1. Subscribermoonbus
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    21 Oct '15 18:44
    ... I noticed something which puzzled me. Normally, when energy is added to a closed system, heat is produced. Yet when a deodorant can is shaken (adding kinetic energy), it gets colder. Can someone explain this phenomenon?
  2. Subscribersonhouse
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    21 Oct '15 19:44
    Originally posted by moonbus
    ... I noticed something which puzzled me. Normally, when energy is added to a closed system, heat is produced. Yet when a deodorant can is shaken (adding kinetic energy), it gets colder. Can someone explain this phenomenon?
    Can you do that with another kind of aerosol can, like whipped cream in a can? Have you tried it with another deodorant can? Usually it will get cold when stuff is sprayed out just like the action in a refrigerator but without that, not a clue.
  3. Cape Town
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    21 Oct '15 21:151 edit
    Originally posted by moonbus
    ... I noticed something which puzzled me. Normally, when energy is added to a closed system, heat is produced. Yet when a deodorant can is shaken (adding kinetic energy), it gets colder. Can someone explain this phenomenon?
    Its all about entropy. Basically you transformed it from a more ordered state to a less ordered state. In this case it was what is also known as evaporation. Evaporation causes cooling, even if it happens inside a can.
  4. Subscribermoonbus
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    21 Oct '15 21:49
    Entropy for fun and profit. Very interesting. Thanks.
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    21 Oct '15 22:09
    https://en.wikipedia.org/wiki/Ideal_gas_law

    PV=nRT

    The can has constant volume, as the pressure goes down, so does the temperature.

    The converse is that trying to pump gas INTO a can/small volume generates lots of heat.

    Which is why when you try to pump up a bicycle tyre the pump and the thin tube to the tyre will get very hot.

    If you want to think of it in terms of entropy...

    Having all the gas in a concentrated small volume is a low entropy/highly ordered state.

    Creating such order takes energy.

    Releasing the gas into a large ill defined/infinite volume is high entropy/low order state.

    Relaxing into such disorder releases energy.

    Such a release will produce heat, but very low intensity/temperature heat, as the kinetic energy concentrated
    in the can is spread out over a much greater volume.
  6. Subscribermoonbus
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    22 Oct '15 05:36
    Thanks, gf. Good explanation.
  7. Cape Town
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    22 Oct '15 05:481 edit
    Originally posted by googlefudge
    The can has constant volume, as the pressure goes down, so does the temperature.

    The converse is that trying to pump gas INTO a can/small volume generates lots of heat.
    He was not talking about what happens when you spray some on yourself (causing pressure to go down), but rather what happens when you simply shake it. In my opinion the can contains both liquid and gas, and when you shake it, some of the liquid evaporates into gas and that will in fact increase the pressure.

    The reason why evaporation causes cooling is because you are breaking the bonds that exist in the liquid and that uses energy.

    For more see:
    https://en.wikipedia.org/wiki/Enthalpy_of_vaporization
  8. Standard memberDeepThought
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    22 Oct '15 06:04
    Originally posted by twhitehead
    Its all about entropy. Basically you transformed it from a more ordered state to a less ordered state. In this case it was what is also known as evaporation. Evaporation causes cooling, even if it happens inside a can.
    Why does evaporation happen? I'm wondering about this as the obvious explanation is that the surface area of the liquid vapour boundary is increased if you shake the bottle. The problem is that that should only affect the rate of evaporation, not the saturated vapour pressure. So if the vapour is already saturated you need to increase the saturated vapour pressure and that depends on temperature and what the substance is, not on the surface area of the fluid vapour boundary. I don't see why shaking the can should cause evaporation.
  9. Cape Town
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    22 Oct '15 06:16
    Originally posted by DeepThought
    I don't see why shaking the can should cause evaporation.
    I believe you are supersaturating the gas when you shake it.
  10. Cape Town
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    22 Oct '15 06:261 edit
    After doing some research I have come across another possible explanation: The can is transferring heat away from your hand to other parts of the can more efficiently when the liquid inside is moving. If this is correct, you should feel exactly the same effect if you cut the top off and filled it with water, and stirred it rather than shaking.
    I personally think the main effect is due to evaporation.
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    22 Oct '15 14:58
    Originally posted by twhitehead
    After doing some research I have come across another possible explanation: The can is transferring heat away from your hand to other parts of the can more efficiently when the liquid inside is moving. If this is correct, you should feel exactly the same effect if you cut the top off and filled it with water, and stirred it rather than shaking.
    I personally think the main effect is due to evaporation.
    The can is mostly full of gas rather than a liquid.

    The gas is a thermal insulator, and the liquid is a conductor.

    When you shake the can the liquid should come into contact with more of the surface
    cooling it and making the can feel colder.

    Remembering that our sense of hot and cold is not a measure of temperature, but a measure
    of rate of heat loss/gain relative to our own internal thermostat.

    So I think the second explanation is much more plausible.

    Because while evaporation will cool the liquid, it does so by heating the gas, while both
    are in the can, the net effect should be neutral.
  12. Cape Town
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    22 Oct '15 15:32
    Originally posted by googlefudge
    Remembering that our sense of hot and cold is not a measure of temperature, but a measure
    of rate of heat loss/gain relative to our own internal thermostat.
    Actually heat loss is directly proportional to temperature difference. Our sense of hot or cold actually is a measure of temperature, but it measures the temperature of the substance directly contacting our skin.

    So I think the second explanation is much more plausible.
    It does sound plausible, but doing my own testing, I am not convinced. If I have two cans, shake one without touching it, and then hold one in each hand, the previously shaken one fells cooler. The greater conductivity explanation doesn't explain that well. The effect is subtle so I suggest more people try it and we compare results.

    Because while evaporation will cool the liquid, it does so by heating the gas, while both
    are in the can, the net effect should be neutral.

    But the liquid is contacting the metal and the whole better conductor story starts all over again. But more importantly, you are wrong. There is a net loss of heat energy in the system as it gets converted into potential energy when the bonds are broken. This is no different from a chemical reaction in which bonds are broken.
  13. Cape Town
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    22 Oct '15 15:35
    If anyone happens to have a thermometer that can measure the temperature of spray cans, please try some experiments and let us know the results. Ideally either a stick on thermometer (temperature sensitive strip for example) or an infra-red thermometer.
  14. Standard memberDeepThought
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    22 Oct '15 16:20
    Originally posted by googlefudge
    The can is mostly full of gas rather than a liquid.

    The gas is a thermal insulator, and the liquid is a conductor.

    When you shake the can the liquid should come into contact with more of the surface
    cooling it and making the can feel colder.

    Remembering that our sense of hot and cold is not a measure of temperature, but a measure
    of rate of ...[text shortened]... d, it does so by heating the gas, while both
    are in the can, the net effect should be neutral.
    That seems to fit. I have a tin of lighter fluid. I put my hand on the bottom, where the lighter fluid is liquid, the bottom of the can was not noticeably cooler than the rest of the tin. I shook it, the cooling effect was significant. If it was just that my hand was warming the liquid and causing an increased rate of evaporation I should get the same effect by gently lying the can on it's side (more of the liquid in contact with the can) and holding it so my hands are under the can and warming the liquid. I tried that and I think I got cooling but am not sure if was from the earlier shaking or not. I'll give it an hour to let the can settle down and try again. I'll also try shaking the tin with it thermally isolated from my hand and see if I get the cooling effect then.
  15. Subscribersonhouse
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    22 Oct '15 16:242 edits
    Originally posted by twhitehead
    If anyone happens to have a thermometer that can measure the temperature of spray cans, please try some experiments and let us know the results. Ideally either a stick on thermometer (temperature sensitive strip for example) or an infra-red thermometer.
    IR might work but remember, they are specific to a particular material. If you measure hot wood with one it will read a different temperature than a similar temperature hot metal. The thermal emission is different for different materials so you would have to measure metal for metal and even the paint or paper on top would change the readings.
    We know this for a fact because of our use of IR thermometers, those hand held units that looks like a toy gun. The good ones have charts of the emissivity of various materials and you have to consult that chart to make an accurate assessment of the actual temperature.

    The best way to measure the can temperature would be with a contact thermocouple, a lot of modern DVM's have a K type thermocouple input which reads temperature from the K type since the thermocouple produces a known (tiny) current depending on temperature. Omega Engineering has them:

    http://www.omega.com/pptst/M12molded.html

    Some of them are bare thermocouple, a small dot of melted metal with 2 leads coming off it, that would be best for your purposes since they react very quickly to variations in temperature changes.

    So you would tape a thermocouple to the can, let the reading stabilize then shake the can say, wearing gloves or something to minimize contact with your hotter hand, and read before and after. Most thermocouple's read pretty quickly so you would be able to make a definitive statement about temperature changes on the can, shaking V still.

    Of course if you have mega bucks you could buy an FLIR but that might be a bit out of the range of this kind of amateur science thing😉
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