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B Why is it so much easier to increase the temperature of something vs. decreasing it?

  1. May 10, 2017 #1
    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?
     
  2. jcsd
  3. May 10, 2017 #2

    phinds

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    What are your thoughts on this?
     
  4. May 10, 2017 #3
    Does heat flow spontaneously from hot to cold, or from cold to hot?
     
  5. May 10, 2017 #4
    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..
     
  6. May 10, 2017 #5

    CWatters

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    Perhaps start with what you mean by "harder"?

    The specific heat capacity of water is the same if you are heating or cooling it :-)
     
  7. May 10, 2017 #6

    sophiecentaur

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    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?
    A refrigerator needs to produce a cold sink by pumping heat away in a thermodynamic cycle; in the case of the domestic refrigerator, the temperature of that sink can't be much more than -30°C.
    BUT - go out into deep space and it is 'easy' to cool things down!
     
  8. May 10, 2017 #7
    What is your personal experience with heat spontaneously flowing from hot to cold vs cold to hot?
     
  9. May 10, 2017 #8

    ZapperZ

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    There is some faulty logic and definition here in this question.

    I don't know what it means to be "easier". I find a stove to be more complicated than, say, a bucket of ice. I can easily cool something down by simply dipping it in ice. Isn't this easier than using a stove? Your comparison is vague, because one can easily use another "equipment" either to heat something or to cool something. Relying on such an external device makes the question and the premise ambiguous.

    Now, one can ask why it takes less time to heat a fixed amount of a substance to some final temperature, versus the amount of time to get that substance from that final temperature back down. That has a bit more clarity to the question since you are using the time taken as a measure of something being "easier".

    But without such explanation, there is no clear way to answer such a vague question.

    Zz.
     
  10. May 10, 2017 #9
    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?
     
  11. May 10, 2017 #10
    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.
     
  12. May 10, 2017 #11

    Nugatory

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    There are several parameters at work here:
    - The specific heat of water: This is the amount of heat that needs to be added to a given mass of liquid water to increase its temperature by one degree, or equivalently the amount of heat that must flow out of that mass to lower its temperature by one degree.
    - The latent heat of fusion of water. This is the amount of heat that must be removed from a given mass of water at 0 degrees Celsius for it to turn into ice at the same temperature.
    Google will find these values, and along with the initial temperature of the water they are sufficient to answer your question about freezing versus heating to boiling.

    As for how long it takes to heat to boiling or freeze solid? That will depend on the rate at which heat moves into or out of the water and that in turn is dependent on the temperature difference between the water and the stove flame or the cold air in the freezer. This is the point that @sophiecentaur was making in post #6 above.
     
  13. May 10, 2017 #12
    Also there is a big difference that hasn't been mentioned yet. A stove generates heat but a fridge transfers heat.
     
  14. May 10, 2017 #13
    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?"
     
  15. May 10, 2017 #14

    Nugatory

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    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.
     
  16. May 10, 2017 #15

    ZapperZ

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    Again, you are confusing the efficiency of heating and cooling with the notion that doing one thing is easier than the other.

    I can freeze a cup of water FASTER by dropping it into a vat of liquid nitrogen than to boil it by heating it with a candle. Any arguments here?

    So using your logic, cooling water is easier than heating it.

    Zz.
     
  17. May 10, 2017 #16
    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.
     
  18. May 10, 2017 #17

    ZapperZ

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    But indicating that a stove is simpler than a fridge has no relevance in whether heating is simpler than cooling. The OP simply picked two arbitrary examples. This is not an apples-to-apples comparison.

    What is a closer comparison is this: put 1 cup of water in a metal beaker.

    1. Place the metal beaker on a stove and heat it.

    2. Place the metal beaker in a liquid nitrogen bath and cool it.

    Those two are closer apples-to-apples comparison.

    Zz.
     
  19. May 10, 2017 #18
    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?
     
  20. May 10, 2017 #19
    The second. And, is the premise of my question erroneous? Is it or isn't it easier ( I.e., requires less energy) to increase temperature of something rather than lower it?
     
  21. May 10, 2017 #20
    Well, I can quote the three laws but I clearly don't grasp them or I'd be able to answer my own question.

    I'm no scientist, this much is clear or I'd be able to answer my own question. It is the second law I'm trying to better grasp. If entropy always increases and the universe is going from hot to cold, then it seems that going from hot to cold is "natural" ( my word) and cold to hot is both "unnatural " and in opposition to the second law.

    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?

    And, is the premise of my question wrong? (Why is it so much easier to increase the temperature of something vs. decreasing it?)
     
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