at the posts around that one you will see why I was feeling 'got at'.

So no hard feelings there.

However I cannot accept a statement that a battery is equivalent in any way to pump.

A battery is a primary energy souce, which is the reason I chose it.
A pump is not.
Period.

I have expressed no objection to saying to a 12 year old

"Voltage is a sort of electrical pressure that forces current through a circuit, but there are differences."

But, as Sophie says, Teachers of 12 year olds should understand the fundamental differences between the two systems and be prepared to offer suitable explanations thereof.

One view is that the most fundamental difference is that pumping fluid is about mass transport. The fluid mass by itself is just that. It requires an external potential field to obtain or absorb work. A pump has to be supplied with an external energy source. A gravitational or other field could also be used.

By contrast electric circuits are about transport of energy or perhaps charge in some circumstances. Once a source of EMF is connected no further energy/work input is required.

A slight different view is that in an odd sort of way the two are (almost) duals. Both are about energy transport.

One the one hand you have lots of mass and have to supply a potential field to transport energy.

On the other you have a potential field and have to supply lots of charge to transport energy.

It is OK to shift lots of mass from A to B and leave it there.
It is far from OK to shift lots of charge from A to B and not return it back to A.

 

[Dale]

However I cannot accept a statement that a battery is equivalent in any way to pump.

A battery is a primary energy souce, which is the reason I chose it.
A pump is not.
Period.

I think this statement goes way too far. The pump is not equivalent to a battery in that way (being a primary energy source). But as I have pointed out above there are plenty of other ways in which they are equivalent. As long as I stick to those other ways in which they are equivalent then my use of the analogy is fine.

Also, so what? Why would a circuit care if it were driven by a primary energy source like a battery or a secondary energy source like a waveform generator with identical voltage and impedance?

 

[Studiot]

OK, nitpick accepted - in fact I did say in a post backalong that in some models a battery is equivalent to a pump.

Having got that out of the way, how about answering my comment on grounds?
I have even provided a nice clean thread for the purpose.

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

 

[russ_watters]

This thread has run its course. Locked.