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TreyBien
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Why is it so much easier to increase the temperature of something than it is to decrease the temperature?
Why are refrigerators more complex than stoves?
Why are refrigerators more complex than stoves?
What are your thoughts on this?TreyBien said:Why is it so much easier to increase the temperature of something than it is to decrease the temperature?
Why are refrigerators more complex than stoves?
Hot to cold.Chestermiller said:Does heat flow spontaneously from hot to cold, or from cold to hot?
The rate of heat transfer is proportional to the temperature difference. Even using liquid N2, the temperature difference can't be more than around 200 degrees. The temperature of a flame or red hot electrical element can be many hundreds of degrees in the other direction. And how easy is it to produce liquid Nitrogen? compared with turning on a heater?TreyBien said:Why is it so much easier to increase the temperature of something than it is to decrease the temperature?
Why are refrigerators more complex than stoves?
What is your personal experience with heat spontaneously flowing from hot to cold vs cold to hot?TreyBien said:Hot to cold.
The universe was hot, then cooled. Right? So going from hot to cold would seem more natural than going from cold to hot. Obviously, I'm missing something..
TreyBien said:Why is it so much easier to increase the temperature of something than it is to decrease the temperature?
Why are refrigerators more complex than stoves?
If I make a cup of tea and forget to drink it, when I come back later it has cooled to room temperature.Chestermiller said:What is your personal experience with heat spontaneously flowing from hot to cold vs cold to hot?
CWatters said:Perhaps start with what you mean by "harder"?
The specific heat capacity of water is the same if you are heating or cooling it :-)
There are several parameters at work here:TreyBien said:Does it require more energy to freeze 8oz of water than to heat it to boiling?
Ahh. So you are not someone who is unfamiliar with thermodynamics. What you are asking is "what is it about heat flow from cold to hot that is mechanistically so much different from heat flow from cold to hot?"TreyBien said:If I make a cup of tea and forget to drink it, when I come back later it has cooled to room temperature.
If I have a glass of ice and leave it on the counter, when I come back later it has melted.
It takes longer for the same amount of water to melt (cold to hot) than for the tea to cool (hot to cold).
I can boil water on the stove in minutes. It takes 1-2 hours to freeze the same amount of water in my refrigerator's freezer compartments.
My freezer's components and technology are more complicated (and more expensive) than my stove.
My curiosity relates to trying to understand entropy. Is hot to cold going from low to high entropy and cold to hot the opposite?
Not so - in both cases you have a mass of water at a given temperature in contact with a volume of gas at a different temperature. The source of the temperature difference is irrelevant to its effect on the water.SlowThinker said:Also there is a big difference that hasn't been mentioned yet. A stove generates heat but a fridge transfers heat.
TreyBien said:Does it require more energy to freeze 8oz of water than to heat it to boiling? I know that in the kitchen my more complicated and expensive refrigerator (freezer compartment) takes longer to freeze water than my stove requires to boil it.
This thread isn't really about heat flow. At least the OP isn't.Nugatory said:Not so - in both cases you have a mass of water at a given temperature in contact with a volume of gas at a different temperature. The source of the temperature difference is irrelevant to its effect on the water.
SlowThinker said:This thread isn't really about heat flow. At least the OP isn't.
It is much easier to generate heat than to transfer it. That IMHO explains why a stove is simpler than a fridge.
Of course there are other issues at play, as being discussed by others.
Chestermiller said:It seems to me that there are two issues here:
1. When we put something hot in contact with something cold, why is the final temperature part way between the hot and cold?
2. When we put something hot in contact with something cold, why do we say that heat flowed from the hot thing to the cold thing, rather than cold flowing from the cold thing to the hot thing?
Trey: which, of these is closer to what you are asking?
Well, I can quote the three laws but I clearly don't grasp them or I'd be able to answer my own question.Chestermiller said:Ahh. So you are not someone who is unfamiliar with thermodynamics. What you are asking is "what is it about heat flow from cold to hot that is mechanistically so much different from heat flow from cold to hot?"
Chestermiller said:Ahh. So you are not someone who is unfamiliar with thermodynamics. What you are asking is "what is it about heat flow from cold to hot that is mechanistically so much different from heat flow from cold to hot?"
No. In terms of the energy transferred into the "thing", raising and lowering the temperature the same amount requires exactly the same amount of energy.TreyBien said:Is it or isn't it easier ( I.e., requires less energy) to increase temperature of something rather than lower it?
In practice you heat up things by converting some other energy form (which is easier to transport and has high energy density) into heat, were you need it. The reverse process requires doing work, because heat doesn't spontaneously concentrate and convert itself into useful stored energy.TreyBien said:So, yes, what is it chemically and physically about heat flow form hot to cold that is so much different than from cold to hot?
Wondering if you wouldn't entertain one more question on this concerning your experiment? In part 1, in the energy state of the water molecules at 20 C they are moving at some speed V20 and if you increase their temperature their kinetic energy goes up and their velocity changes to V30. In part 2, we start at V20 and end up with slower molecules moving V10. My question is and just to illustrate, suppose V30=30 and V20=20 then the change in kinetic energy during the part 1 is 900-400 = 500. In part 2, given the same ΔT, the kinetic energies changes from V20 to V10. Suppose V10 = 10 then the change in KE is 400-100 = 300. so, given the same ΔT we arrive at 2 different KE's. On one hand, ΔT gives a change of 500, on the other hand, the same ΔT gives a change of 300. Is the difference in these changes from V20 somehow related to the magnitude of the inter-molecular forces at these temperatures? If not, would you please clarify? Would these difference in velocities have some impact on the rate heating or cooling? (Also note, I have no idea what actual velocities are. I just picked some numbers to illustrate the question). Appreciate your time in advance. (EDIT: I guess I asked more than 1 question)ZapperZ said:With that, I'm done!
You are trying to explain the results of an experiment that you have not run using hypotheses that you have pulled from thin air and mechanisms that you do not understand.Vector1962 said:Wondering if you wouldn't entertain one more question on this concerning your experiment?
[snip a bunch of hypotheticals about molecular kinetic energy and intermolecular forces]
You are certainly correct, hence my questions. Thank you for your reply.jbriggs444 said:mechanisms that you do not understand.
What question?Vector1962 said:You are certainly correct, hence my questions. Thank you for your reply.
Thank you for taking to time to respond to my inquiries.Chestermiller said:It seems to me that there are two issues here:
1. When we put something hot in contact with something cold, why is the final temperature part way between the hot and cold?
2. When we put something hot in contact with something cold, why do we say that heat flowed from the hot thing to the cold thing, rather than cold flowing from the cold thing to the hot thing?
Trey: which, of these is closer to what you are asking?
As far as my knowledge on thermodynamics in nature;TreyBien said:Why is it so much easier to increase the temperature of something than it is to decrease the temperature?
Why are refrigerators more complex than stoves?
TMT said:Therefore, "it is so much easier to increase the temperature of something than it is to decrease the temperature"
ZapperZ said:The OP seems to have ignored many of the points that I've made in this thread, so this will be my last intrusion into this thread.
The problem here in the very beginning is (i) the faulty starting point, i.e. the OP made arbitrary comparison between the process of heating and cooling, and (ii) the conclusion derived from that faulty starting point.
This can be a lesson to many people, especially non-scientists, in how we devise a valid experiment to test a hypothesis. This is especially true when we want to make a comparison between two different processes, in this case, heating and cooling. Is it a fair comparison to heat something on a burner, and then compare that to cooling it in a refrigerator? I've posted on why it is not.
IF the whole issue here is to see how "easy" or "difficult" it is to heat or cool something, then the process of cooling and heating must be as close to be identical as possible. Otherwise, other factors will come in that has nothing to do with the physics of heating and cooling that substance. In other words, we want the heating and cooling of the object to be INDEPENDENT of the process that we use, so that we can get to the actual physics and detect if there are differences in the two processes that are NOT due to how we heat and cool it.
So, similar to what I've suggested before, here's what we can do:
1. Get a 1 kg mass of water in a metal vessel.
2. Let it come to thermal equilibrium at room temperature, say 20 C.
3. Immerse it in a heat bath that is at 30 C.
4. Record the time it takes for the water to get to 30 C.
5. Repeat the experiment, but this time, immerse it in a cold bath that is at 10 C.
6. Record time it takes for the water to come to 10 C.
7. Compare the time. If they are the same, then the process of heating and cooling is identical. If they are not, then you have something.
I pick the same temperature difference to heat and cool to make sure that the rate of heat loss or heat absorption due to surrounding temperature will be similar, i.e. one must make sure that external factors do not significantly affect the result.
If there is a noticeable difference in the time taken for cooling and heating, then this thread is valid. If not, then this thread is moot, because we will be trying to discuss why the unicorn has pink horn.
With that, I'm done!
Zz.
SWB123 said:Something which is NOT being considered here is that different materials have a fixed thermal coefficient of conductivity, and the rate, at which a materail 'gives up', or 'takes on' thermal energy is GREATLY affected, by the surface area of contact between the two materials in question.