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Why is voltage like pressure?

  1. May 4, 2010 #1
    :smile: Hello everyone,

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

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

    This is the circuit. Now lets 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 :smile:
     
  2. jcsd
  3. May 4, 2010 #2

    mgb_phys

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    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.
     
  4. May 4, 2010 #3
    Thanks for the reply :smile: 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!!
     
  5. May 4, 2010 #4

    mgb_phys

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    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.
     
  6. May 4, 2010 #5
    Thanks again for the help :smile: 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.
     
  7. May 4, 2010 #6

    mgb_phys

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    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
     
  8. May 4, 2010 #7
    Thanks for the help :smile: 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.
     
  9. May 4, 2010 #8

    rcgldr

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    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.
     
    Last edited: May 4, 2010
  10. May 4, 2010 #9
    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.
     
  11. May 4, 2010 #10

    russ_watters

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    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.
     
  12. May 4, 2010 #11
    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?
     
  13. May 5, 2010 #12
    Thanks for the reply :smile: 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!!
     
    Last edited: May 5, 2010
  14. May 5, 2010 #13
    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.)

    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?).
     
    Last edited: May 5, 2010
  15. May 5, 2010 #14

    sophiecentaur

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    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.
     
  16. May 5, 2010 #15
    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.
     
  17. May 5, 2010 #16
    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?)
     
    Last edited: May 5, 2010
  18. May 5, 2010 #17
    I'm getting confused now.

    http://home.earthlink.net/~mmc1919/venturi.html [Broken]

    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?
     
    Last edited by a moderator: May 4, 2017
  19. May 5, 2010 #18

    sophiecentaur

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    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.
     
  20. May 5, 2010 #19
    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 :smile:

    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.
     
  21. May 5, 2010 #20

    sophiecentaur

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    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.
     
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