Hot and cold

[Resha Caner]

Maybe this should have gone in the "ask a physicist anything" thread, but I was afraid it would get lost there.

Does a hot object have more mass than a cold object? I think the answer is yes, but thought I would check.

Suppose we have two equal quantities of steel at 0 C. By whatever means (for example, we weigh them and use a gravitational law to equate that to mass) we find that both have a mass of 1 kg. We then heat one of the steel masses to 1000 C. Does it now have more mass (trivial as the difference may be)?

And, as it radiates heat and cools back to 0 C, does it lose that mass until it is again equal to the mass we did not heat?

 

[Delphiki]

Heat itself is not a substance. It's merely the 'excitement' of matter. Think of heat as the activity of atoms bouncing around like mexican jumping beans. So when something is losing heat, think of it as the atoms are 'calming down'. When the "beans" are sitting completely still, that is 0 Kelvin degrees.... absolute zero.

The only mass that might get lost is actually as a result of heat... like evaporation and vaporization.

 

[Resha Caner]

Yes, I understand what heat is. I've had thermodynamics classes. Heat is energy.

And E = mc^2. As relativity indicates, when the velocity of an object increases (by gaining energy), it's mass increases.

So, when an object is heated, the activity of the molecules within the object increases. Therefore, shouldn't their mass increase?

 

[Gishin]

Wouldn't density just decrease?

(I'm not a physics guy at all )

 

[leftrightleftrightleft]

I'm not entirely sure if that is actually correct. It kind of makes sense and I don't know if it's been empirically tested. Even so, if you added 500 J of energy to an object to heat it up, you would raise the mass by 5.55x10^-15 kg. So its not very significant because remember, MASS contains lots ENERGY (ex nuclear power) but ENERGY contains very little MASS.

One reason I'm not sure this would work is because I'm not sure that you can say:

∆E = ∆m(c^2)

This would imply that if you add energy to a vacuum (set E1 = 0, E2 = 500 and m1= 0) then you could produce mass. And this seems somewhat impossible.

 

[keith99]

Yes, I understand what heat is. I've had thermodynamics classes. Heat is energy.

And E = mc^2. As relativity indicates, when the velocity of an object increases (by gaining energy), it's mass increases.

So, when an object is heated, the activity of the molecules within the object increases. Therefore, shouldn't their mass increase?

Might I rephrase?

When heated the individual atoms are excited, this increases their velocity (Easiest for me to picture with gas). This means the average speed of indivdual atomms has increased. There should be the corresponding increas of mass due to relativistic effects.

Final point, this should be very very small, perhaps to small to detect.

It has been far to long since Physics for me to even begin to crank the numbers on this.

 

[Michael]

Maybe this should have gone in the "ask a physicist anything" thread, but I was afraid it would get lost there.

Does a hot object have more mass than a cold object? I think the answer is yes, but thought I would check.

It would be "easier" (and probably more correct) to simply suggest that the hot object has more overall "energy" than than a cold object, not necessarily more mass.

In other words, heating a cast iron pot on a stove doesn't increase or decrease the weight of the pot, it increases the net energy. There are the same number of molecules in the pot (therefore the same mass), those molecules just "vibrate" a little faster. The mass is the same, the vibration kinetic energy is greater in the hot object however. That energy is likely to radiate away as photons (heat), and the pot's weight/mass didn't change because the total number molecules in the pot never changed. You could try to equate energy with mass, but it's really more in the form of "internal kinetic energy" that simply radiates away over time as "heat".

 

[Nostromo]

The short answer is yes it does.

I'm sure if you google mass/energy equivalence you'd come up with a better explanation than my waffling.

 

[Michael]

Yes, I understand what heat is. I've had thermodynamics classes. Heat is energy.

And E = mc^2. As relativity indicates, when the velocity of an object increases (by gaining energy), it's mass increases.

So, when an object is heated, the activity of the molecules within the object increases. Therefore, shouldn't their mass increase?

You're on the right track in terms of the total number of molecules IMO. The number of molecules (and therefore the "rest" mass) stays the same. The "energy" is in the form of "kinetic" energy which radiates away again as "heat" (kinetic energy).

A photon has "kinetic energy", but it has no resting mass. You might think of the heat source as "photons" (with no mass but kinetic energy) that creates "kinetic energy vibration" between the molecules (motion), that radiates away again as "photons", with no mass. That's another way you might visualize it IMO.

 

[Resha Caner]

Final point, this should be very very small, perhaps to small to detect.

I realize that, but still think that theoretically the mass would change. As one approaches the quantum level, maybe concepts of mass and temperature start to lose their meaning, but it seems that would be the place to do the experiment if one were looking.

 

[Michael]

I realize that, but still think that theoretically the mass would change. As one approaches the quantum level, maybe concepts of mass and temperature start to lose their meaning, but it seems that would be the place to do the experiment if one were looking.

You might want to look up the term "rest mass" in relationship to photons.

Photon - Wikipedia, the free encyclopedia

You "could" say it has more total momentum/energy and therefore more total mass. I suppose it's valid to look at it either way, I just prefer to see that extra energy as "kinetic energy" rather than "mass" since the total number of molecules never changes.

 

[Resha Caner]

You "could" say it has more total momentum/energy and therefore more total mass. I suppose it's valid to look at it either way, I just prefer to see that extra energy as "kinetic energy" rather than "mass" since the total number of molecules never changes.

I also understand what photons are. I've been through both undergraduate and graduate science classes. And I get your point, but I think you are associating mass with number rather than quantity.

And it is quanta that modern physicists work with. So, I could challenge you by asking, "What is a molecule?" Take water, for example. The molecule is two hydrogen and one oxygen atom. And what is the hydrogen atom? It is one electron and one proton. And what is an electron? It is a wave packet of energy ... I think that's the right way to say it.

Anyway, concepts of mass are a problem if one digs deep enough. That's why I put the qualifier in my OP. How does one measure mass? Hmm. You put it on a scale, and knowing the gravitational constant ... but wait, though good enough for measuring out the water for baking a cake, that's not technically correct.

Anyway. I recently saw bits and pieces of "Absolute Zero" on PBS (bits and pieces because I have children). I remember catching a comment about the difficulties of macro properties at absolute zero, but didn't get to hear the whole thing. So, it set me to wondering ...

 

[keith99]

I realize that, but still think that theoretically the mass would change. As one approaches the quantum level, maybe concepts of mass and temperature start to lose their meaning, but it seems that would be the place to do the experiment if one were looking.

I would start with a container of gas. There the classical model is well known. I'd run the theroetical numbers on an ideal gas and see what increase in mass would be expected due to relativistic effects before even thinking of any experiment. It would not surprise me in the slightest to find they are so small that there is no way to detect them.

 

[Michael]

I also understand what photons are. I've been through both undergraduate and graduate science classes. And I get your point, but I think you are associating mass with number rather than quantity.

And it is quanta that modern physicists work with. So, I could challenge you by asking, "What is a molecule?" Take water, for example. The molecule is two hydrogen and one oxygen atom. And what is the hydrogen atom? It is one electron and one proton. And what is an electron? It is a wave packet of energy ... I think that's the right way to say it.

Some "wave packets" have "mass" and others don't. An electrons for instance has a measurable rest mass, whereas a photon does not. That's pretty much the difference between energy and mass inside the molecule (as I see it).

I don't think it's invalid to claim it has more moment and therefore more total mass/energy, I simply prefer to see it as a stored form of kinetic energy inside a mass lattice. You're mileage may vary.

The absolute zero thing I believe relates right back to that "kinetic energy". Once there isn't any more kinetic energy, it reaches absolute zero. Since nothing can block all forms of energy (like neutrinos), it's virtually impossible to keep anything at that energy state for long, even if you achieve it momentarily.

 

[Resha Caner]

I would start with a container of gas. There the classical model is well known. I'd run the theroetical numbers on an ideal gas and see what increase in mass would be expected due to relativistic effects before even thinking of any experiment. It would not surprise me in the slightest to find they are so small that there is no way to detect them.

Yeah, probably. That's so disappointing. I like these little mental experiments, but I hate they come to an end because it probably couldn't be tested.

Hence the Copenhagen Interpretation ... or at least my expression of it ... that there is no point speculating about what can't be known. Why spend time fretting over Schrodinger's Cat and such things? We know what we know. Nothing more, nothing less.

And yet there is still something about that which bothers me. It seems to leave the physicist with the biggest stick in charge rather than settling it by demonstrating one's theory.

 

[sfs]

You're on the right track in terms of the total number of molecules IMO. The number of molecules (and therefore the "rest" mass) stays the same. The "energy" is in the form of "kinetic" energy which radiates away again as "heat" (kinetic energy).

A photon has "kinetic energy", but it has no resting mass. You might think of the heat source as "photons" (with no mass but kinetic energy) that creates "kinetic energy vibration" between the molecules (motion), that radiates away again as "photons", with no mass. That's another way you might visualize it IMO.

A single photon has no rest mass, but a system of photons does (in general) have a rest mass. The general expression for rest mass is
m[sup]2[/sup]c[sup]4[/sup] = E[sup]2[/sup] - p[sup]2[/sup]c[sup]2[/sup]. If you heat an object, its energy increases but its net momentum is unchanged. So yes, the rest mass of the object increases (and so does its weight). I doubt it's possible to measure such a small increase, but I don't know.

 

[sfs]

Yeah, probably. That's so disappointing. I like these little mental experiments, but I hate they come to an end because it probably couldn't be tested.

Hence the Copenhagen Interpretation ... or at least my expression of it ... that there is no point speculating about what can't be known. Why spend time fretting over Schrodinger's Cat and such things? We know what we know. Nothing more, nothing less.

There's a big difference between "can't be measured" and "can't be measured with today's equipment". The former generates philosophical debates, while the latter generates the development of new technology.

 

[Resha Caner]

There's a big difference between "can't be measured" and "can't be measured with today's equipment". The former generates philosophical debates, while the latter generates the development of new technology.

Sure. My statement was aimed at the outcomes of those philosophical debates.

 

[Resha Caner]

A single photon has no rest mass, but a system of photons does (in general) have a rest mass. The general expression for rest mass is
m[sup]2[/sup]c[sup]4[/sup] = E[sup]2[/sup] - p[sup]2[/sup]c[sup]2[/sup]. If you heat an object, its energy increases but its net momentum is unchanged. So yes, the rest mass of the object increases (and so does its weight). I doubt it's possible to measure such a small increase, but I don't know.

Uh. There's something very intriguing in what you've said here, but I'm not sure I've grasped it well enough yet to ask a question.

A "system" of photons has a rest mass. Hmm. Can you elaborate on what that "system" is?

And net momentum is unchanged while energy increases. If momentum is mass*velocity and energy is 0.5*mass*velocity^2, how did one remain the same while the other increased ... are my terms too classical?

Because, say we start with m = 0.5 and v = 0.5. So, p = 0.25. Then say after heating m = 0.6. If p doesn't change, then v = 0.25 / 0.6 = 0.4167. So, initially E = 0.0625 and finally E = 0.0521. So energy decreased, not increased.

What am I missing? Is it the "system" part? Maybe a system of particles would produce a different type of result. I'll need to ponder this a bit.

 

[Michael]

A single photon has no rest mass, but a system of photons does (in general) have a rest mass. The general expression for rest mass is
m[sup]2[/sup]c[sup]4[/sup] = E[sup]2[/sup] - p[sup]2[/sup]c[sup]2[/sup]. If you heat an object, its energy increases but its net momentum is unchanged. So yes, the rest mass of the object increases (and so does its weight). I doubt it's possible to measure such a small increase, but I don't know.

That may very well be true, but do you have a reference for the suggestion that a "system" of photons has rest mass? I haven't read anything like that before.