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I did an experiment when I was a teenager. I want to prove/see what really went on in that experiment.

When taking a bath, take a warm water in bucket and start taking a bath. You will notice that the water will get cold in lets say 10 minutes. Because the water is bucket is stagnent.

In second instant take the same water and stir the water in bucket in whirlpool motion. You will notice that this water keeps warmer for longer? Now the heat from my hands could get the water a bit warmer but I think it might have to do with the motion of the molecules of water. If you stir them, the water will keep warmer longer?

Is there any truth in it?

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  • $\begingroup$ By stirring the water, you're adding kinetic energy to it. Due to friction, this energy is then converted to heat. $\endgroup$
    – Harry Johnston
    Commented Nov 30, 2011 at 2:56
  • $\begingroup$ That is what I was thinking but I am not entirely sure if this is how it works. $\endgroup$
    – TheTechGuy
    Commented Nov 30, 2011 at 3:17
  • $\begingroup$ This is a related question that could interest you: physics.stackexchange.com/questions/5265/… $\endgroup$
    – Arnoques
    Commented Nov 30, 2011 at 5:28
  • $\begingroup$ ""When taking a bath, take a warm water in bucket and start taking a bath. "" Do You take the bath in that bucket or what is the relevance of the bathing? $\endgroup$
    – Georg
    Commented Nov 30, 2011 at 9:20
  • $\begingroup$ @Georg: your esteemed suggestion regarding "how to bath" was not needed here! $\endgroup$
    – Vineet Menon
    Commented Nov 30, 2011 at 9:25

3 Answers 3

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In answer to the title question, "yes, but...." (it's not practical and the effect is too small to be noticed in the sort of situation you describe)

1 - whatever energy you use to stir the water, ends up as heat pretty soon (which raises the temperature)

2 - it is a good idea to develop an intuitive feel for the magnitude of mechanical versus heat energy. There is a lot of mechanical work in a "small" (in the everyday sense) amount of heat. For example let's say your bucket has 10 litres (10kg) of water, and its temperature falls by 20 degrees-C in 10 minutes (for the purpose of illustration). That's an average temp drop of 2degrees-C per minute, which means that via the various heat loss mechanisms (conduction, convective transfer to air, vapourization and transport, etc), heat is leaving the water in the bucket at rate of 83680 Joules per 60 seconds (2 degrees C times a heat capacity of 4184 Joules/deg-C/kg times 10kg of water), which is 1395 Joules per second which is 1395 Watts. So for you to keep the temperature of the water constant you would have to add mechanical work to the water at this rate, which is an absolutely huge amount for a person to deliver. This is about 1.8 horsepower; perhaps a topnotch cyclist, delivering sprint-level output on an exercise bike, might be able to produce this power level for a short period of time. The important point is that this is several orders of magnitude more than what vigourous stirring by hand might deliver.

For what it's worth heat from pumping is real, for example my neighbour has a small hot tub and its heating comes solely from the circulation pump motor. I haven't been able to find a photo on the internet of that sort of setup but here is mention ("pump friction") of the method:

http://www.precisionspa.com/Precision-Spa/Spa-Heater-Repair/spa-repair-hot-tub-heating.html

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It does, if your water bucket does not allow any water or vapor to escape. Work is being done on the system and since the system is closed, the work done will manifest as heat.

Actually, this king of heating is very common when you want to heat a large volume of heating and structurally placing a dedicated heater is not possible. You start to pump the liquid and move the liquid in a closed circuit. This would heat up the liquid; the rate depending on the rating of the pump.

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    $\begingroup$ Should this not be a non heat conducting bucket? In a cold room stirring the hot water in a metal bucket should cool it faster, in my opinion. $\endgroup$
    – anna v
    Commented Nov 30, 2011 at 5:00
  • $\begingroup$ that depends on the rate of stirring. As I said, the closed loop pumping for heating water, the pumps are rated 8MW! So, whatever the vessel(pipes) is made of, it gets heated pretty fast! The thing which matter is the equilibrium for heat transfer. If stirring is slower than the heat which is escaping thru metal bucket, you might get a cooler water. $\endgroup$
    – Vineet Menon
    Commented Nov 30, 2011 at 5:11
  • $\begingroup$ Vineet, read about the temperature increase in a practical eyperiment for demonstration of mech. heat eqiuvalence! And "common kind of heating" is such a nonsense, incredible! $\endgroup$
    – Georg
    Commented Nov 30, 2011 at 9:24
  • $\begingroup$ I have witnessed it myself, and don't annotate a thing as nonsense if you haven't come across it.... $\endgroup$
    – Vineet Menon
    Commented Nov 30, 2011 at 9:28
  • $\begingroup$ ""I have witnessed it myself, "" Where? $\endgroup$
    – Georg
    Commented Nov 30, 2011 at 11:13
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You are essentially correct and it was this phenomenon that lead to Joule claiming an equivalence between heat and mechanical work. Wiki has details: Link.

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