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

[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.