Silly plumbing (or maybe physics) question

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My bathtub is located some distance from the water heater. I turn on the hot faucet only and wait. The tub starts to fill, with cold water initially. After a while, the water becomes hot. That's all fine and as expected.
However, I can tell when the water is hot by *listening* to the flow. The sound of the water changes quite significantly and I've been trying to figure out why.
Clearly, the hot water will be slightly less dense than the cold but I have a hard time imagining this would cause an audible change in the sound. Also the metal faucet will expand and that might change the sound of the water whistling through the valve. Again, it's hard imagine the thermal expansion of fractions of a millimeter causing such an audible effect -- think about the size of changes between two notes on a piano, guitar, flute or whatever.
This happens in my current home but I have a vague feeling I've seen (heard) the effect at other homes too.
Any other theories/inputs? Anyone else even observed this effect?
--
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| Malcolm Hoar "The more I practice, the luckier I get". |
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Malcolm Hoar wrote:

Would you have well water? After filling for a while, would you be hearing more water pressure when the well pump kicks in? Do you get the same sound with just the hot water running?...cold water off.
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avid_hiker wrote:

Have you every noticed how the pitch changes when you stir sugar into your coffee?
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No, I only add coffee to my coffee ;-)
But I suspect you're describing a different manifestation of the same effect. If so, the change is likely in the reverse direction because I presume the sugar would make the coffee more dense.
Your data suggests the effect is indeed density related I think.
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No well (or pumps) here. Regular city water supply.
I've generally noticed it with the hot faucet full on and the cold faucet off.
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Malcolm Hoar wrote:

I've always thought it was the sound of the rubber gasket in the faucet itself. As it heats up, it gets more pliable and thus its vibration characteristics would change. It may even pop in or out, reducing water flow....
Pure conjecture on my part, though.
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I have no more authoritative a take, but am leaning toward it being the warmer faucet bits than the actual water modulating the sound as well.
-- Todd H. http://www.toddh.net /
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Follow my logic here:
- Cold is the absence of heat. - Cold water is just water. - Hot water is water with heat added. - Therefore, hot water is thicker than cold and squeezing it through the pipes makes a different sound.
QED
Malcolm Hoar wrote:

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DerbyDad03 wrote:

And ice is the thickest water of all...
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wrote:

Ice should actually be less thick, considering that water expands when freezing.
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Ah, but, (to continue this physical science mental masturbation session), it depends on how you define "thick." :-)
Per mass of water, ice is going to be thicker dimensionally because of its lower density. Which was probably Bub's clever point.
Best Regards, -- Todd H. http://www.toddh.net /
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On 12 Jan 2007 13:33:58 -0600, snipped-for-privacy@toddh.net (Todd H.) wrote:

Sorry, I noticed the ambiguity too late.
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wrote:

This already has most of the heat of hot water (the TOTALLY ARTIFICIAL) 0-point used in both Fahrenheit and Celsius temperature scales hides this fact.

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Oh the pain! The heartache! My "thickness of water" theorem has been disproved!
And I thought I would be joining Mssrs. Smoot and Mather in Stockholm next year.
Mark Lloyd wrote:

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I will follow your logic as soon as you tell me how much heat "weighs". Heat is just the average kinetic energy of the molecules of the substance. More heat=more energy. More energy=larger distance between molecules. Larger distance between molecules=Less Density.
That is the correct logic you are looking for. Doug
DerbyDad03 wrote:

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Re: how much heat "weighs".
I'll get back to you as soon as I finish reading the article that the following abstract describes:
On the Weight of Heat and Thermal Equilibrium in General Relativity
Richard C. Tolman Norman Bridge Laboratory, California Institute of Technology, Pasadena, California Received 30 December 1929
In accordance with the special theory of relativity all forms of energy, including heat, have inertia and hence in accordance with the equivalence principle also have weight. The purpose of the present article is to investigate the thermodynamic implications of the idea that heat has weight. In particular an investigation is made to see if a temperature gradient is a necessary accompaniment of thermal equilibrium in a gravitational field, in order to prevent the flow of heat from regions of higher to those of lower gravitational potential.
A preliminary non-rigorous treatment of this problem is first given by attempting to modify the classical thermodynamics only to the extent of associating with each intrinsic quantity of energy an additional amount of potential gravitational energy. In this way an expression is obtained for increase in equilibrium temperature with decrease in gravitational potential which, however, could in any case only be correct as a first approximation in a weak gravitational field. A discussion of the uncertainties and lack of rigor of this preliminary treatment is then given and the necessity pointed out for a rigorous treatment based on the principles of general relativity.
A rigorous relativistic treatment is then undertaken using the extension of thermodynamics to general relativity previously presented by the author. The system to be treated is taken as a static spherical distribution of perfect fluid which has come to gravitational and thermodynamic equilibrium. The principles of relativistic mechanics are first applied to such a system in order to obtain results needed in the later work. And it is then shown that these mechanical principles themselves are sufficient to determine the temperature distribution as a function of potential in the simple case of black-body radiation. The principles of relativistic thermodynamics are then applied to this same case of pure black-body radiation and the same expression for temperature as a function of potential obtained by the thermodynamic as by the mechanical treatment. This may be regarded as giving some measure of check on the validity of the proposed relativistic thermodynamics.
Following this, a thermodynamic treatment is given for the temperature distribution in the more general case of matter and radiation and a result found which harmonizes with that for radiation alone. A treatment is then given to the distribution of a perfect monatomic gas in a gravitational field both on the assumption that the total number of atoms must remain constant and on the assumption of the ready interconvertibility of matter and radiation. In the latter case the same dependence of concentration on temperature is obtained as was found by Stern and by the author for the case of flat space-time.
Using a system of coordinates such that the line element for the sphere of fluid takes the form
ds2=-eu(dr2+r2dθ2+r2sin2θdφ2)+eνdt2 the general result for the relation between gravitational potential and equilibrium temperature T0 as measured by a local observer in proper coordinates can be given by the equation d lnT0/dr=-1/2dν/dr
This equation reduces in the case of a weak field to that obtained by the preliminary non-rigorous treatment, and gives a very small change of temperature with position in fields of ordinary intensity. The result, however, is one of great theoretical interest, since constant temperature throughout any system which has come to thermal equilibrium has hitherto been regarded as an inescapable thermodynamic conclusion. It is also not out of the question that the effect might sometime be of experimental or observational importance.
©1930 The American Physical Society
Doug wrote:

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DerbyDad03 wrote:

Reducible to each individual moving particle's mass going up a skosh due to its velocity?
What is the actual peak (between collisions) velocity of, say, a molecule in some given solid at some given temp?
Dave
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On 15 Jan 2007 10:56:36 -0800, snipped-for-privacy@yahoo.com wrote:

I bet it has more to do with the washer and valve seat expanding than anything
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Malcolm Hoar wrote:

I've noticed this as well. Regular copper pipe.
Chris
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Chris Friesen wrote: - Malcolm Hoar wrote: - - - However, I can tell when the water is hot by *listening* - - to the flow. The sound of the water changes quite significantly - - and I've been trying to figure out why. - - I've noticed this as well. Regular copper pipe. - - Chris
Chris,
What were you doing at Malcolm's house?
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