# Why does EVERYONE claim voltage is electrical pressure?

leright
Why does EVERYONE claim voltage is "electrical pressure?"

Nearly everyone I talk to who says they know "a lot" about electricity say voltage is electrical pressure...

However, I fail to realize how voltage has anything to do with pressure...in fact, I am quite certain it has NOTHING to do with pressure, and everyone who claims the contrary is completely wrong.

Can someone provide some insight that possibly validates the claim that voltage is like electrical pressure? From what I understand, voltage is the potential energy (force exerted through a distance) per unit charge a given charge has due to an electric field. The force would depend on the charge producing the e-field.

## Answers and Replies

leright
ok, I guess I can see why voltage is like pressure...it just seems like a half assed way of thinking about it, and I don't like it.

davidbenari
leright
and if someone else uses the analogy of water flowing through a pipe to explain electrical flow, I am going to flip.

Cyrus
When you have something under pressure, you can make a hole in the tank and let the water flow out of the hole. This flow can be put to use, (turn a paddle wheel, run a generator). The more pressure you have, the faster the wheel or generator will turn.
Likewise, the more voltage you have, the more work you can possibly do. Its just an analogy so that you can have a tangible understanding of it.

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amt
You need a pressure to push the electrons in a conductor. If not, what will force the electrons(current) to move? With no pressure(electro motive force or potential difference), the electrons will simply idle around at a small pace.

Don't flip- But, think of the Ocean wave as the Voltage and the water as the current.

So, if you take a voltage source, then there has to exist a difference between the two points in order for a voltage to be generated. Potential difference is defined as how much work is needed to move a charge from one point to another. You can integrate the Electric field from one point to another to obtain the Voltage.

leright
amt said:
You need a pressure to push the electrons in a conductor. If not, what will force the electrons(current) to move? With no pressure(electro motive force or potential difference), the electrons will simply idle around at a small pace.

Don't flip- But, think of the Ocean wave as the Voltage and the water as the current.

So, if you take a voltage source, then there has to exist a difference between the two points in order for a voltage to be generated. Potential difference is defined as how much work is needed to move a charge from one point to another. You can integrate the Electric field from one point to another to obtain the Voltage.

I'm aware of what voltage is defined as and I know you can integrate the electric field over a given distance to get voltage. Since e-field is N/c, you can integrate that with respect to distance to get J/c. This wasn't really my question.

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.

Thanks for the quick reply.

leright
cyrusabdollahi said:
When you have something under pressure, you can make a hole in the tank and let the water flow out of the hole. This flow can be put to use, (turn a paddle wheel, run a generator). The more pressure you have, the faster the wheel or generator will turn.
Likewise, the more voltage you have, the more work you can possibly do. Its just an analogy so that you can have a tangible understanding of it.

I've heard that analogy before, but it still REALLY doesn't make a strong connection between work that CAN be done on a charge due to an E-field and the pressure of water hitting a paddle wheel...does it?

A better analogy would be that voltage is kinda like the work the water does due to the force of gravity pulling the water a distance equal to the height of the tank after it is dumped PER unit volume of water. The voltage is just like this, except instead of the force being a gravitatial field, it is electric field, and instead of water, it is charge. Also, instead of the force being only attractive (as in gravity), it can be both attractive and repulsive.

No physics professor I have ever had has used the "voltage is like pressure" analogy...because it is wrong.

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Mentor
leright said:
and if someone else uses the analogy of water flowing through a pipe to explain electrical flow, I am going to flip.
I don't know why you're going to flip - the reason people compare these phenomena is because they really are exactly the same thing. A watt is a watt is a watt whether it's electrical or mechanical or chemical. If you want to calculate how much electrical energy you can get from a hydroelectric dam, for example, you calculate how much mechanical energy is released by the water flowing through it (and multiply by the efficiency of the turbines). It should not be surprising that the equations for describing what happens in the wires look a lot like the equations for what happens in the pipes.

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davidbenari
Homework Helper
leright said:
and if someone else uses the analogy of water flowing through a pipe to explain electrical flow, I am going to flip.
I can kind of understand your frustration with that analogy. But it's not a totally useless analogy. In fact, it helped me understand how water flows through a pipe. :rofl:

Staff Emeritus
Gold Member
leright said:
Nearly everyone I talk to who says they know "a lot" about electricity say voltage is electrical pressure...

However, I fail to realize how voltage has anything to do with pressure...in fact, I am quite certain it has NOTHING to do with pressure, and everyone who claims the contrary is completely wrong.

Can someone provide some insight that possibly validates the claim that voltage is like electrical pressure? From what I understand, voltage is the potential energy (force exerted through a distance) per unit charge a given charge has due to an electric field. The force would depend on the charge producing the e-field.

Ohm's Law for cuurent in a conductor : V = IR
where
V : voltage or potential drop across ends of conductor
I : current (or rate of flow of charge)
R : resistance of wire/conductor

Power drop in a resistor = IV

Effective resistance of conductors in series, R(eff) = R1 + R2 + ... + Rn

Effective resistance of conductors in parallel, 1/R(eff) = 1/R1 + 1/R2 + ... + 1/Rn

Flow of incompressible fluid through a pipe : P = Qr
where
P : head or pressure drop across ends of pipe
Q : fluid flow rate
r : impedance of pipe

Hydraulic power drop in pipe = PQ

Effective impedance of pipes in series, r(eff) = r1 + r2 + ... + rn

Effective impedance of pipes in parallel, 1/r(eff) = 1/r1 + 1/r2 + ... + 1/rn

Well, do you now see the reason for drawing up the analogy ?

quasi426
leright said:
No physics professor I have ever had has used the "voltage is like pressure" analogy...because it is wrong.

Actually Professor Feynman used that analogy. He's a physics professor I think.

Theelectricchild
Correction: *was*

kirovman
It's an analogy. Analogies by defination are not entirely exact and specific. I suggest you deal with it. I don't think anyone said "Voltage IS pressure", they said "Voltage is similar to pressure because the potential difference drives the current, just as the pressure drives the flow of water...although they are different things"

A lot of analogies work on a similar basis, although the more knowledgeable physicists will of course know better. Analogies are just used to help people understand otherwise complicated things.

Antiphon
The analogy can be made much deeper.

If you measure the attractive force between the plates of a capacitor they
are proportional to the voltage. (If you measure the repulsive force between the
windings of an inductor they are proportional to the current in the inductor. The
energy in the inductor is exactly analgous to the kinetic energy of moving water in a pipe.)

Pressure is force/unit area.

For a fixed geometry of a given capacitor attractive pressure on the plates will be related by a constant of
proportionality to the voltage.

For a fixed geometry of a given vertical pipe, outward pressure on a cross section of pipe will be related
by a constant of proportionality to the height of the fluid colum in the pipe.

While not exatly the same they are similar enough that you could for many
problems solve the equations for one and get solutions to the other. That is
by definiton a good and useful analogy.

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kleinjahr
Read some of the history about electricity and you will find that Volta et al. thought of it as, literaly, a fluid/aether. Is it any wonder that they applied what they knew of hydraulics to describe it? No, the wonder is that the description actually works so well, within it's limits.

Antiphon
Yes Kleinjahr. Good point.

And to this day the transmission of electrical power is done over
"high tension wires". They aren't talking about the mechanical tension of the
cables but rather the "tension" means voltage in another analogy to pressure.

marlon
leright said:
Nearly everyone I talk to who says they know "a lot" about electricity say voltage is electrical pressure...

Actually, there is quite some confusion on exactly how stuff like EMF, voltage,...should be defined. The link will give you an article of IOP, illustrating the various contradictory definitions. You will need a subscription to download the file so you should open it at your university-library. It really is a great article that also explains in a very easy way the quasi Fermi Level (though not really stringent)

http://www.iop.org/EJ/abstract/0031-9120/3/5/001

regards
marlon

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Antiphon
marlon said:
You will need a subscription to download the file so you should open it at your university-library.

I don't have convenient access to it. Can you give en example of one of the
contradictory definitions for us?

leright
Antiphon said:
The analogy can be made much deeper.

If you measure the attractive force between the plates of a capacitor they
are proportional to the voltage. (If you measure the repulsive force between the
windings of an inductor they are proportional to the current in the inductor. The
energy in the inductor is exactly analgous to the kinetic energy of moving water in a pipe.)

Pressure is force/unit area.

For a fixed geometry of a given capacitor attractive pressure on the plates will be related by a constant of
proportionality to the voltage.

For a fixed geometry of a given vertical pipe, outward pressure on a cross section of pipe will be related
by a constant of proportionality to the height of the fluid colum in the pipe.

While not exatly the same they are similar enough that you could for many
problems solve the equations for one and get solutions to the other. That is
by definiton a good and useful analogy.

ok...this statement does nothing but further strengthen my point. Electrical pressure is NOT the same as voltage...electrical pressure is merely a good analogy to think of voltage...however, I don't entirely agree with this either. Something being linearly related to voltage DOES NOT make that something an analogy...

Electrical pressure can be found using voltage, but they are in no way the same thing. Voltage is a VERY specific physical quantity.

Staff Emeritus
Nobody ever said that voltage *was the same* as pressure.

We *are* saying that you can construct a reasonably good simile, metaphor, or analogy for how electric charges flow through a conductor by considering incompressible fluid flow.

We *are not* saying that that unit of charge is the liter. We are saying that when you make a metaphor to compare electric current flow to fluid flow, you treat charge *like* the volume of an incompressible fluid. (The motivation for this is firstly that charge is conserved, secondly that the amount of charge in a given volume of conductor is constant).

The metaphor extends to say that pressure is *like* voltage. (Energy / unit charge translates to energy per unit volume in the *anaology*, which translates to pressure).

leright
pervect said:
Nobody ever said that voltage *was the same* as pressure.

We *are* saying that you can construct a reasonably good simile, metaphor, or analogy for how electric charges flow through a conductor by considering incompressible fluid flow.

We *are not* saying that that unit of charge is the liter. We are saying that when you make a metaphor to compare electric current flow to fluid flow, you treat charge *like* the volume of an incompressible fluid. (The motivation for this is firstly that charge is conserved, secondly that the amount of charge in a given volume of conductor is constant).

The metaphor extends to say that pressure is *like* voltage. (Energy / unit charge translates to energy per unit volume in the *anaology*, which translates to pressure).

read my above post. I am aware of the point you are making in this post. People are CONSTANTLY claiming voltage to be EQUAL to electrical pressure...in fact, voltage was at one point referred to as electrical pressure, but I think they stopped referring to it as pressure, because it is NOT pressure.

Renge Ishyo
It is true that voltage and pressure are not the same thing. You cannot have a voltage unless you can do work (it has units of J/C after all). You CAN have a pressure even if you are doing no work (units of N/M^2). Voltage only exists if the charges are able to move through a distance.

Having said that, you can toss both words in the garbage and the water analogy still works. In the case with water, you have excess molecules on one end that are able to flow to the other end that has less molecules. In the case with electricity, you have excess electrons on one end that are able to flow to the other end that has less electrons. We can call either of these processes and the various ways we measure them whatever we want, the same basic underlying principles are at work here in either process. That is the *point* of making an analogy that links the two concepts together, so that if you understand one of the processes it will help you develop a feel for the other process.

Of course, you can always disregard the analogy and treat water flowing and electricity flowing as two completely separate things...whatever works for you.

daveed
and uncertainty doesn't EXACTLY mean that some man up there is throwing a pair of dice every half second...

it just helps people make sense of it

timeformation
going back to beginning of thread, the hole in the pipe, makes water shoot out, and could turn a wheel.. blah blah...

The flow exists because at one point of the pipe, and the other, there is a difference in pressure, hence the its like osmosis, not like, but is.. anyway.. the voltage is like this pressure difference, in the sense thta it causes the water to have more pressure on one side, than the other.. that is why you copnnect one end to a ground and the other not to ground.. but can you see why the pressure is needed for the water to flow? If yes, then the same logic applies to current... no voltage = no current.

mikerodi

Hi there ... I realize that this thread dates back some years, but have come accross the same problem and I'm sure that after all this time you might have found the answer, but in the thread I noticed that nobody mentioned resistance, which I believe is what causes 'pressure'... that is, in the case of electrons and the quatity of these, we can only determine that there is a quantity if we present something that can 'group' these electrons so as to determine that quantity... in this case... a resistance.

If we force electrons to come close together (I know almost nothing about chemistry), so, like a cube of water, where the cube IS the resistance and the water the AMOUNT of 'energy' (current), then a pressure automatically forms at the base of this cube.
Now we have all three elements... in this order... resistance, current AAAAND voltage (pressure) which is the product of restance and current... (sounds a bit like Ohm's law?).

But all this is pretty useless unless we use it, but we do definitely have a potential difference (in the cube... the pressure). That's why it just doesn't seem logical to say that we apply a 'voltage' to something, we are actually applying current to something where ITS resistance has determined at what speed (voltage) that current should flow.

Gold Member

I hate hate hate the idea that Potential difference could be looked upon as a 'sort of pressure or force'. (The old term 'emf' / electro motive force doesn't help, either)
But, sometimes you can find yourself using the analogy - before quickly washing your mouth out.
It can't be 'all bad' - in the same way as saying Resistance is just like Friction. I have so many times seen a student's eyes glaze over when I have answered the question "What IS resistance" and I have replied "V/I".

Like me, it's sloppy but can be useful, on occasions.

cabraham

It's an analogy. Analogies by defination are not entirely exact and specific. I suggest you deal with it. I don't think anyone said "Voltage IS pressure", they said "Voltage is similar to pressure because the potential difference drives the current, just as the pressure drives the flow of water...although they are different things"

A lot of analogies work on a similar basis, although the more knowledgeable physicists will of course know better. Analogies are just used to help people understand otherwise complicated things.

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.

FWIW, using "pressure" to explain voltage is not a good idea. There are always analogies between different things that may help a beginner visualize new concepts, but sooner or later these analogies break down. Students should be taught the basics right up front and not rely on analogies. Voltage is energy per charge, and also it is electric scalar potential. It is also the time derivative of magnetic flux. It is that and that and all that.

Pressure is not a good way of viewing voltage. Water flowing in 2 pipes in close proximity in time varying fashion exhibits no induction or "transformer-like" behavior whatsoever. Wave propogation from antennae cannot be likened to pressure, because fields are involved. Also, voltage is both potential and rate of change. In the electric energy domain, it is potential, or "scalar electric potential". In the magnetic energy domain it is -N*d(phi)/dt, phi=flux.

Current is rate of change in the electric domain, i.e. i = dq/dt, q=charge. Current is scalar magnetic potential in the magnetic domain.

These concepts have no equivalent in pressure and water flow. I recommend learning the basics in their most elementary form, and try not to lean on analogies. Pressure is not a good way of looking at circuits. Peace.

Claude

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.

FWIW, using "pressure" to explain voltage is not a good idea. There are always analogies between different things that may help a beginner visualize new concepts, but sooner or later these analogies break down. Students should be taught the basics right up front and not rely on analogies. Voltage is energy per charge, and also it is electric scalar potential. It is also the time derivative of magnetic flux. It is that and that and all that.

Pressure is not a good way of viewing voltage. Water flowing in 2 pipes in close proximity in time varying fashion exhibits no induction or "transformer-like" behavior whatsoever. Wave propogation from antennae cannot be likened to pressure, because fields are involved. Also, voltage is both potential and rate of change. In the electric energy domain, it is potential, or "scalar electric potential". In the magnetic energy domain it is -N*d(phi)/dt, phi=flux.

Current is rate of change in the electric domain, i.e. i = dq/dt, q=charge. Current is scalar magnetic potential in the magnetic domain.

These concepts have no equivalent in pressure and water flow. I recommend learning the basics in their most elementary form, and try not to lean on analogies. Pressure is not a good way of looking at circuits. Peace.

Claude

I've never heard the water pressure analogy used to describe transformers or antennae - only very simple series and parallel circuits.

It's good for beginners and only for those simple circuits...going beyond that it's worthless to electrical engineers.

CS

Gold Member

Hitting 12 year olds with the subtleties of Maxwell's equations would not be a good idea.

I, personally, use the analogy of height above ground rather than pressure, with school students, to give a handle on Potential Difference. This ties in with Kirchhoff II and voltage measurements round a circuit and it also deals with the transfer of energy on the way down a slope - and you can use water wheels in the analogy.

I guess the only people who should be allowed to use analogies are those who actually understand what they're trying, to a reasonable level, what it is they're trying to explain.

Studiot

The head or height analogy for voltage has much to commend it over the pressure model.
It allows the concept of completing the circuit to be developed and does not beg the question
"Why can't voltage 'force' current down a single wire like pressure can force fluid down a single pipe?"
You can even develop a pressure free model if you use water wheels and reservoirs as you say.
It also avoide the issue of Bernouilli's equation for flow and energy calculations.

Stick with it Sophiecentaur.

Per Oni

I just had a similar discussion here: https://www.physicsforums.com/showthread.php?t=377359
I go along with leright (5 yrs ago)
“and if someone else uses the analogy of water flowing through a pipe to explain electrical flow, I am going to flip..”
Power in electrical conduction is not transported through electrons pushing length ways just like water in a pipe. It is transported by electro magnetic fields which flow partly inside, partly outside the wire.

Gold Member

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? It is necessary to talk in terms of 'something' flowing and we can all detect flowing charges with an Ammeter. If there aren't any flowing then our lamp doesn't light, so it seems reasonable to use the concept when describing 'electricity' at an elementary level.

It's all very well stating what 'we' all know (or should know) but does that help a beginner? What we should really be after is the least worst analogy - which, when you get down to it, is all that Physics can hope for when describing anything.

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

Gold Member

It's all very well stating what 'we' all know (or should know) but does that help a beginner? What we should really be after is the least worst analogy - which, when you get down to it, is all that Physics can hope for when describing anything.

This thread / question is actually a very difficult one to resolve satisfactorily.
When teaching someone the basics of electricity, it is a very useful analogy. Draw a tank to hold water, and explain how the potential energy (head) can do work, and how sizing the output piping (resistance) can control the flow rate. It doesn't take too much to get to a simple circuit with a battery and resistor(s). Show a student a circuit with two resistors in parallel and ask them to think of the analogy a bit (lower R value = larger pipe) and predict how electricity will flow through the resistors.

I carry the analogy further when explaining to someone how a vacuum tube (valve) operates in a circuit and how small changes in bias can lead to large output variations. (I once repaired tube amps as a side-line, and sometimes had to explain why output tubes should be biased properly for pleasing output sounds.) Most people can grasp that if you keep things simple and use diagrams. Like any analogy, comparing hydraulic and electrical systems falls apart at some level, but if you want to give a newbie a handle on electricity, it can be very useful.

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