Why is voltage like pressure?

[scilearner]

:-)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 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

[ewmon]

The flow rate is the same through both resistors because there's no "leaks" or other paths. Think of it as a closed system of pipes, like in a house. If you have a very simple house with only a kitchen faucet, the flow at the faucet equals the flow at the meter.

 

You're right in that both resistors resist the water flow. In your analogy, water flow is "current", so the two resistors in series both resist the current.

 

This should help you understand your next question about two energies. That is, electricity doesn't have two energies (unless maybe you're talking about lasers).

 

For your last question, electrons have the same charge, so they repel each other. Think of a crowded building where everyone's pushing and shoving each other equally. No one really goes anywhere until there's some "give" (that is, difference in pressure) in the situation ... let's say an exit door opens. Flow occurs when someone pushes harder that someone else. I think we've all experienced this.

[scilearner]

The flow rate is the same through both resistors because there's no "leaks" or other paths. Think of it as a closed system of pipes, like in a house. If you have a very simple house with only a kitchen faucet, the flow at the faucet equals the flow at the meter.

 

You're right in that both resistors resist the water flow. In your analogy, water flow is "current", so the two resistors in series both resist the current.

 

This should help you understand your next question about two energies. That is, electricity doesn't have two energies (unless maybe you're talking about lasers).

 

For your last question, electrons have the same charge, so they repel each other. Think of a crowded building where everyone's pushing and shoving each other equally. No one really goes anywhere until there's some "give" (that is, difference in pressure) in the situation ... let's say an exit door opens. Flow occurs when someone pushes harder that someone else. I think we've all experienced this.

 

Thanks a lot for the reply All though I'm still uncertain about some bits (not your fault) in general I found the response very helpful. Continuing on that exit door bit you mentioned, is that the reason pressure drops, as more and more people exit. Could you further explain why pressure drops along the tube using that analogy. Thanks

[swansont]

The flow rate is the same through both resistors because there's no "leaks" or other paths. Think of it as a closed system of pipes, like in a house. If you have a very simple house with only a kitchen faucet, the flow at the faucet equals the flow at the meter.

 

In addition, there are no voids or compressions in the fluid in the pipe. For each molecule of water that flows in, one must flow out, and vice-versa — this is known as continuity. Once you have this condition, then you know the flow rates have to be the same throughout the system.

[scilearner]

In addition, there are no voids or compressions in the fluid in the pipe. For each molecule of water that flows in, one must flow out, and vice-versa — this is known as continuity. Once you have this condition, then you know the flow rates have to be the same throughout the system.

 

Thanks for the reply swansoft Could you please tell me how exactly does a resistor slow the current using pressure analogy. Thanks

[ewmon]

Continuing on that exit door bit you mentioned, is that the reason pressure drops, as more and more people exit. Could you further explain why pressure drops along the tube using that analogy.

 

By pressure (voltage) drop, we mean the pressure (voltage) drops from one side of the resistor to the other. The doorway is the resistance. You can actually measure it with a volt or pressure meter, both in the circuitry or in a water pipe. The pressure drop is represented by the difference between people inside the door screaming, "STOP SHOVING", and the people outside the door sighing, "What a relief." The pressure drop = STOP SHOVING – What a relief. The pressure inside the building never drops because it's connected to a voltage (pressure) supply.

 

Likewise, we're not talking about the pressure "along the tube". The pressure is one level from the source to the first resistor, a second level between the first and second resistor, and a third level after the second resistor. This would also occur in a water pipe system.

 

Having said that, there actually is a very slight gradient to the pressure along the tube itself because the tube offers a very slight resistance to the water (and the wire offers a very slight resistance to the current). Make sense? But circuits are designed so the power consumed by the wiring is insignificant compared to the power consumed by the elements of the circuit.

 

And that's why you don't want to use very long extension cords to power an appliance, because the larger voltage drop in the cord means that the appliance isn't running on the full 120 volts (or whatever), which can cause problems.

[swansont]

Thanks for the reply swansoft Could you please tell me how exactly does a resistor slow the current using pressure analogy. Thanks

 

It doesn't slow the current. We've established that the flow maintains a constant rate. The current is the same everywhere in the circuit.

[scilearner]

Thanks again for everyone who replied I'm still trying to get this. Ok I can sort of understand how pressure would drop when there are resistors, let's say the same circuit I mentioned with no resistors, then is there no pressure drop along the wire. Resistance decreases current. So when the current is established why is it slow (ok constant but slow), in the pressure circuit above. If there is a circuit with no resistance does the voltage drop or not? Does the voltage and pressure always drop?

 

Ok can anyone explain pressure drop in molecular level to me.

 

 

p=120------------------------------------------------------------------p=0

 

Ok now if the pressure drops, how does this happen. The molecules bound off each at the start and then move, then why does the pressure drop along? How can relate this to voltage drop?

 

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?

 

Ok can anyone explain pressure drop in molecular level to me.

 

Edit: 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.

Edited May 5, 2010 by scilearner
Consecutive posts merged.

[ewmon]

let’s say the same circuit I mentioned with no resistors, then is there no pressure drop along the wire.

p=120------------------------------------------------------------------p=0

No. The difference in pressure always exists. The pressure drops. As I said, wire has a very slight resistance to it. The pressure drop occurs, just like when you have a water system in your house at 100 psi, and you turn the faucet wide open and let the water flow into the air, which has 0 psi. Even with a battery sitting unused on the shelf, the voltage "pressure" will cause electrons to flow at an extremely small rate (current) out one terminal, through the air, and into the other terminal ... because the air presents an extremely high resistance to the current.

 

Resistance decreases current.

Yes. So with very little resistance, the opposite happens — a lot of current flows! Like a faucet wide open. NOTE: Never lay a wrench across the terminals of a car battery!!

 

If there is a circuit with no resistance does the voltage drop or not? Does the voltage and pressure always drop?

For example, there's always a pressure difference/drop between one end of the battery and the other. What current flows then depends on the resistance.

 

Ok can anyone explain pressure drop in molecular level to me.

Sorry, no (and I’m embarrassed ).

 

Ok now if the pressure drops, how does this happen. The molecules bound off each at the start and then move, then why does the pressure drop along? How can relate this to voltage drop?

The number of electrons and the speed that they travel remain constant in your circuit, so maybe it’s easiest to think of voltage as energy or excitation.

 

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

Generally, I think of "flow" as movement in a single direction, and I think of "diffusion" as movement in all directions. So, if electrons are flowing, they are in a sense diffusing, but in a single direction. All sorts of stuff naturally diffuses along a gradient. When I think of diffusion, I think of an expanding 3-D movement, and the further out the substance moves, the more it fills wider and wider volumes (think of adding new sections onto the base of a cone), so the more dilute it becomes and the weaker the diffusion pressure. Electrons do not flow through circuits like this. So, to my mind, the idea of "diffusion" seems to have electrons behaving in ways that they wouldn't behave in a simple wire circuit.

 

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.

The hydraulic analogy to electrical circuitry occurs in a zero gravity world.

 

This article is okay. It's funny that the famous Oliver Heavyside referred to the hydraulic analogy as the "drain-pipe theory".

[scilearner]

Hey thanks ewmon for the detailed response. I appreciate it. Going back to that people in the room and opening the door analogy. When you open the door and some molecules hit,collide (call these A) and send some molecules through the dooor (call these B). Then molecules at A have lost momentum, meaning pressure, is this what happens along a pipe. Your room analogy is the one that is the easiest for me to understand.

 

"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'm asking this as a pressure question. I mean in a pipe if hydrostatic pressure is acted upon by friction, wouldn't it regain the energy because the gravity is always acting and it would give it back.

[swansont]

It's a little more subtle than that. Hydrostatic pressure difference are due to having a column of a certain height in a gravitational field. A horizontal pipe by itself will only have atmospheric pressure, assuming it's open to the air.

[ewmon]

Yeah, it's really important to assume a zero-gravity world. The idea of a pipe filled by gravity from a reservoir doesn't work here, and there's no new energy source that makes water in a pipe speed up. Instead, think of a battery or a wall outlet as a constant voltage/pressure source.

 

For all practical purposes, the source is always able to supply the voltage/pressure required. So, think of an indoor rock concert in a building located over the gates to hell ... kinda like Heavy Metal meets Ghostbusters. Let's say the concert ends and the producers open a door to a corridor leading to an exit. The concert room remains at a constant pressure (that is, pushing and shoving). But, as you move along the corridor, you notice a gradient (that is, people are pushing less and less as you approach the exit).

 

Wow. Oliver Heavyside would roll over in his grave!

[scilearner]

It's a little more subtle than that. Hydrostatic pressure difference are due to having a column of a certain height in a gravitational field. A horizontal pipe by itself will only have atmospheric pressure, assuming it's open to the air.

 

 

I don't understand, how can hydrostatic pressure drop along this vessel, if gravity is always acting, even if molecules lose energy they will get it back from gravity.

[swansont]

Are you sure the source of the hydrostatic pressure in this case is gravity? Capillaries aren't always vertically oriented, and they are really small.

[scilearner]

Hi swansoft

 

Wiki says Hydrostatic pressure is the pressure exerted by a fluid at equilibrium due to the force of gravity

 

Yes it could be the pressure generated by the heart here but may be I'm getting confused with the definition. Is the hydrostatic pressure only due to gravity, that is what wiki says.

[swansont]

Hi swansoft

 

Wiki says Hydrostatic pressure is the pressure exerted by a fluid at equilibrium due to the force of gravity

 

Yes it could be the pressure generated by the heart here but may be I'm getting confused with the definition. Is the hydrostatic pressure only due to gravity, that is what wiki says.

Which Wikipedia article states this? http://en.wikipedia.org/wiki/Capillary does not mention gravity.

 

Well, you have the possibility of pressure exerted by a column for a capillary in e.g. the legs, but I would think that a large source of pressure is due to the heart compressing and pumping the blood.

[scilearner]

Which Wikipedia article states this? http://en.wikipedia.org/wiki/Capillary does not mention gravity.

 

Well, you have the possibility of pressure exerted by a column for a capillary in e.g. the legs, but I would think that a large source of pressure is due to the heart compressing and pumping the blood.

 

Thanks swansoft I have seriously confused the wiki articles. I typed in google hydrostatic pressure

 

http://en.wikipedia.org/wiki/Fluid_statics

 

and this article came and then I quickly skimmed to the hydrostatic pressure section. Now I read the first line in the article

 

Fluid statics (also called hydrostatics) is the science of fluids at rest

 

Sorry I really missed it

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Merged post follows:

Consecutive posts merged

Hey but this topic is not over yet!! If the answer to this question is yes I will understand what voltage is.

 

In a circuit with no resistance and energy loss. The positive end is A and negative end is B

 

If find the number of electrons travelling in the whole circuit at one point in time. One electron would take a long time to move from A to B so what I mean is all the electrons in the circuit from A to B in an established current.Then add all their kinetic energies up that should equal to voltage.

 

Is that right?

[ewmon]

Hold on. All circuits have a resistance, otherwise, it's a hydraulic analogy where the water moving along the wall of the pipe does not create any friction, and we know that can't happen. More realistically, as the electrons move through a circuit, they expend energy, but not in the kinetic sense because that would mean they slow down, and they don't do that.

[scilearner]

Hey I have a new analogy. There is a hill, the bicycle is going up (charge) and chain is moving (electrons).

 

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 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?

[ewmon]

But the charge is the electrons.