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random Physics question

B

Biggerhammer

Golden Member
How would one determine how quickly a heated liquid will regress to its environmental temperature? If I put a mug of coffee in the freezer (thus avoiding paying Drunkin's four bucks for an iced coffee 🙂 ) for an hour, there's going to be a predictable temperature loss from the 90C or so of fresh coffee to the -5C of my freezer. Is there some way (short of timing and measuring temps every five minutes) to determine the rate of temperature drop?
 
Yes, but it's pretty hard to get exact numbers. You can use Newton's law of cooling to get a good estimate though. Google it 🙂
 
i could let you know if you sent me your thermo book.. but obviously you don't have one.. umm.. yah... i dont' know..
 
You could just take the layman's approach and stick the damn coffee in the freezer and check on it every fifteen-twenty minutes or so 😛
 
I did a physics experiment in college where we had to determine the most efficient time to drop an ice cube in the coffee to get the maximum cooling effect. (Coffee cools much much faster at first, from the air, then gradually cools slower and slower) It was like 8 min. after you pour it, you put the ice cube in.
 
thermodynamics is your friend :thumbsup:

i dont remember the morbid details, but there is a formula that you can use to determine this. too lazy to look up the formula though, so tough luck!
 
thats a real easy heat transport problem...

Just model it as a typical pipe with insulation problem.

you have the temperature of the fluid, you have the insulation material (cup), get the K value for the cup, and approximate the coffee as water.

http://www.engineersedge.com/heat_transfer/overall_heat_transfer_coef.htm


Find the rate of heat transfer since you know all temps, and should be able to find all k values. Time = amount of heat divided by Rate of heat transfer.

you will also have to take into account the open top of your mug, which will be part of your equation.
 
Well, in real-world testing I discovered that 750ml of coffee can go from fresh-brewed temperature (assumed 80C) to nicely chilled (approx 5c) in just under two hours in a freezer. Success!

:cookie: for me.

Thank you all for the thermo advice.
 
Originally posted by: Biggerhammer
Well, in real-world testing I discovered that 750ml of coffee can go from fresh-brewed temperature (assumed 80C) to nicely chilled (approx 5c) in just under two hours in a freezer. Success!

:cookie: for me.

Thank you all for the thermo advice.
Click to expand...

I bet you can cut down the time if you agitate the coffee while its being chilled.

A magnetic stirer, a few baffles, and an ice bath :laugh:
 
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