# B Is Voltage a measurement of pressure?

#### frankin garcia

Summary
Volts
Summary: Volts

When measuring an electrical currents voltage, are you actually measuring the pressure emitted in electricity

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Staff Emeritus
No. Voltage and pressure are two different things.

#### Nugatory

Mentor
No, you are measuring the voltage difference between two points in the circuit.

In some situations this is somewhat analogous to pressure in a water system, so you’ll see it described that way in some introductory explanations - but as with all analogies, there’s only so far that it can go. You can use it as long as it helps you form an intuitive mental picture of what’s going on, but any time that it’s not helping you understand you should give it up.

#### Andrew Mason

Homework Helper
Summary: Volts

When measuring an electrical currents voltage, are you actually measuring the pressure emitted in electricity
Voltage between two points, A and B, in a circuit is a measure of the potential energy difference between a unit charge at A and a unit charge at B. Voltage has dimensions of energy/charge (units such as joules/coulomb).

Pressure is force per unit area or energy per unit volume (a kind of potential energy density). A pressure difference between two points A and B in a fluid circuit signifies a difference in potential energy of a unit volume of fluid at A compared to a unit volume at B.

So voltage and pressure are dimensionally different but conceptually similar.

AM

#### Henryk

Gold Member
Umm, measuring the potential difference is a bit tricky.
The quantity a voltmeter measures is actually a difference of chemical potential of electrons divided by the elementary charge. The chemical potential is affected by electric field but, if you look at it closely, it is not the same.

#### sophiecentaur

Gold Member
Summary: Volts

pressure emitted in electricity
Perhaps you are trying to use 'scientific words' to make your point but ask yourself what you mean by emitted pressure. If you try to make up a satisfactory simplified model for some physical process or situation then you have a serious problem when you don't start off with a rigorous, accepted model*.
Pressure in a fluid and volts around a circuit do have some degree of equivalence and there are a number of mathematical equations for each that have a common form. But you cannot go further than that. People just get things wrong when they try to take the pressure model into circuit theory. So why do people keep offering that approach? It does no one any favours.
*this is NOT the Spanish Inquisition approach, btw.

#### frankin garcia

Voltage between two points, A and B, in a circuit is a measure of the potential energy difference between a unit charge at A and a unit charge at B. Voltage has dimensions of energy/charge (units such as joules/coulomb).

Pressure is force per unit area or energy per unit volume (a kind of potential energy density). A pressure difference between two points A and B in a fluid circuit signifies a difference in potential energy of a unit volume of fluid at A compared to a unit volume at B.

So voltage and pressure are dimensionally different but conceptually similar.

AM
Maybe volts and pressure are conceptually different and dimensionally similar, because of the fact that volts is the force in the movement electrons(so it seems)and pressure is the force in movement, please correct where i am wrong, if.

#### sophiecentaur

Gold Member
Maybe volts and pressure are conceptually different and dimensionally similar, because of the fact that volts is the force in the movement electrons(so it seems)and pressure is the force in movement, please correct where i am wrong, if.
By "dimensionally similar", all you can mean is the linear relationship between those two quantities and 'another two' quantities. The word "flow" is a sort of verbal link but the dimensions are totally unrelated.

You seem determined to hang on to an intuitive connection and some very familiar ideas (not always correct, in fact) associated with fluid flow, rather than to give Electricity the respect it deserves as a totally separate part of Science.

Water will always let you down when you use it in trying to extend any present knowledge you have of Electricity. One of the aims of PF is to improve understanding of Science and dodgy short cuts really don't help.

#### Andrew Mason

Homework Helper
Maybe volts and pressure are conceptually different and dimensionally similar, because of the fact that volts is the force in the movement electrons(so it seems)and pressure is the force in movement, please correct where i am wrong, if.
The other term for voltage or potential difference is "emf" which is an acronym for "electro-motive force". The actual electro-motive "force" per unit charge is the electric field, which is the gradient or space derivative (spatial rate of change) of the potential. So thinking of voltage as a force leads to incorrect physics.

The better term would be "electro-motive pressure". If you think of voltage as a kind of electro-motive pressure and charge as a flow volume, you can model electric circuits in your head as somewhat analogous to volume of water flow doing work between two points due to pressure difference between the two points - such as water turning a hydro-electric turbine:

AM

#### sophiecentaur

Gold Member
The better term would be "electro-motive pressure".
Something that people seem to forget in trying to hop freely between Potential and Force (water or electricity) is the fact that the Field (force on each charge) can be anything you want for a given PD. A long 1W resistor has entirely different Fields along its length from a short 1W resistor. The two resistors are electrically identical.
The relevant Potential Difference, in the case of the hydroelectric system above is all across the Turbine (we assume the pipes are all wide enough and no work is done in getting the water to the turbine input port). The Pressure on the turbine is more or less ρhg and the Force depends entirely on the design of the blades etc. etc. When you get down to it, the water model is at least as complicated as the electrical model because the Kinetic Energy of the moving water is also relevant. KE doesn't come into it when you are talking about Electrical Power.
I could go on further to demonstrate that the 'simplicity' of the Water Model is a snare and a delusion. The fact that there are so many Electrical Engineers who can operate at a reasonable level of competence yet very few Fluid Engineers who are as 'good' at the theoretical level, goes to prove my point.
It may be time, I think, for the Electrical Model to be used to 'explain' Water Flow problems. (But only the simple ones)

#### Andrew Mason

Homework Helper
The Pressure on the turbine is more or less ρhg and the Force depends entirely on the design of the blades etc. etc.
The pressure difference or potential difference is $\rho g\Delta h$ (potential energy / unit volume).
When you get down to it, the water model is at least as complicated as the electrical model because the Kinetic Energy of the moving water is also relevant. KE doesn't come into it when you are talking about Electrical Power.
Kinetic energy is not really a factor in the water model either, except that there has to be some movement of the water through the turbine. The work done in passing through the turbine (assuming no change in kinetic energy through the system ie. steady flow rate with no difference between the size of the inlet and outlet pipe to/from the turbine) is pressure (difference) x volume or potential difference x volume.

AM

#### sophiecentaur

Gold Member
Kinetic energy is not really a factor in the water model either, except that there has to be some movement of the water through the turbine.
I think that statement only applies to well designed turbines but you always need to shift the exit water out of the way of the next lot coming out. I have seen water coming out from an old hydro generator and it was pretty fast and furious but mechanical energy is always 'impressive' subjectively so my intuition may have failed me there.

If you look at water wheels where the PD is low, the KE can be a significant contribution to the equation because they are not efficient. Wheels where the water flows over the top are more efficient (iirc) than when the water flows under. The KE makes a difference there and I don't think there's an electrical power transfer where KE actually counts in the same way.

#### sophiecentaur

Gold Member
I know I keep batting on with my objections to the water analogy but I feel very strongly that when you give someone an analogy without all the necessary caveats (and no one gets them in school) they will (human nature) run off with them and try to apply them way outside their valid range. I guess the ultimate analogy is Maths and we all know how careful we have to be in interpreting the results, even of mathematical models.
I see little point in trying to justify the water analogy except at a very superficial level but people seem to cling to it like a drowning man and a lifeboat. We all (I think) use a sort of intuitive water model in our heads at times but that's 'private' and internal.

#### Stephenk53

From my experience the water model of electricity is only useful if you are explaining something to someone who has no intention of studying electricity for example it works well in high school but not so much in college if you are studying electricity for a degree of some kind, since you will see times where the analogy does not work well.

#### sophiecentaur

Gold Member
the water model of electricity is only useful if you are explaining something to someone who has no intention of studying electricity
I agree totally. Which is what surprises me about all the posts on this and other threads which try so hard to justify the model. Which of the members of PF that they are trying to help are not studying Electricity?

#### frankin garcia

Um guys i never said anything about water..
I am a welder and when i put the voltage up in my tig welder the flame gets thicker like more pressure was applied or more air was in the same volume applying a stronger force in every direction, and when i put up or down the amps in my tig welder, it changes how fast the flaming air is coming out, so i would assume amps is measuring the rate of electricity, or electrons moving thru any given point. Assist me where im wrong, if.

#### frankin garcia

Also guys, dont forget uncertainty has been proven to be real, so All of our thoughts must be questioned to account for what is uncertain.

Emitted means produce. The potential of my pure electric welder is real but only possible with some pressure moving thru, we measure that part, on the voltage knob aside from the on and off button. So i wonder if what is really happening here is a simple measurement of pressure under certain conditions.

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#### sophiecentaur

Gold Member
Assist me where im wrong, if.
Analogous relationships can be found all over the place. Something increases and that causes something else to increase. Your income and the number of goodies you can afford are related in the same way but would you draw a meaningful parallel between that and the way electricity behaves (except the obvious one)?
The point of Science is to find and describe the way quantities are related in as precise a way as possible. That makes it more and more possible to predict things and to improve Technology and lives in general. it can't really surprise you to find adverse reactions on PF to your attempt to muddy the waters by applying your own home brewed models. There's nothing wrong with holding all sorts of personal views but don't expect them to automatically have validity in the world of Physics. The 'rules of the game' are tried and tested; only occasionally can a very well informed person validly change them. The reason for the way Physics works as well as it does is not 'faith' but pragmatism.

#### frankin garcia

Sophiecentaur, i had thought the purpose was to summarize the universe? I feel like there are spiritual aspects strongly involved with that approach that science keeps pushing. if you follow the paths of analogous relationships, you will find them all over the place leading to one single subject. I might call this subject god but thats a preference, and the paths themselves are still distinguished except now more so. Ive always felt as a child to now that maybe somethings Matter that the science community and I were neglecting or not acknowledging, and since the question was real and i learned of uncertainty i say 'there is uncertainly something uncertain about what we say deciding what actually Matters' or materalizes for a more vivid approach.

#### sophiecentaur

Gold Member
I don’t know what “summarise” means in this context. I don’t know of any uses of the simile in Science.
The common language appears to be Maths.
I really can’t comment on the rest of your post. I don’t do mystical.

#### Stephenk53

Sophiecentaur, i had thought the purpose was to summarize the universe? I feel like there are spiritual aspects strongly involved with that approach that science keeps pushing. if you follow the paths of analogous relationships, you will find them all over the place leading to one single subject. I might call this subject god but thats a preference, and the paths themselves are still distinguished except now more so. Ive always felt as a child to now that maybe somethings Matter that the science community and I were neglecting or not acknowledging, and since the question was real and i learned of uncertainty i say 'there is uncertainly something uncertain about what we say deciding what actually Matters' or materalizes for a more vivid approach.
By the way PF does not allow for spiritual talk nor talk about what we "feel like" due to the subjectivity of that and lack of the ability to test such things. Also I think you meant describe the universe not summarize the universe. A lot of scientists say that science is meant to create a universal theory of everything but that is a bit off topic.

#### frankin garcia

Well my question is answered i guess. I also never knew the spiritual as something untestable, i actually came to such useful thought by asking an unusual amount questions to any and everyone, carefully. Mark this day, i will come back to pf with my research done and completed on this 'meta-physical' with the standards you guys hold to be valued in science. Yall inspire mee!!!

#### sophiecentaur

Gold Member
with the standards you guys hold to be valued in science. Yall inspire mee!!!
How embarrassing.

#### Lord Crc

I used the water analogy in reverse to understand the hydraulic ram. "Oh this is just like a boost converter but with water". Then the device made perfect sense.

#### sysprog

I think that Blaise Pascal was a bright guy.