What is the difference between adding heat and adding cold?

[D H]

Hmmm... wonder why there is the demand on small systems and short time scales.

The reason is that the second law of thermodynamics is a probabilistic statement.

A simple example: Imagine a gas chamber that contains a certain number of gas molecules. Finding that all of the molecules are in one half of the chamber should not be all that surprising if the number of molecules is small. It would be incredibly surprising (never happens) if the chamber contains a mole of molecules.

There are lots of other examples where something is essentially impossible statistically purely by virtue of the large number of molecules / large number of interactions. Those essentially impossible events are the basis of the second law of thermodynamics.

 

[dreimd]

In this paper I examine the debate regarding the positive reality of cold: whether it is merely an absence of heat, or a quality or entity in its own right.

Our entire debate..

 

[alphawolf50]

From the link:

Marc-Auguste Pictet stimulated this debate by showing that radiation from a cold object apparently could be focused by concave mirrors to cool another object some distance away from it.

This baffles me -- how can radiation from a cold object cool another object? I'm familiar with laser cooling, but this sounds like a different animal.

I haven't seen the setup, but I would venture to say they've got the causality reversed. IE, the mirrors are isolating the system somewhat, and the warm object is radiating energy, which gets absorbed by the cold object. The cold object naturally radiates less energy, so the warm object absorbs less energy than it is radiating. Eventually the two should reach equilibrium. Just my take.

 

[Drakkith]

So what I am getting from this thread is:

Heating = adding thermal energy.
Cooling = Removing thermal energy.

Seems pretty straightforward to me. Whats the problem? Adding cold is removing heat and vice versa. Obviously you cannot add a million degrees of Cold to something that didnt already have at least a million degrees of heat already.

Do i have all this right?

 

[D H]

No. "Adding cold" sounds even worse than "adding heat". The term "adding heat" is a bit of a throwback to the old and falsified caloric theory. Objects do not contain heat.

The temperature of an object can increase (or decrease) with zero heat transfer. Think of a stellar nursery, a relatively high concentration (high compared to interstellar space) of hydrogen and other other atoms/molecules in space. Temperatures in such nurseries are often quite cool, 10 K or so. As the cloud collapses to form a protostar gravitational collapse makes the protostar increase in temperature to millions of Kelvins. There is no heat transfer here. The heating is solely from conversion of gravitational energy to kinetic energy.Addendum
Since "adding heat" is a bit of a misnomer, it is best not to complicate things by adding the even uglier phrase "adding cool" to the mix. Yech.

 

[Drakkith]

No. "Adding cold" sounds even worse than "adding heat". The term "adding heat" is a bit of a throwback to the old and falsified caloric theory. Objects do not contain heat.

The temperature of an object can increase (or decrease) with zero heat transfer. Think of a stellar nursery, a relatively high concentration (high compared to interstellar space) of hydrogen and other other atoms/molecules in space. Temperatures in such nurseries are often quite cool, 10 K or so. As the cloud collapses to form a protostar gravitational collapse makes the protostar increase in temperature to millions of Kelvins. There is no heat transfer here. The heating is solely from conversion of gravitational energy to kinetic energy.


Addendum
Since "adding heat" is a bit of a misnomer, it is best not to complicate things by adding the even uglier phrase "adding cool" to the mix. Yech.

I'm not 100% sure of the mechanics of a gas, but arent you are either adding heat through gravity, or you are compressing the gas and its heat into a smaller space, raising the temperature but not adding any heat? (Not sure which one)

Edit: Also, adding Heat is adding energy to something, isn't it? I realize that it might be a bit questionable to say heat = thermal energy, but everything I've read has used the 2 terms interchangeably. Seems pretty straightforward to me.

 

[Drakkith]

From the wikipedia article on heat:
Heat is also loosely referred to as thermal energy, although many definitions require this thermal energy to be in transfer between two systems to be technically called heat, otherwise, many sources prefer to continue to refer to the internal quantity as thermal energy.

 

[russ_watters]

No. "Adding cold" sounds even worse than "adding heat". The term "adding heat" is a bit of a throwback to the old and falsified caloric theory. Objects do not contain heat.

The temperature of an object can increase (or decrease) with zero heat transfer.

This looks to me like two separate issues assumed to be connected in this thread but not actually being what the OP was driving at. Yes, the temperature of an object can change without heat transfer or heat transfer can occur without the temperature of an object changing - but I don't see that as being part of the issue here. Temperature isn't heat or energy. The question is regarding the heat transfer itself.

"adding heat" is just another way of saying "adding energy". Heat is a form of energy - at least that's how the word is used.

I see the issue raised by the OP as simply being whether you can have negative heat transfer. Not heat in (positive) or heat out (negative) but negative heat in or negative heat out. Whether it is physically possible to have a negative BTU of energy (it isn't), it is treated that way both colloquially and mathematically by engineers and it works fine.

 

[russ_watters]

You are playing semantic games, dreimd. The purpose of an air conditioner or a refrigerator is to cool things off. What happens to the external environment is a side-effect. Heating is heat transfer with a positive sign, cooling is heat transfer with a negative sign. Objects don't contain heat. They transfer heat as a process. We call it that process heat transfer, positive or negative, by convention.

I get into this argument a lot with physicists because I'm somewhat as a grammar Nazi. Earlier it was stated several times that in physics, heat is not a noun, but grammatically, heat is a noun. In the sentence "They transfer heat as a process", heat is a noun. You could replace the word "heat" with "thermal energy" - or "apples", for that matter - and the sentence is still gramatically correct. Same goes for the next sentence: Replace "heat" with "thermal energy" and the sentence works the same: "We call [] that process thermal energy transfer..."

The way it looks to me, physicists get very picky about "heat" being a verb, but still use it as a noun! If people want to call it "work" when it is mechanical and in motion, "heat" when it is non-mechanical and in motion, and "energy" when it is stationary, that's fine, but it is still just different forms of the same thing. A BTU or kWh can be any of the three.

Note also the term "heat pump", where "heat" replaces "water". "Heat" is a noun, a quantity that can be containerized and moved around.

But again, I don't think arguing over definitions was the point of the OP. I think the point of the OP was to ask if there is such a thing as a negative BTU.

 

[D H]

I'm not 100% sure of the mechanics of a gas, but arent you are either adding heat through gravity, or you are compressing the gas and its heat into a smaller space, raising the temperature but not adding any heat? (Not sure which one)

The latter.

Edit: Also, adding Heat is adding energy to something, isn't it? I realize that it might be a bit questionable to say heat = thermal energy, but everything I've read has used the 2 terms interchangeably. Seems pretty straightforward to me.

To a physicist, heat is the quantity Q in \Delta U = Q - W (or in terms of derivatives, dU/dt = \partial Q/\partial t - \partial W/\partial t). Heat (Q) and internal energy (U) are not the same thing. Internal energy is not even the same as temperature; temperature is but a component of the internal energy of some system.

Changes in temperature can result from

• Heat transfer (non-zero Q or \partial Q/\partial t),
• Work (non-zero W or \partial W/\partial t) or
• Changes in the components of internal energy itself.

Star collapse is an example of temperature change in which neither heat transfer nor work is involved. Here's another example: Suppose we have a very sturdy, thermally insulated gas chamber outfitted with a spark plug. Obviously closing the circuit on the spark plug will transfer heat to the gas in the chamber, but we can make this heat transfer quite negligible by making the time interval over which the circuit is closed very small.

Now fill the chamber with O₂ and close the circuit briefly. Not much happens. The O₂ gas will increase in temperature by a tiny amount due to the spark. But at least we have quantified how much heat transfer is involved with triggering the spark. Now empty the chamber and fill it with H₂. Once again, not much happens when the circuit is closed. Now add some O₂ to the H₂ already in the chamber such that there is one molecule of O₂ for every two H₂ molecules in the chamber. Now close the circuit.

Kaboom! This time there is a large change in temperature. We've just quantified how much energy the spark adds (not much). The chamber is thermally isolated, so except for that tiny amount of transfer from the spark the chamber is essentially adiabatic. The chamber is very sturdy and rigid, so no work is involved. The temperature has changed solely because of the conversion of chemical potential energy to thermal energy.

 

[alphawolf50]

The opposite, "heating" with no temperature change is observed quite frequently, though most don't notice. Melting ice can absorb a lot of "heat" -- but just as the ice was 0 degrees C before the state change, the water is 0 degrees after. As long as the heating is slow and even, the ice/water mixture will remain at 0 degrees C until all the ice has melted.