# Hydraulic analogy, pressure and voltage.

Discussion in 'Physics' started by studiot, May 21, 2008.

1. ### studiot Thread Starter AAC Fanatic!

Nov 9, 2007
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My barometer measures atmospheric pressure in inches or millimetres.

This is actually a unit of length, not pressure.

But it has been common practice to refer to pressure in this way since before electric circuits were invented and is still in common use today.

This usage of 'static head' to represent pressure gave rise to the original hydraulic analogy for electric circuits. In these terms the analogy is pretty exact. Unfortunately many modern authors have changed this analogy to pressure flow in pipes, which is a poor analogy, especially when using the force/area definition of pressure.

As it is a deal of work to prepare a description of the static head analogy I will proceed if there is enough demand for a sensible discussion.

2. ### beenthere Retired Moderator

Apr 20, 2004
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There is a reason why atmospheric pressure is sometimes measured in units of length. A Italian experimenter (might have been Bernoulli) was trying to create a vacuum. He filled a glass cylinder, closed at one end, with mercury and turned it up with the lower end in a container of more mercury. His reasoning was that the heavy mercury would run out and leave the cylinder filled with a vacuum.

He was surprised to find that the cylinder did not empty out fully - instead a column of mercury remained in place. He correctly reasoned that the pressure of the air balanced the suction of the vacuum, leaving almost 30" of mercury in the cylinder.

So we can measure barometric pressure by the mercury column height. Standard sea level pressure is 29.97" Hg. That is also the definition on one Bar (you have also seen barometric pressure given in millibars).

3. ### AlexR Well-Known Member

Jan 16, 2008
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No you barometer measures atmospheric pressure in "inches/mm of mercury" not plain inches/mm and that is a measure of pressure

4. ### mrmount Active Member

Dec 5, 2007
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AlexR has a point. Only mm/inches of Hg or H2O (or other liquid) indicates pressure and simply inches/mm does not indicate anything wrt pressure.

5. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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Standard pressure is usually 29.92" of Hg at 70°F (101.325 kPa).
John

6. ### Caveman Active Member

Apr 15, 2008
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Isn't all of this explained quite clearly on wikipedia? http://en.wikipedia.org/wiki/Hydraulic_analogy

If we disagree with that article, then perhaps this thread would act as a discussion for what should be improved. Then the result could be fed back to wikipedia for the benefit of all.

7. ### studiot Thread Starter AAC Fanatic!

Nov 9, 2007
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Nice to see someone at least is reading the main 95% of a post rather than quibbling with the other 5%.

The Hydraulic Analogy I was talking about predates Heaviside by at least 75 years. The Wiki article uses pressure as the driving element and relates it to velocity.

I don't

Unfortunately the equations for pipe flow in relation to pressure are nothing like the equations for the flow of current in circuits. The simplest ignores friction, so how can we use this to model resistance?
This is Bernoulli's equation, which contains a square term

Z + p/$\rho$ + v$^{2}$/2g = A Constant

Z = ht above datum, v = velocity, rho = density.

The equation for frictional flow is even worse (Darcy's law).

As a matter of interest to our American cousins

Bernouilli was Swiss, and there were two of them (brothers).

The Italian gent who 'invented' the mercury barometer was Toricelli.

8. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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Didn't mean it as a quibble. Just as minor technical correction. In most cases, it (0.05 " = approximately 50 feet altitude) doesn't make a difference. John