Why is the flow rate constant in a circuit with two resistors?

[sameeralord]

Hello everyone,

First of all I have to say I never understood what voltage is? So this is my final hurrah at understanding this I'm hoping to understand this using pressure analogy.

Pin= 120 mmHg -----res1-------res2---Pout= 0mmHg

This is the circuit. Now let's think water is flowing. Now I want to know when water passes through the resistor 1 and resitor 2 why is the flow rate the same. I mean if res 1 slows down water, then res 2 should slow it down further.Why is it constant?

The way I have come up with an answer is like this, please tell me if this is right. Water has 2 energies = Hydrostaic pressure energy + kinetic energy pressure.

So each time the resistor only loses the hydrostatic energy of the water so flow remains constant. Now I don't know how to transfer this analogy to electrons, is it the same thing, does electrons also have 2 energies like this.

Also one last question. Why does a pressure gradient make something flow, is it simply that 120 mmHg pressure side in this case has more collisions, and molecules simply move due to these collisions? I know voltage is electrical potential difference between 2 points, but why exactly does this make electrons move? Do they move due to the collision like water or due to opposite charges attracting the electrons?

Thanks

 

[mgb_phys]

You can't really stretch the analogy that far.
In fact electrons aren't really a good way to picture voltage and current in a circuit

Voltage is like pressure in that it adds and to increase the flow you use more pressure just like using more voltage to increase the current.
Water flow is also like current, you can't have water appear or disappear in a pipe just like you can't have different current in different parts of the same circuit.

 

[sameeralord]

You can't really stretch the analogy that far.
In fact electrons aren't really a good way to picture voltage and current in a circuit

Voltage is like pressure in that it adds and to increase the flow you use more pressure just like using more voltage to increase the current.
Water flow is also like current, you can't have water appear or disappear in a pipe just like you can't have different current in different parts of the same circuit.

Thanks for the reply This is what I'm thinking. When the water for the first time starts to flow, it passes through resistor 1 slows down and then pass through resistor 2 slows down again. How is this different when a flow is established? Also why does high pressure make something flow. May be what I'm thinking about this is wrong. Thanks!

 

[mgb_phys]

It's probably only valid to think of the steady state - when water is flowing, not when you are filling up the system!

If you have a pipe with a narrow section.
The flow must be the same all the way along - you can't pour a gallon/min in one end and not have a gallon/min come out of the other!
Where there is a resistance (the narrow bit) a pressure difference forms - as water is being pushed into the constriction it pushes back.
This is exactly the same as Ohm's law for a resistor.

You need to think of speed as gallons/min not the speed of an individual water molecule. This is the same as electricity, the individual electrons don't all move through the circuit and the ones that do move only go at walking pace - it's the flow of electrical current that is very fast (almost the speed of light).
This is the same as water - if you push one end of a hydraulic piston the other end moves (almost) instantly - it's the pressure that moves through the fluid (at the speed of sound)- not the water molecules flowing.

 

[sameeralord]

It's probably only valid to think of the steady state - when water is flowing, not when you are filling up the system!

If you have a pipe with a narrow section.
The flow must be the same all the way along - you can't pour a gallon/min in one end and not have a gallon/min come out of the other!
Where there is a resistance (the narrow bit) a pressure difference forms - as water is being pushed into the constriction it pushes back.
This is exactly the same as Ohm's law for a resistor.

You need to think of speed as gallons/min not the speed of an individual water molecule. This is the same as electricity, the individual electrons don't all move through the circuit and the ones that do move only go at walking pace - it's the flow of electrical current that is very fast (almost the speed of light).
This is the same as water - if you push one end of a hydraulic piston the other end moves (almost) instantly - it's the pressure that moves through the fluid (at the speed of sound)- not the water molecules flowing.

Thanks again for the help I'm bit confused about the resistance you mentioned in the narrow bit, is this resistance due to walls coming closer, or the water in that narrow area is providing resistance (in a flow that is established). Maybe if I can understand what exactly the resistance does to the flow, I can get closer to undersatnding this.

 

[mgb_phys]

In a pipe it's due to the walls coming closer so the water experiences more friction so it takes more force (pressure) to push the same amount of water through.

In electricity resistance means electrons are more tightly held to the atoms and so it takes more force (voltage) to push the same amount of current through

 

[sameeralord]

In a pipe it's due to the walls coming closer so the water experiences more friction so it takes more force (pressure) to push the same amount of water through.

In electricity resistance means electrons are more tightly held to the atoms and so it takes more force (voltage) to push the same amount of current through

Thanks for the help Ok I can understand that if there is a flow, the flow rate must be the same at both sides, or would would clog up someplace, but how does the system natually comes up with a particular flow rate, I don't think I understand the difference of flow rate when a system is filling up and established flow.

 

[rcgldr]

Voltage is a potential. A better analogy would be height and the intensity of gravity (V = gh). Since the intensity of gravity is constant near the surface of the earth, then height would be a reasonable analogy, a point at 50 meters height has double the potential of a point at 25 meters.

 

[sameeralord]

Thanks everyone. I want to know why the pressure drops to zero in the wire. Wire does the voltage drop to 0 as it goes along the wire.

 

[russ_watters]

Neither pressure nor voltage necessarily drop to zero. They are sometimes pre-defined to be zero at a particular reference point. For a pump or a battery, it is often useful to define the pressure/voltage to be zero on one side and some positive number at the other.

 

[cesiumfrog]

Pin= 120 mmHg -----res1-------res2---Pout= 0mmHg

I want to know why the pressure drops to zero in the wire.

Say the pressure in the water mains is 4atm. The pressure in a puddle is 1atm. If you let water through a hose (from the mains to the puddle), its pressure will drop from 4 to 1 atm (do you really need this explained?). If you use a water pressure gauge, it may be configured to subtract 1 atm from every measurement, in such a way that it measures 0 pressure near the end of the hose and 3 atm near the top. By placing a number of kinks (of varying tightness) at different places along the hose, you can control the resistance (friction) of each point along the hose, and hence control the total current of water going through. (This is just Ohm's law, Poiseuille's law, etc.) You can thereby alter the pressure-drop across any particular segment of the hose (which can be between zero and three atm). But you cannot alter the pressure-drop across the total hose (which always remains 3 atm). Does that help?

 

[sameeralord]

Say the pressure in the water mains is 4atm. The pressure in a puddle is 1atm. If you let water through a hose (from the mains to the puddle), its pressure will drop from 4 to 1 atm (do you really need this explained?). If you use a water pressure gauge, it may be configured to subtract 1 atm from every measurement, in such a way that it measures 0 pressure near the end of the hose and 3 atm near the top. By placing a number of kinks (of varying tightness) at different places along the hose, you can control the resistance (friction) of each point along the hose, and hence control the total current of water going through. (This is just Ohm's law, Poiseuille's law, etc.) You can thereby alter the pressure-drop across any particular segment of the hose (which can be between zero and three atm). But you cannot alter the pressure-drop across the total hose (which always remains 3 atm). Does that help?

Thanks for the reply It certainly helped me. I understand the voltage difference a bit better now but I think I will need that bit explained. Sorry

"If you let water through a hose (from the mains to the puddle), its pressure will drop from 4 to 1 atm (do you really need this explained?)."

Why does the puddle have a pressure to begin with. Let's say I'm pouring water for the first time to an empty puddle, what would happen.

Is flow and diffusion the samething in this case. Is that why molecules are flowing, are they simply diffusing. If they are diffusing why do they diffuse along a gradient?

Thanks!

 

[cesiumfrog]

Why does the puddle have a pressure to begin with. Let's say I'm pouring water for the first time to an empty puddle, what would happen.

The top of the puddle will have 1 atm of pressure, whether empty or not. This is because there is one atmosphere of air pressure (not more and not less) exerted against the puddle. (Basically, if this were not the case, then the water would flow and slosh and redistribute itself, and never reach steady-state until such time as it is the case.)

(However, you risk breaking the water/electricity analogy when you start to consider hoses that are initially empty except for air.)

Is flow and diffusion the samething in this case. Is that why molecules are flowing, are they simply diffusing. If they are diffusing why do they diffuse along a gradient?

No.. There's several velocities that you shouldn't confuse. 1- The pressure waves move at the speed of sound, which is hundreds of metres per second (so if you turn on a long hose pre-filled with water, water begins spurting out the end nearly instantly), just as the voltage can change through a circuit at up to the speed of light. 2- The water current moves fairly slowly (probably about one metre per second, so if you inject dye at one end of the hose it takes a while to come out the other end), just like the net drift velocity of electrons in a circuit. 3- Each individual water molecule (or electron) moves about randomly but quickly and at a different speed again (this is called thermal motion), which we normally ignore. Note 2 is simply the average of 3. Diffusion speed is a slightly different concept again (namely, if you inject a small amount of concentrated dye into the water, how quickly does it mix to produce a large amount of faded colour?).

 

[sophiecentaur]

Water flow analogies are full of pitfalls. Talking in terms of a water circuit just consisting of pipes with different diameters is confusing. Much better to consider a model with very wide pipes which connect between a series of turbines. The turbines transfer energy and the pipes don't transfer significant energy. This is the analogy between light bulbs and connecting wires.
No energy is lost, in an ideal case, in getting the charges to flow through the wires of getting the water to flow through the pipes.
The energy transfer in the turbines is 'visible' and identifiable. If you look at the pressure difference across the turbines and the rate of flow, you will know the energy converted by each turbine. The total energy per kg of water flowing will be equal to the sum of the energy transferred by all the turbines. Just like the volts in a series circuit add up to the supply volts.
So it's less confusing if you talk not in terms of pressure but in terms of Energy - then you have a much nearer analogy to circuits and Kirchoff's second law. If You Really Want To Use A Water Analogy in the first place.
The preceding posts show just how cloudy the issues are when you try to identify the similarities between volts and water pressure in what may, at first sight, to be two trivial, analogous problems. The models are not complete.

 

[mikeph]

I don’t like pressure as an analogy. Do you use an inviscid flow? Presumably it must be laminar, incompressible and steady. Then Bernoulli’s equation will tell you that as you follow a streamline into a kink (with smaller cross sectional area) the velocity increases to keep the flux constant, so the pressure will drop and then rise again.

When you follow an “electron streamline” through a resistor, the voltage does not drop and rise again.

I think this demonstrates the difference between the two – pressure differences cause internal forces which vary along the flow due to the flow characteristics, whereas in the case of an electron, the electric field (resulting from voltage differences) leads to an external force on all electrons. Introduce viscosity and your resistance analogy breaks down even further since drag is a phenomenon acting on the pipe walls, whereas electrical resistance occurs uniformly throughout the conductor in the first approximation.Much better IMO is to use an external force, eg. gravity, where the voltage can directly be compared to the gravitational potential per unit mass.

 

[cesiumfrog]

I don’t like pressure as an analogy. Do you use an inviscid flow? Presumably it must be laminar, incompressible and steady. Then Bernoulli’s equation will tell you that as you follow a streamline into a kink (with smaller cross sectional area) the velocity increases to keep the flux constant, so the pressure will drop and then rise again. When you follow an “electron streamline” through a resistor, the voltage does not drop and rise again.

Sorry, could you explain that? Under what circumstances are you saying the pressure rises again?
(Or perhaps, do you have something to say about fluctuating electron drift velocities along a wire of fluctuating gauge?)

 

[sameeralord]

Sorry, could you explain that? Under what circumstances are you saying the pressure rises again?

I'm getting confused now.

http://home.earthlink.net/~mmc1919/venturi.html

This link is showing that. Why is pressure not dropping in the page of this diagram. What I mean is if there is an empty pipe and I pump water at 100mmHg when it comes out of the other end is the pressure the same if there is no resistance in the pipe?

 

[sophiecentaur]

I don’t like pressure as an analogy. Do you use an inviscid flow? Presumably it must be laminar, incompressible and steady. Then Bernoulli’s equation will tell you that as you follow a streamline into a kink (with smaller cross sectional area) the velocity increases to keep the flux constant, so the pressure will drop and then rise again.

When you follow an “electron streamline” through a resistor, the voltage does not drop and rise again.

I think this demonstrates the difference between the two – pressure differences cause internal forces which vary along the flow due to the flow characteristics, whereas in the case of an electron, the electric field (resulting from voltage differences) leads to an external force on all electrons. Introduce viscosity and your resistance analogy breaks down even further since drag is a phenomenon acting on the pipe walls, whereas electrical resistance occurs uniformly throughout the conductor in the first approximation.


Much better IMO is to use an external force, eg. gravity, where the voltage can directly be compared to the gravitational potential per unit mass.

Precisely!
You have to ignore that stuff about how fluids actually flow and only involve yourself with mote tangible energy transfers such as making turbines work or raising weights. The problem is that flowing water is such a familiar phenomenon that people are just not aware of the complexity and think that it's as simple as electrical charge flow.

Volts dropped is ENERGY transferred- and that's the bottom line.

 

[sameeralord]

Precisely!
You have to ignore that stuff about how fluids actually flow and only involve yourself with mote tangible energy transfers such as making turbines work or raising weights. The problem is that flowing water is such a familiar phenomenon that people are just not aware of the complexity and think that it's as simple as electrical charge flow.

Volts dropped is ENERGY transferred- and that's the bottom line.

I suppose if you only consider static pressure in the bernoulli case, the analogy still stands but quite clearly I'm certainly not the person who should comment on stuff like this when I don't even understand it properly. lol

Another question raised just now. If hydrostatic pressure in a pipe is acted upon by resistance, how can it ever be zero if hydrostatic pressure is due to gravity and gravity is always there.

 

[sophiecentaur]

Google Bernouli Effect and there are many links which will show how the pressure in a narrow constriction is lower than the pressure at either end when fluid is flowing through it. I remember finding a brill animation in which you could change the shapes of the pipes and get all sorts of results.

 

[sameeralord]

Google Bernouli Effect and there are many links which will show how the pressure in a narrow constriction is lower than the pressure at either end when fluid is flowing through it. I remember finding a brill animation in which you could change the shapes of the pipes and get all sorts of results.

Ok I will do that. Hey could you help me with my new question ,

If hydrostatic pressure in a pipe is acted upon by resistance, how can it ever be zero if hydrostatic pressure is due to gravity and gravity is always there. I mean if pressure drops to zero won't gravity give back energy.

 

[sophiecentaur]

That's introducing another factor. It would be best to understand what's going on in a horizontal run of pipe circuitry first. Your additional energy source is a bit like having 1.5V cells dotted strategically around an electrical circuit.

 

[rcgldr]

Even without Bernoulli effect, pressure is a bad analogy, since it doesn't have to change with distance in the direction of force, but a potential does change value with distance in the direction of force.

Once again, a direct comparason is another potential, such as gravitational potential. Near the surface of the earth, gravitational potential = V = g h, which is the intensity of the gravity (expressed as an acceleration) times height above surface of the earth.

 

[sophiecentaur]

I think we're all arguing in the same direction. Best to avoid "Volts are like Pressure" and leave it at that, I reckon.

 

[cesiumfrog]

If I have an electrical current conducted through two parallel wires, which then converge down to one wire (of the same metal, so resistance is doubled, but we reserve the option whether to dismiss this by letting the metal superconduct), then diverges into two parallel wires again...

By conservation of current, isn't the net drift velocity faster in the narrow than elsewhere? Does this (analogous to the Bernoulli effect) require some opposite "electromotive force" to decelerate the charge-carriers after the narrow?

 

[rcgldr]

If I have an electrical current conducted through two parallel wires, which then converge down to one wire (of the same metal, so resistance is doubled, but we reserve the option whether to dismiss this by letting the metal superconduct), then diverges into two parallel wires again...

By conservation of current, isn't the net drift velocity faster in the narrow than elsewhere? Does this (analogous to the Bernoulli effect) require some opposite "electromotive force" to decelerate the charge-carriers after the narrow?

Assuming zero resistance, then voltage remains constant in this case, but for pressure, it would decrease when speed increased. Drift velocity is just deviation from pure random motion, the actual speed of the electrons is changed little, if any, by a net current flow. Another example of why pressure is a bad analogy.

 

[russ_watters]

As is often the case, the main problem with the water analogy is that people don't understand the fluid dynamics part and thus apply it wrong:

I don’t like pressure as an analogy. Do you use an inviscid flow? Presumably it must be laminar, incompressible and steady. Then Bernoulli’s equation will tell you that as you follow a streamline into a kink (with smaller cross sectional area) the velocity increases to keep the flux constant, so the pressure will drop and then rise again.

Google Bernouli Effect and there are many links which will show how the pressure in a narrow constriction is lower than the pressure at either end when fluid is flowing through it.

There isn't one type of pressure, there are three in the most common form of Bernoulli's equation. The relevant pressure here is total pressure and Bernoulli's principle, simply stated, is that total pressure is constant along a streamline. It is often the case that people drop the one-word qualifier stating which pressure they are talking about. In the case of a venturi tube, it is static pressure that drops, but velocity pressure rises, keeping total pressure constant.

Even more important, none of this has any relevance to the water/electricity analogy, since the most common form of Bernoulli's equation is a conservation of energy statement, so it isn't saying anything about energy dissipation. It's not a restriction that is needed for the description, but a device to dissipate flow energy. This dissipation of flow energy shows up as a decrease in total pressure (ie, voltage) for a certain constant flow rate (ie, amperage).

 

[cesiumfrog]

There isn't one type of pressure, there are three in the most common form of Bernoulli's equation. The relevant pressure here is total pressure and Bernoulli's principle, simply stated, is that total pressure is constant along a streamline. It is [..] static pressure that drops[..]
Even more important, none of this has any relevance to the water/electricity analogy

Guess I'm asking, must there also be a concept of "static voltage" (independent from total voltage) to halve the mean velocity of electrons wherever the wire doubles?

It's not a restriction that is needed for the description, but a device to dissipate flow energy.

Poiseuille's law?

 

[russ_watters]

Guess I'm asking, must there also be a concept of "static voltage" (independent from total voltage) to halve the mean velocity of electrons wherever the wire doubles?

No.

Poiseuille's law?

Yes, but there are other things that will create a drop in total pressure (flow energy). Every real valve or orifice (or corner or other obstruction) has an efficiency coefficient and causes a total pressure drop proportional to the velocity pressure through the orifice.

Also, a turbine converts flow energy to mechanical energy.

 

[cesiumfrog]

No.

Wait, do you agree the net drift velocity varies depending on the cross section of the conductor?
Would you agree that any such change in velocity involves some corresponding potential gradient?
Are saying an analogy with static pressure (as an extension of the analogy of external voltage with total pressure) is invalid, or just not necessary?

Also, a turbine converts flow energy to mechanical energy.

Just like a motor. (Or maybe analogous to a light bulb.) But a resistor just makes heat, which is why I think it is a close analog to any (simple or real) restriction (or obstruction) in the flow.

 

[russ_watters]

Wait, do you agree the net drift velocity varies depending on the cross section of the conductor?
Would you agree that any such change in velocity involves some corresponding potential gradient?
Are saying an analogy with static pressure (as an extension of the analogy of external voltage with total pressure) is invalid, or just not necessary?

Well, I guess I should just say I've never heard of the analogy used in that way...and the electrical part is beyond my understanding anyway.

Just like a motor. (Or maybe analogous to a light bulb.) But a resistor just makes heat, which is why I think it is a close analog to any (simple or real) restriction (or obstruction) in the flow.

If we're using the analogy primarily as a teaching tool, I'd prefer using similar looking things together, so I'd prefer to use the losses through a pipe to be analagous to losses in a wire and something like an orifice plate to be analagous to a a resistor. After all - all an orifice really does is dissipate energy as heat too.

People don't tend to remember that in a closed circuit with no mechanical output, all input energy in a piping system ends up as heat. For my job, it sometimes matters. I recently designed a heat recovery loop where calculating the energy savings required subtracting the pump energy twice - once for the cost of the electricity itself and once for the heat energy put into the loop by the pump.

 

[cesiumfrog]

I think we're in agreement. I'm not sure I'd heard of the analogy used that way either, until MikeyW raised the issue.

Was that design for cooling recover or heat recovery? (For the latter I would have thought you would desire the heat from the pump, but not the spending of power for it, contributing opposing sign.)

 

[russ_watters]

Sorry, I only skimmed the thread and picked out a couple of major issues...

 

[sophiecentaur]

It's interesting to read that the more that a poster on this thread actually knows about fluid flow, the less likely he is to go along with the analogy with electron flow. Perhaps the less informed posters should take note of this.

 

[sameeralord]

In a pipe it's due to the walls coming closer so the water experiences more friction so it takes more force (pressure) to push the same amount of water through.

In electricity resistance means electrons are more tightly held to the atoms and so it takes more force (voltage) to push the same amount of current through

I think I have almost understood voltage but if I can understand this I'll post my new understanding of what voltage is.

+----------A---resistor-B----------C--_

I'm confused with the fact that you said not all electrons move through the whole circuit. Ok so in this case if the electrons from the resistor move from B to C. Wouldn't electrons accumulate in the A area. Also what happens if the resistor loses all the electrons it is holding. I'm getting confused here. I'm thinking that resistor is also supplying electrons apart from the battery, I'm thinking of a resistor as metal that has delocalised electrons,I don't know if this is wrong, but I want to know why you said not all electrons are moving from positive end to negative end.

 

[mgb_phys]

Electrical current isn't really electron flow in the same way water flows down a pipe.
The first electron pushes the next electron which pushes the next electron and so on - they are all negatively charged so repel each other. That's how 'electricity' travels down a wire at near the speed of light while electrons themselves move through a conductor at walking pace.

It's more like fluid flow in a hydraulic system, the brake fluid under the brake pedal pushes the next molecule of fluid, which pushes the next and so on until the last molecule pushes the brake caliper piston.

 

[sameeralord]

Electrical current isn't really electron flow in the same way water flows down a pipe.
The first electron pushes the next electron which pushes the next electron and so on - they are all negatively charged so repel each other. That's how 'electricity' travels down a wire at near the speed of light while electrons themselves move through a conductor at walking pace.

It's more like fluid flow in a hydraulic system, the brake fluid under the brake pedal pushes the next molecule of fluid, which pushes the next and so on until the last molecule pushes the brake caliper piston.

Thanks I'm close to posting my new understanding of voltage but I have one more clarification before that. Do electrons move through the resistor? What I mean is you said that resistors hold on to their electrons move tightly, so when these electrons leave do other electron come and enter the resistor to fill the gap? Otherwise electrons would accumulate in the circuit.

 

[sameeralord]

Ok I think the answer for my previous question is yes, or else it won't work. Ok this is my final understanding of voltage. I'm sure there are many faults in this.

Ok there is a circuit and there is a battery. The positive and negative end of the battery creates an electric force field. If I put a coulomb of charge and let it move along the electric field right to the end, it would gain some energy due to its movement, this energy is generated by the electric force W=fx. Now voltage is equal to this "W" energy. It is the energy before the electric field is acting upon the charge. Once the electric field acts upon the charge the voltage is converted to energy of movement of charge (may be kinetic energy I don't know). When the charge finally travels along the whole circuit, the charge has energy (kinetic energy equal to W and voltage would be zero)

Also since individual electrons take time to move from + to -. My explanation is bit lacking but I think if find the number of electrons traveling in the whole circuit(meaning from A to B) at one point in time and add all their kinetic energies up that should equal to voltage. Is that right?

For resistors what I think is they weaken the electric force field, so molecules have to go slower (low current) to reach the other end.

 

[sophiecentaur]

Electrons move VERRRRY slowly (a few mm per second) and the conduction electrons constitute a very tiny fraction of the mass of the bulk metal (1/100000 ish). I don't think you need count the KE of the electrons in the energy transferring process. Contrast this with the whole mass of water flowing in a pipe.

Would you say the Kinetic Energy of the links in a bicycle chain was a major factor in what gets you up a hill? It's the same with electrons only much much more so.

 

[sameeralord]

Electrons move VERRRRY slowly (a few mm per second) and the conduction electrons constitute a very tiny fraction of the mass of the bulk metal (1/100000 ish). I don't think you need count the KE of the electrons in the energy transferring process. Contrast this with the whole mass of water flowing in a pipe.

Would you say the Kinetic Energy of the links in a bicycle chain was a major factor in what gets you up a hill? It's the same with electrons only much much more so.

Hey thanks for the response. I think your last analogy was excellent Tell me if this is right. Ok due to electron movements,very small speed(the charge is pushed along, so it is like the chain meaning it itself doesn't travel the whole distance in quick time, but like spins along, or as you said pushes the charge along), the charge(the bicycle moves). If there is a movement of something there must be energy expended. So that is voltage. Did I get this? If I did you analogy did it for me.

 

[sophiecentaur]

Pretty ok. I should think.
As you say, when something is moved (I would say changed, perhaps) then energy is transferred.
In the case of Electrical energy, the energy per Coulomb of charge is, indeed, the voltage drop.

1Volt = 1Joule/Coulomb is what we learn.

You can rewrite this as Energy = QV
or, more familiarly, by dividing both sides by time,
Power = V I
(Power being Energy per second and Current being Coulombs per second)

Energy per coulomb is really nothing like Newtons per square metre is it?

 

[rcgldr]

Energy per coulomb is really nothing like Newtons per square metre is it?

Multiply Newtons per square meter by meter/meter and you get energy per meter^3, energy per unit volume as opposed to energy per unit mass or energy per unit charge. However, I would consider static pressure as actual energy per unit volume, while voltage as potential energy per unit charge.

 

[sophiecentaur]

I see where you're coming from.

 

[russ_watters]

It's interesting to read that the more that a poster on this thread actually knows about fluid flow, the less likely he is to go along with the analogy with electron flow. Perhaps the less informed posters should take note of this.

I see it the opposite way - most people who object to the analogy seem to object based on misunderstandings of the fluids part.

 

[sophiecentaur]

Dunno about misunderstandings (or perhaps I've actually 'misunderstood' your wording).
'Fluid' people are often dealing with masses of stuff, on the move, carrying energy. With the exception of Hydraulic rams ('high impedance' systems), possibly, they are mainly considering Kinetic Energy. Once you have fast circulating water, you get further and further away from a valid analogy with the situation with charge flow in metals and I seriously believe that is the picture in most people's heads when they think of the water analogy. That's why I don't like it, despite rcgldr's last post.

 

[rcgldr]

believe that is the picture in most people's heads when they think of the water analogy. That's why I don't like it, despite rcgldr's last post.

My last post was to explain pressure as energy per unit volume, as well as the fact that it's not a potential energy. Unlike static pressure, voltage is position sensitive, the position in a field, or the "position" in a circuit (at least where resistance exists in the circuit). In a static field, such as the field between two plates, the force per unit charge is constant within the field, but voltage varies with position within the field. Static pressure doesn't have this position aspect of potentials like voltage or gravitational potential.

 

[Antiphon]

The water analog is no good but the water analogy is almost perfect. The analog cannot be used to solve problems the way you can use conductivity potentials as an analog to solve electric field problems using carbon paper.

The analogy is excellent. You can use the intertia of water to make inductors where there is an induced pressure across a pipe section due to the time derivative of the flow. In fact you can build a real water-and-pipe transformer! It works by eletrogravitational induction (too weak to measure except by gravity probe B but real nonetheless).

 

[sophiecentaur]

I agree again. (Edit: with last post but one!)The work done by shifting a column of water against a piston is pressure times volume - that's all. The dimensions are correct and you can't say much more than that. It's not a 'source of energy' whereas, an emf (owch, the force word) implies as much energy as you want.
As all the last posts have been more or less in agreement, why don't we all go to another thread where, perhaps, we can not agree and verbally tear each other's throats out?

 

[sophiecentaur]

Oh.
I spoke too soon

 

[Studiot]

The two systems (water flow in pipes and electrical circuits) are not identical. Each has characteristics unrepresented in the other.
A good teacher will understand both systems, their differences and similarities, and know when to offer the analogy and when to draw the line.