Undergrad General Question about Dissipation of Energy

[crastinus]

Conceptual question here: In the case of something that dissipates energy, how do we describe the transition between the range along which the system can continue to dissipate energy more or less in the same way and the point at which, given increasing temperature, the system can no longer dissipate energy in the same way? Take a radiator. When in normal range, the radiator dissipates energy according to some formula given its makeup, etc. But consider when the radiator itself begins to get too hot and its shape breaks down (the loss of which seems to decrease the rate at which it can dissipate heat) and then the matter of the device itself begins to break down into its elements: how can we describe this "breakdown" in the clearest physics terms?

 

[anorlunda]

Do you mean the radiator melting?

It is hard to understand want you are asking.

 

[Nidum]

You may be trying to describe thermal runaway ?

 

[crastinus]

Yeah. Sorry. I do mean the radiator melting.

But I'm looking for a general way to describe situations like when the radiator melts. I presume there are others instances of that sort of thing.

Does that make sense?

 

[anorlunda]

I still don't understand. Can you rephrase your question in terms of the sun instead of a radiator?

 

[sophiecentaur]

If you are taking about a hot body, out in space, the rate that it loses heat will depend on the surface temperature and the surface area. If it gets so hot that it melts then its area will go to a minimum so its temperature will increase if the power of the heat source inside is the same.

 

[crastinus]

I still don't understand. Can you rephrase your question in terms of the sun instead of a radiator?

Here's the same idea: The sun radiates heat; it does this today within a range; imagine that the sun undergoes some change in the near future such that it won't radiate heat within that range anymore because the matter of the sun disperses somehow, and the sun in any meaningful sense is no longer there. Where before we had a strong dissipator of energy (the sun as it is today), we now have only a very large widely dispersed array of material.

So, the question I have is: What is the best way to describe that kind of change in general? Physicists usually have some elaborate and precise way of picking out changes, so I am wondering how best to describe this sort of change. Does that make sense?

 

[sophiecentaur]

because the matter of the sun disperses somehow

What mechanism are you suggesting here? If you want to get a handle on the Physics of Stars then read around the subject. "Somehow" isn't good enough.
It is true to say that, as a star expands, if the rate of heating remains the same, the surface temperature drops but there is more than just that involved with the changes stars undergo.

a strong dissipator of energy

What do you mean here? The interior of the Sun is a Source of energy, which radiates away, according to various laws. The final surface temperature is reached when equilibrium is achieved between rate of production and rate of dissipation. See this link about the Stefan Boltzman Law.

 

[anorlunda]

Here's the same idea: The sun radiates heat; it does this today within a range; imagine that the sun undergoes some change in the near future such that it won't radiate heat within that range anymore because the matter of the sun disperses somehow, and the sun in any meaningful sense is no longer there. Where before we had a strong dissipator of energy (the sun as it is today), we now have only a very large widely dispersed array of material.

So, the question I have is: What is the best way to describe that kind of change in general? Physicists usually have some elaborate and precise way of picking out changes, so I am wondering how best to describe this sort of change. Does that make sense?

I don't know any physics that sounds like your description.

 

[crastinus]

Maybe I'm just not thinking or articulating myself clearly enough here.

Thanks for the help, though!

 

[sophiecentaur]

Maybe I'm just not thinking or articulating myself clearly enough here.

Thanks for the help, though!

Try to do some more reading about these things. I think you are having problems articulating your question because you have insufficient basic knowledge. Reading could help and it could be entertaining too.

 

[crastinus]

Try to do some more reading about these things. I think you are having problems articulating your question because you have insufficient basic knowledge. Reading could help and it could be entertaining too.

Can you recommend anything on, say, thermodynamics? I am developing my calculus, stats, etc., so I don't want anything that's just popular, but beyond that I would proabably need something toward the less advanced end of the spectrum. So, serious, but introductory! Thanks.

 

[sophiecentaur]

I don't know your level so it would be difficult to give a specific recommendation. There are a number of A Level Physics textbooks for the UK pre University exams. The A level course includes Stefan's law. The AstroPhysics and Cosmology courses have their own course books which may be separate from the mainstream textbooks.
But the problem with self drive education in a subject like Physics is planning a suitable path through it all, Dipping into a textbook can leave you with more questions than answers but it's a really good idea to have ownership of some actual hard copy of the information.

 

[gmax137]

... how do we describe the transition between the range along which the system can continue to dissipate energy more or less in the same way and the point at which ... the system can no longer dissipate energy in the same way?

There are some objects in some configuration in the real world. You develop a model of the configuration, that allows you to calculate the heat loss from the object. Then, something changes in the real world, such that your model is no longer accurate. You want to find a generalized method for determining when the model becomes inadequate?

I think this is an impossible task. You can do it for a given model, by assessing the model's range of applicability. But there's no way to generalize the method, since all of the models are quite different (the radiator in your living room and the sun both dissipate heat, but the mechanisms by which this happens are different, as are the models and the reasons for the models' limits).

 

[Drakkith]

So, the question I have is: What is the best way to describe that kind of change in general? Physicists usually have some elaborate and precise way of picking out changes, so I am wondering how best to describe this sort of change. Does that make sense?

I think I understand what you're asking, but I don't think there's an easy way to answer your question. The precise details of what happens during this transition will greatly affect the description you'd need to use. For example, if your radiator melts, the heat dissipation will change because the shape is changing, the composition may be changing if parts of the radiator are vaporized before other parts, etc. If this same radiator is rapidly spinning in space as it melts, then it will lose all cohesion and fly apart, which will need a very different description from the previous example.

In short, there's just no easy answer we can give you.

 

[Grinkle]

Given your statements about being relatively new to studying physics ...

You may have learned about matter phase changes, eg water changes from gas to liquid to solid, and perhaps you expect that some similar set of common states exists across a wide body of different energy sources that warrants a specific vocabulary being created to describe it.

This is not the case in general, but it is the case for specific energy sources, for example stars. Stars have well defined and well modeled life stages that each have names and boundary conditions and state descriptions which definitely involve how much energy is output, but this vocabulary is specific to stars.

Earthbound nuclear reactions have different possible states with descriptive vocabulary, at least I recall sub-critical and critical.

Combustion engines have different states - turbo boost on or off, clutch engaged or disengaged, etc that each can lead to different energy outputs.

Energy sources as a general category with commonly defined states / state models / transition etc do not exist - different energy sources do not have enough in common to make such a vocabulary / set of models meaningful.

 

[russ_watters]

...imagine that the sun undergoes some change...
...so I am wondering how best to describe this sort of change.

What happens as a result of the "change" will depend on what the "change" is. You are going to have to describe it to us, specifically, in order for us to answer.

However, be mindful that if you propose an impossible or catastrophic change, there may not be an easy answer. For example, before a radiator gets hot enough to melt, it will burn down your house. But I don't suppose that is the answer you are looking for.

Edit; I'm assuming by "radiator", you are referring to the heating device common in houses, but I don't actually know if that is what you mean...