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?
Originally posted by moonbusCan 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.
... 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?
Originally posted by moonbusIts 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.
... 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?
Originally posted by googlefudgeHe 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 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.
Originally posted by twhiteheadWhy 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.
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.
Originally posted by twhiteheadThe can is mostly full of gas rather than a liquid.
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.
Originally posted by googlefudgeActually 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.
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.
Originally posted by googlefudgeThat 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.
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.
Originally posted by twhiteheadIR 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.
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.