5 ms·

The point is taking advantage of matter phase transitions: a lot of energy goes into converting a solid into a liquid, without changing the temperature. Ergo, y

The point is taking advantage of matter phase transitions: a lot of energy goes into converting a solid into a liquid, without changing the temperature. Ergo, you can dump a lot of heat into something without then having to move the something away fast to keep the heat source cool. Only works insofar as it only takes so much energy to melt the solid (wax, ice, whatever), after which you need to either resume a classic moving-coolant process or otherwise stop generating so much heat and let the now-melted substance cool via ambient surroundings and re-solidify. An interesting variant on this is Coffee Joulies http://www.joulies.com http://www.joulies.com which are metal balls you drop in your too-hot coffee to rapidly cool it to a desirable temperature, then keep it at that temperature much longer than it normally would stay. The initial heat is absorbed to melt the ball's contents, then is re-released back during the reversed phase transition. Kinda neat. A limited solution, but easily quantified and quite applicable to a narrow but popular range of use. I first thought the title's point was Apple would be stuffing the phone's interior with wax to either make it waterproof or further frustrate reverse-engeineering attempts.

samatman 13y ago
Joulies do not in fact work: http://en.wikipedia.org/wiki/Coffee_Joulies http://en.wikipedia.org/wiki/Coffee_Joulies Basically, if your vessel isn't insulated, you lose the same amount of heat to the walls and air. The part where they cool your beverage to a certain temperature and then stop cooling it works fine. The part where they retain heat is thermodynamically senseless. Fortunately, when you replace "beverage" with "chip", you have something that cools the chip for awhile and then stops. Hence the article.
- computer 13y ago
  Why is it senseless? You lose the same amount of heat in joules/sec, but the total heat stored in the system is higher, since there's a buffer in it with a higher volumetric heat capacity than coffee. So unless I'm missing something, it would indeed cool slower in degrees/sec, but the effect is likely very small. (Note however that the extra volume of the ball would slightly increase surface of the coffee, losing slightly more joules/sec)
  - pa5tabear 13y ago
    I agree with you.
- pa5tabear 13y ago
  Most coffee vessels ARE insulated, or at least have low thermal conductivity. u/computer explained the buffer effect nicely. You're right that the keeping it hot part might be insignificant.
- Retric 13y ago
  In a 30c room a cup of 80C coffee loses heat energy a lot faster than 60C coffee so there is a faire amount of free energy when the coffie is to hot to drink. Phase changes store and release identical amounts of energy. So, while the increased drinking time may not be significant it is there assuming that increasing the liquids volume does not overly increase the heat loss. Edit: look at the graph on page 2 of this PDF. http://web.eecs.umich.edu/~martinjs/math115/coffee_expl.pdf http://web.eecs.umich.edu/~martinjs/math115/coffee_expl.pdf
  - samatman 13y ago
    The former point is correct as far as it goes. The latter is not. A cup of coffee loses heat around the perimeter, not the middle, and the rate of heat loss is entirely determined by that perimeter. The only thing the Joulies can keep warmer than that equilibrium is themselves. If they start warmer than the coffee, they may contribute to slowing the rate of heat loss. If they do not, they cannot. I have come to think of this as the Monty Hall problem of thermodynamics. People with degrees similar to my own disagree with me all the time.
    - samatman 13y ago
      To forestall the most common objection: Yes, the joulies store more heat than water/coffee, by virtue of being denser. The phase change is irrelevant except as a trick; think of it as extra density you don't have to carry. You also don't get to drink it; it's more heat, but less coffee. Water/coffee stores more heat per gramme than either the phase change or the joulies, so unless you're short of volume, just insulate your coffee.
    - Retric 13y ago
      If they start warmer than the coffee, they may contribute to slowing the rate of heat loss. If they do not, they cannot. Except people don't drink 80C coffie so the absolute time it takes to reach 30C is less important than how long it spends in the drinkable range. Now I don't know what wax there using or how much, but Paraffin wax for example has about 1/2 the heat of fusion of water. So it would store enough energy to keep ~35 times as much water at a constant temperature rather than failing one degree c at which point the was would freeze and provide negligible benifit. On the other hand if the wax water ratio was 1:5 then the temperature would stabilize though 7 degrees worth of cooling when the coffie was 80C and then 7C of warming when the coffie was 37C which would be noticeable. Granted, 37C is a little to cold for coffie, other waxes have a higher melting point. PS: I still don't think this would be all that useful in practice. I am just pointing out it's not physically impossible.

samatman 13y ago

Joulies do not in fact work: http://en.wikipedia.org/wiki/Coffee_Joulies http://en.wikipedia.org/wiki/Coffee_Joulies Basically, if your vessel isn't insulated, you lose the same amount of heat to the walls and air. The part where they cool your beverage to a certain temperature and then stop cooling it works fine. The part where they retain heat is thermodynamically senseless. Fortunately, when you replace "beverage" with "chip", you have something that cools the chip for awhile and then stops. Hence the article.

computer 13y ago
Why is it senseless? You lose the same amount of heat in joules/sec, but the total heat stored in the system is higher, since there's a buffer in it with a higher volumetric heat capacity than coffee. So unless I'm missing something, it would indeed cool slower in degrees/sec, but the effect is likely very small. (Note however that the extra volume of the ball would slightly increase surface of the coffee, losing slightly more joules/sec)
- pa5tabear 13y ago
  I agree with you.
pa5tabear 13y ago
Most coffee vessels ARE insulated, or at least have low thermal conductivity. u/computer explained the buffer effect nicely. You're right that the keeping it hot part might be insignificant.
Retric 13y ago
In a 30c room a cup of 80C coffee loses heat energy a lot faster than 60C coffee so there is a faire amount of free energy when the coffie is to hot to drink. Phase changes store and release identical amounts of energy. So, while the increased drinking time may not be significant it is there assuming that increasing the liquids volume does not overly increase the heat loss. Edit: look at the graph on page 2 of this PDF. http://web.eecs.umich.edu/~martinjs/math115/coffee_expl.pdf http://web.eecs.umich.edu/~martinjs/math115/coffee_expl.pdf
- samatman 13y ago
  The former point is correct as far as it goes. The latter is not. A cup of coffee loses heat around the perimeter, not the middle, and the rate of heat loss is entirely determined by that perimeter. The only thing the Joulies can keep warmer than that equilibrium is themselves. If they start warmer than the coffee, they may contribute to slowing the rate of heat loss. If they do not, they cannot. I have come to think of this as the Monty Hall problem of thermodynamics. People with degrees similar to my own disagree with me all the time.
  - samatman 13y ago
    To forestall the most common objection: Yes, the joulies store more heat than water/coffee, by virtue of being denser. The phase change is irrelevant except as a trick; think of it as extra density you don't have to carry. You also don't get to drink it; it's more heat, but less coffee. Water/coffee stores more heat per gramme than either the phase change or the joulies, so unless you're short of volume, just insulate your coffee.
  - Retric 13y ago
    If they start warmer than the coffee, they may contribute to slowing the rate of heat loss. If they do not, they cannot. Except people don't drink 80C coffie so the absolute time it takes to reach 30C is less important than how long it spends in the drinkable range. Now I don't know what wax there using or how much, but Paraffin wax for example has about 1/2 the heat of fusion of water. So it would store enough energy to keep ~35 times as much water at a constant temperature rather than failing one degree c at which point the was would freeze and provide negligible benifit. On the other hand if the wax water ratio was 1:5 then the temperature would stabilize though 7 degrees worth of cooling when the coffie was 80C and then 7C of warming when the coffie was 37C which would be noticeable. Granted, 37C is a little to cold for coffie, other waxes have a higher melting point. PS: I still don't think this would be all that useful in practice. I am just pointing out it's not physically impossible.