Why Are Refrigerators More Complex Than Stoves?

[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.