Why does stirring a hot liquid cool it down, but stirring a cold liquid warms it up?
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Why does stirring a hot liquid cool it down, but stirring a cold liquid warms it up?
You can check the answer of the people under the question at Quora “mercury boat engine coffee stirrer“
Let me rephrase the question. “Why does stirring a liquid shorten the amount of time it takes to become the same temperature as the room?” It’s not a matter of the temperature weirdly going up sometimes and down other times. It’s always going toward the middle. If it’s just sitting around, the part of the liquid in the center of the cup or whatever is protected from the room temperature to a certain degree. When you stir it, you keep mixing the hot and cold parts together, evening it all out, and that helps it get to room temperature faster.
If you go into a bar and stir up a hot dude, he is likely to cool down ‘ caus he ‘dont know where your com’in from. But if you stir up a cool dude then he likely to get hot, cause you stirring him. You know what I mean?
Actually, according to the Clausius statement of second law of Thermodynamics : Heat can transfer from a hot body to a cold body , but it cannot transfer from a cold body to a hot body unless any external work has been done on the system.
Thus when you stir a hot liquid,due to the phenomenon of convection (It is a heat transfer process that can occur only in fluids), part of heat from hot liquid goes to the medium which you are stirring (Eg. spoon) and another part of heat flows to the surrounding which is at a room temperature. Thus due to these things heat energy is liberated at a higher rate
But when you went on to stir a cold liquid (let’s say it is at 19 °C), and you are giving a mechanical work on the system. As mentioned in the second law of Thermodynamics the heat gets transferred from cold liquid to the surrounding air ( lets say 30 °C ) due to the work given (stir). But the transfer of heat is very small that could make the liquid to warm only to a little amount due to which the temperature of the liquid increases (let’s say it is at 25 °C), and convection (heat transfer process in fluids. In here it is from cold liquid to at. air) also occurs a little amount. So to make this hot even more work has to be done on the system (cold liquid) if the system is closed the work to be done can also be reduced.
This answer is given in terms of Thermodynamic laws and principles. I think so this will clear your doubt.
Matter exists in three states or phases: solid, liquid, and gas.
A combination of the motion of molecules and their attraction for one another determines whether a substance is a solid, liquid, or gas.
Adding heat energy increases the motion of molecules.
Removing heat energy, or cooling, decreases the motion of molecules.
If heat is added to a solid, the molecules can move fast enough to change from a solid to a liquid. (Melting)
If heat is added to a liquid, the molecules can move fast enough to change from a liquid to a gas. (Evaporation)
If heat is removed from a gas, the molecules can slow down enough to change from a gas to a liquid. (Condensation)
If heat is removed from a liquid, the molecules can slow down enough to change from a liquid to a solid. (Freezing)
Heat always moves from an object at a higher temperature to an object at a lower temperature.
In order to cool down liquid,you will either increase the wind speed(in order to help surface molecules to escape constantly and fast.),decrease humidity(so there will be enough space for surface molecules to escape.) or stir it(as it will add an additional speed to the speed of molecules and make them to escape faster.)
When the speed of the molecules increase,they start escaping to air decreasing the temperature of the liquid.
Now…..
Cold liquid’s molecules are moving slower than usual speed. In order to increase their speed to heat the liquid,up you will either use external heating or you will stir it up. Here,strirring up will add an additional speed to the speed of molecules and make the liquid heat up.
All the Best.
Due to temperature difference between water and the atmosphere, the heat will be transfered from hotter medium to cooler medium. In case if water is already at room temperature no heat will be transferred
When stirring we are just allowing it to go to room temperature.
First one, it cools down because you are getting more hot particles in contact with the cooler air. Plus, part of the heat is leaving through the spoon.
Second one, if you stir quick enough, with an electric blender or something, the liquid may get slightly warmer due to friction. And same thing, hear is coming in from through the metal parts in the blender, spoon…
If you leave, say, a cup of drink that is at a different temperature to the surrounding air it will gradually warm or cool to match the room temperature as heat moves from a warmer to a cooler zone.
The surface of the drink will change first as this is what is in contact with the air. Because of this the surface of the drink will be at a different temperature than the bulk of the drink and this will slow the transfer of hear.
Stirring mixes the surface layer into the bulk maintaining the temperature gradient and speeding the heat transfer.
Note. I have ignored heat gain/loss through the container which is more of the same process and convection within the drink which is minimal if not stirred. There is also conduction within the drink and between the drink and container and radiation from or to the container from the room.
Victor Allen’s