# Why does EVERYONE claim voltage is electrical pressure?

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.

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

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.

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.

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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russ_watters
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
BobG
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:

Gokul43201
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 ?

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.

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 knowledgable physicists will of course know better. Analogies are just used to help people understand otherwise complicated things.

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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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.

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.

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)

IOP Article

regards
marlon

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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

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.

pervect
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).

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.

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.

and uncertainty doesnt EXACTLY mean that some man up there is throwing a pair of dice every half second...

it just helps people make sense of it