Temperature increase with stirring?

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  • Thread starter Thread starter Gabriele Pinna
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Discussion Overview

The discussion revolves around the effects of stirring on the temperature of coffee, exploring whether stirring increases or decreases the temperature due to energy transfer and friction. Participants reference classic experiments and consider various factors influencing the outcome, including heat loss mechanisms and the initial temperatures of the objects involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that stirring introduces energy into the coffee, potentially increasing its temperature due to the conversion of kinetic energy into thermal energy.
  • One participant notes that the temperature increase from stirring would be minimal and not practical for heating coffee.
  • Another participant highlights that the spoon's lower temperature compared to the coffee could affect the overall temperature change.
  • Concerns are raised about heat loss from the coffee through its container and evaporation, suggesting that stirring may not effectively maintain the coffee's temperature.
  • A humorous suggestion is made that using microwave radiation could be a more effective method for heating coffee than mechanical stirring.
  • It is mentioned that the initial temperatures of the cup and surrounding air may also influence the outcome of the stirring process.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of stirring in increasing coffee temperature, with some arguing it may have a negligible effect while others emphasize the importance of heat loss factors. No consensus is reached on the overall impact of stirring.

Contextual Notes

Participants acknowledge various assumptions, such as the initial temperatures of the spoon, coffee, and surrounding environment, which may affect the discussion's conclusions. The discussion also touches on the limitations of practical heating through stirring versus other methods.

Gabriele Pinna
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Let's say that I have a spoon and a cup of coofee, I stir it with the spoon does its temperature decrease or increase ? (There is friction force so a bit of energy should turn into heat but it's strange).
 
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Gabriele Pinna said:
Let's say that I have a spoon and a cup of coofee, I stir it with the spoon does its temperature decrease or increase ? (There is friction force so a bit of energy should turn into heat but it's strange).
What do you think happens? There is a classic experiment where you put energy into the liquid by stirring and measure the starting and finishing temperatures. Have you seen info about that experiment?
 
As the macroscopic fluid flow of the stirred coffee diffuses because of internal viscosity and so forth, the energy has to go somewhere, and it does. The large scale motion of clumps of fluid diffuses into small scale molecular motion, which we usually associate with thermal energy.

Assuming all other things constant, the temperature would increase, but by a really tiny amount. It would not be worth trying to heat up coffee by mechanical motion.

2 kilojoules of energy is about what you'd need to heat a glass of water by just 1 degree Celsius.
2 kilojoules of energy is also the amount of kinetic energy in a 4 kilogram bowling ball rolling at highway speeds (100kph)

The amount of kinetic energy in a swirling pot of coffee is a lot less than that of a bowling ball
 
I know Joule's experiment but you need to consider that the spoon has a lower temperature than coofee
 
In the real world, I would think you would have to stir really fast and furious to keep your coffee warm, and even then it would not help much. Afterall, the coffee is loosing heat through its container, and evaporation, and stirring would help that along. I don't think realistically you could keep up.
 
Quantum mechanics is the best way to heat up your coffee. Subject the water molecules to microwave radiation, the increase in rotational energy will warm your coffee just fine. Much better than Joule heating, and no spoon to cool things down, just the cup pulling heat out of the system. (Of course I am being tongue in cheek here.)
 
Gabriele Pinna said:
you need to consider that the spoon has a lower temperature than coofee
The answer also will depend on the initial temperatures of the cup and the surrounding air.
 

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