Why Are Refrigerators More Complex Than Stoves?

[TreyBien]

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?

 

[phinds]

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 are your thoughts on this?

 

[Chestermiller]

Does heat flow spontaneously from hot to cold, or from cold to hot?

 

[TreyBien]

Does heat flow spontaneously from hot to cold, or from cold to hot?

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..

 

[CWatters]

Perhaps start with what you mean by "harder"?

The specific heat capacity of water is the same if you are heating or cooling it :-)

 

[sophiecentaur]

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?

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!

 

[Chestermiller]

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..

What is your personal experience with heat spontaneously flowing from hot to cold vs cold to hot?

 

[ZapperZ]

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?

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.

 

[TreyBien]

What is your personal experience with heat spontaneously flowing from hot to cold vs cold to hot?

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?

 

[TreyBien]

Perhaps start with what you mean by "harder"?

The specific heat capacity of water is the same if you are heating or cooling it :-)

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.

 

[Nugatory]

Does it require more energy to freeze 8oz of water than to heat it to boiling?

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.

 

[SlowThinker]

Also there is a big difference that hasn't been mentioned yet. A stove generates heat but a fridge transfers heat.

 

[Chestermiller]

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?

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?"

 

[Nugatory]

Also there is a big difference that hasn't been mentioned yet. A stove generates heat but a fridge transfers heat.

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.

 

[ZapperZ]

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.

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.

 

[SlowThinker]

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.

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.

 

[ZapperZ]

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.

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.

 

[Chestermiller]

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?

 

[TreyBien]

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?

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?

 

[TreyBien]

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?"

Well, I can quote the three laws but I clearly don't grasp them or I'd be able to answer my own question.

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?"

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?)

 

[russ_watters]

Is it or isn't it easier ( I.e., requires less energy) to increase temperature of something rather than lower it?

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.

And as far as the input energy for the device goes, it is entirely dependent on the heating/cooling mechanism. For example, a gas stove might be 70% efficient, but a refrigerator has a coefficient of performance (similar to efficiency) of 300%! So by that measure, it is much, much easier to cool than heat.

 

[A.T.]

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?

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.

https://en.wikipedia.org/wiki/Laws_of_thermodynamics

 

[ZapperZ]

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.

 

[Vector1962]

With that, I'm done!

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)

 

[jbriggs444]

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 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]

mechanisms that you do not understand.

You are certainly correct, hence my questions. Thank you for your reply.

 

[jbriggs444]

You are certainly correct, hence my questions. Thank you for your reply.

What question?

 

[TreyBien]

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?

Thank you for taking to time to respond to my inquiries.

 

[TMT]

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?

As far as my knowledge on thermodynamics in nature;
Heat flows from hotter region to colder region.
Heat flow from colder region to hotter region request work done on system.
Consider reversibility. A heat engine produce work while heat flows from hotter region to colder region (naturally) if you want to reverse this flow direction;
you have to reverse the work. Instead of taken work from system you have to apply work to system.
In heating you just interact a hotter region with cooler region. you haven't do nothing else. But cooling you have to perform work on system
Therefore, "it is so much easier to increase the temperature of something than it is to decrease the temperature"

 

[Vanadium 50]

Therefore, "it is so much easier to increase the temperature of something than it is to decrease the temperature"

This makes no sense.

If I have a hot object and a cool object and I put them in contact, the hot one cools and the cool one warms. How can one say that one is easier than the other?

 

[sysprog]

Cooling something and heating something are the same, in that when you cool something you're heating something else, and when you heat something you're cooling something else. Whether heating a specific object is easier than cooling that object is dependent, among other factors, on the comparative availability of a heat source (something hotter) and a heat sink (something colder). Heat flows spontaneously with extremely high probability from hotter to colder; analogously: it's easier to collapse a card castle than it is to build one.

 

[Mapes]

Regarding the perceived asymmetry between heating things up vs. cooling them down, note that we're surrounded by sources of low-entropy, non-thermal energy that are specifically intended to drive irreversible processes, which always produce heat. Some examples are batteries, electrical wall outlets, and gas for household use. A battery feels like it's at room temperature, but its thermodynamic temperature is far higher; it's an out-of-equilibrium system specifically designed to supply energy.

As others have implied, if we were in Antarctica with no power sources, we might be pondering why it's so easy to cool things down (e.g., by conduction, convection, and radiation) and so hard to heat them up.

 

[SWB123]

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. When you are heating up a pan of water on the stove, you have a metal pan on a metal burner (or, with the grate, and open flame if you are cooking with gas). There is much quicker transfer of heat from metal to metal, and with quite a bit of surface area of the water with the inside of the pan to make the transfer pretty quickly between the metal, and the water. The heated water sets up a convection current of hot water flowing to the top, and the colder water sinking to the bottom of the pan to be heated; this, also aides the speed at which the whole volume of water heats up. When you put that same volume of water into a plastic ice tray placed in the freezer, then the 'test bed' for this experiment in 'thermal energy exchange' has been changed drastically! Even if the area of the plastic in contact with the frost on the metal freezer floor, and between the plastic, and the water, and the area of water exposed to the cold air in the freezer the rate of thermal exchange between all these diverse materals is MUCH less, than those used in the heating process. So, the whole process of cooling with these sorts of drastic changes in materials, and transfer technology used is EXPONENTIALLY less effecient, than that of the heating process!

Whether it is 'easier', or 'harder' to make a given volume of material 'give up', or 'take on' thermal energy CAN NOT be tested, with such drastically different test beds. Given the proper experimental methods, and materials, I think you will find that, 'that rate' might be measurable; but, hardly stastically significant!

The amount, and complexity, of the mechanism used is NOT, at all a factor in accomplishing the tasks in question. Nor was it even a major concideration to the manufacturer of your kitchen appliances. They were mainly concerned with the cheapest way to make a device, that accomplishes the task, which the average person would consider purchasing!

Try pricing the cost of keeping a liquid nitrogen tank filled, and at your disposal 24/7, for freezing and storing your Eggos, and leftovers; 'IF', freezing things at a similar rate, as you heat them is that important to you. (Assuming, you are independently wealthy.)

 

[craigi]

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.

I don't think this experiment would behave as you expect. Recall that the density of water does not vary linearly with temperature and that there is maximum density at 4 degrees centigrade, hence the rate of convective heat transfer would not be the same in the two cases.

 

[ZapperZ]

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.

But this is why I listed, in my final post before this, a way to test this out so that the method of heating and cooling does NOT significantly effect the outcome. So yes, this HAS been considered.

Many of the posts here, especially the one made by the OP, are being swayed by the process of heating and cooling, and somehow confusing that with the actual physics of heating and cooling an object. Heating water in a pan, and comparing it to cooling it in a refrigerator, are NOT comparing apples to apples.

Zz.

 

[ZapperZ]

I don't think this experiment would behave as you expect. Recall that the density of water does not vary linearly with temperature and that there is maximum density at 4 degrees centigrade, hence the rate of covective heat transfer would not be the same in the two cases.

But do you think the density would vary THAT much so as to significantly affect the result when compared to the heat loss due to cooling and heating? I mean, after all, the OP is comparing heating water in a pan versus cooling it in a refrigerator? Which test is a more valid comparison?

If you don't like water, than use a block of aluminum! Would a 10 C change in temperature be that significant in this type of test?

Zz.

 

[craigi]

But do you think the density would vary THAT much so as to significantly affect the result when compared to the heat loss due to cooling and heating? I mean, after all, the OP is comparing heating water in a pan versus cooling it in a refrigerator? Which test is a more valid comparison?

If you don't like water, than use a block of aluminum! Would a 10 C change in temperature be that significant in this type of test?

Zz.

As you try to heat or cool a container of water at 4 degrees centigrade, convection would halt completely. Convection is the primary mode of heat transfer in liquid water, so it seems reasonable that the rate of convective heat transfer would be significantly different in the 10 and 30 degrees cases.

Stick with the aluminium then you don't need to worry about the details of convection. That makes your point better.

 

[ZapperZ]

As you try to heat or cool a container of water at 4 degrees centigrade, convection would halt completely. Convection is the primary mode of heat transfer in liquid water, so it seems reasonable that the rate of convective heat transfer would be significantly different in the 10 and 30 degrees cases.

Stick with the aluminium then you don't need to worry about the details of convection. That makes your point better.

We have done many physics undergraduate labs using water cooled to around 10 C below room temp, and heating it to way higher, to find, say, the specific heat of water, without much loss in accuracy for that level. I truly doubt that the very small change in density of water over that temperature range will show up and affect the measurement in such a way that one could draw a conclusion that heating water is "easier" than cooling it.

Zz.

 

[craigi]

We have done many physics undergraduate labs using water cooled to around 10 C below room temp, and heating it to way higher, to find, say, the specific heat of water, without much loss in accuracy for that level. I truly doubt that the very small change in density of water over that temperature range will show up and affect the measurement in such a way that one could draw a conclusion that heating water is "easier" than cooling it.

Zz.

I expect that above 20 degrees centigrade, you'll find a linear relationship between temperature and density to be a good approximation, hence a the rate of convective heat transfer to be constant, but that relationship will fail as you approach 4 degrees.

The point that I'm not sure you grasped is that it is the difference in density which drives free convection. At temperatures close to 4 degrees the density of water is almost identical, so convection would cease almost entirely.

 

[ZapperZ]

I expect that above 20 degrees centigrade, you'll find a linear relationship between temperature and density to be a good approximation, hence a the rate of convective heat transfer to be constant, but that relationship will fail as you approach 4 degrees.

You seem to continue to miss the point.

If you do this experiment using standard undergraduate lab equipment, do you think that the change in "convective heat transfer" in going from 10 C when compared to 20 C will actually be detected and be MORE influential than other sources? Ahead of cooling loss and other factors?

I had already mentioned that standard thermo experiments such as measuring the specific heats can already produce accurate-enough values that such a change in either density or such heat transfer does NOT significantly alter values measured.

And I don't understand why we are nitpicking on this when the OP produced a glaringly horrible test for comparison. How about I change the final temperature of the test to ±1 C? Is that better now?

Zz.

 

[craigi]

You seem to continue to miss the point.

If you do this experiment using standard undergraduate lab equipment, do you think that the change in "convective heat transfer" in going from 10 C when compared to 20 C will actually be detected and be MORE influential than other sources? Ahead of cooling loss and other factors?

I had already mentioned that standard thermo experiments such as measuring the specific heats can already produce accurate-enough values that such a change in either density or such heat transfer does NOT significantly alter values measured.

And I don't understand why we are nitpicking on this when the OP produced a glaringly horrible test for comparison. How about I change the final temperature of the test to ±1 C? Is that better now?

Zz.

It's not important as I'm sure that the OP isn't actually going to carry out the experiment which you suggested. I was just concerned that you were giving him a bum steer.

 

[ZapperZ]

It's not important as I'm sure that the OP isn't actually going to carry out the experiment which you suggested. I was just concerned that you were giving him a bum steer.

Sorry, a bum steer?

A starving man is about to eat something rotten out of a dumpster, and you are "concerned" that I'm giving him milk that is one day beyond expiration.

Zz.

 

[Shane Kennedy]

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?

Temperature difference. Cooling something, outside the laboratory, involves the use of a freezer, at all of -4 or -6 degrees C. That is a temperature difference of usually 25 - 30 degrees. Heating (cooking) is the application of maybe +200C. Even in the lab, -200C is extreme, and heating could be several thousand degrees.
As for complexity, very few chemical reactions are endothermic,and those only slightly. Most being exothermic, and many are seriously so.