Heat provider and heat acceptor makes up the system.heat flows

  • #1
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Main Question or Discussion Point

heat provider and heat acceptor makes up the system.

heat flows from high temperature to low temperature. the amount of heat that flows from the heat provider is the heat that goes into the heat provider minus the total work of the heat provider system based on the Carnot cycle.

more specifically, the heat that goes into the heat provider is the heat input in the isothermal expansion which equals the Work of the system to expand the gas in the carnot cycle.
and, the heat that flows out of the heat provider is the heat output in the isothermal compression which equals the Work external to compress the gas in the carnot cycle.

the net heat input of the heat provider is equal to the net work done by the heat provider which is also equal to the heat input minus the heat output. THE RESULT IS A DECREASE IN TEMPERATURE OF THE HEAT PROVIDER AS HEAT FLOWS OUT.

this net work done by the heat provider is the energy lost by the heat provider as heat flows from a hight temperature area to a low temperature area.

is everything correct so far?????
 

Answers and Replies

  • #2
russ_watters
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is everything correct so far?????
Doesn't make a whole lot of sense to me...
the amount of heat that flows from the heat provider is the heat that goes into the heat provider minus the total work of the heat provider system based on the Carnot cycle.
Heat goes into the "heat provider"? So it is also a "heat acceptor?"
more specifically, the heat that goes into the heat provider is the heat input in the isothermal expansion which equals the Work of the system to expand the gas in the carnot cycle.
Ok, so your "heat provider" isn't a single device, it is an entire carnot heat engine?
and, the heat that flows out of the heat provider is the heat output in the isothermal compression which equals the Work external to compress the gas in the carnot cycle.
Isentropic, not isothermal compression, but yeah...
the net heat input of the heat provider is equal to the net work done by the heat provider which is also equal to the heat input minus the heat output.
Yes.
THE RESULT IS A DECREASE IN TEMPERATURE OF THE HEAT PROVIDER AS HEAT FLOWS OUT.
No. The sentence above described a steady-state/conservation of energy situation. There is no energy flow into or out of the system (edit: that would be NET energy flow). Heat flows in, mechanical work and heat of rejection flows out: (Eout+Qout)-Qin=0
this net work done by the heat provider is the energy lost by the heat provider as heat flows from a hight temperature area to a low temperature area.
Minus the rejected heat.
 
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  • #3
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take the example of the melting of ice at 273 degrees kelvin to water at 273 degrees kelvin by a heat source that is infinitesimally higher in temperature to the ice.

using the complete carnot engine model with 2 isothermal and 2 adiabatic processes, you can follow the heat flow from heat source to the ice.

the carnot engine model for the flow out of heat from the heat source into the ice shows us that the heat flow out decreases by an infinitesimally amount the internal energy of the heat source with an infinitesimally lower temperature.

the carnot engine model for the flow in of heat into the ice shows us that the heat flow in increases by an infinitesimally amount the internal energy of the ice with an infinitesimally higher temperature.

because the infinitesimally lower energy state of the heat source after heat flows out into the ice creates a temperature gradient between the area of heat source that lost the heat the the area of the ice that gained the heat. thus, heat from the ice flows back out from that area back into the area of the heat source that lost the heat.

this cycle repeats countless times until the ice melts. thus, 1st law of thermo is observed and the 2nd law of thermo that describes the direction of the heat flow process is also observed.

is this correct so far????
 
  • #4
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also,

how can you explain the sensation you feel when you enter a sauna in terms of entropy. of course the heat of the sauna flows into your body. as the heat flows into your body, your body uses some of the heat energy as work and heat flows out of the heat accepted part of your body into regions in your body that hasn't been heated...this cycle repeats until your body has done lots of work and moved the heat from the sauna into your inner core parts of your body.... thus, due to the input of heat energy into your body that allows your body to do some free work... saunas are healthy for your body because your body doesn't have to create the heat to do the necessary work. we all know that entropy increases MORE in the areas where heat has been accepted due to the lower temperature... but does that mean that the area of the sauna that provided the heat did work to expel the heat into your body??????

what about when you leave a sauna and the heated areas of your body expel the heat to the cooler environment? your inner core that has been heated is moving the heat out to your skin and environment through countless carnot like cycles... more work is being done by your cells as the heat is moved from core to environment... is this the reason we feel rejuvenated??????

I wonder why we feel uncomfortable sitting in the sauna when free energy is being inputted to your body.... that free energy that was inputted to your body escaped your body when you leave to create more work in your cells and we feel rejuvenation... why don't we feel the same way we receive the heat from the sauna???????
 
  • #5
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help!!!!!
 
  • #6
russ_watters
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take the example of the melting of ice at 273 degrees kelvin to water at 273 degrees kelvin by a heat source that is infinitesimally higher in temperature to the ice.

using the complete carnot engine model with 2 isothermal and 2 adiabatic processes, you can follow the heat flow from heat source to the ice.
Could you tell me what the four processes are there, because I only see 1 and it is an isothermal phase change...?
the carnot engine model for the flow out of heat from the heat source into the ice shows us that the heat flow out decreases by an infinitesimally amount the internal energy of the heat source with an infinitesimally lower temperature.

the carnot engine model for the flow in of heat into the ice shows us that the heat flow in increases by an infinitesimally amount the internal energy of the ice with an infinitesimally higher temperature.

because the infinitesimally lower energy state of the heat source after heat flows out into the ice creates a temperature gradient between the area of heat source that lost the heat the the area of the ice that gained the heat. thus, heat from the ice flows back out from that area back into the area of the heat source that lost the heat.

this cycle repeats countless times until the ice melts. thus, 1st law of thermo is observed and the 2nd law of thermo that describes the direction of the heat flow process is also observed.

is this correct so far????
No, it is not correct. You are overcomplicating things. While the ice is melting, you have a steady-state and no parts of your system have a changing temperature. You have, for example, an electric heating element that remains at a constant temperature, slightly above the ice temp, where heat constantly flows in and out in a conservation of energy situation, keeping the temperature of the heat element constant.

Heat transfer in thermodynamics is not modeled incrementally - there is no reason for it. Steady-state power is all you need. Unless this is for an upper-level heat transfer (not thermodynamics) class and you are using somem sort of spreadsheet-based simulation technique to model the development of the steady-state from the time the heating element is first turned-on...
 
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  • #7
russ_watters
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how can you explain the sensation you feel when you enter a sauna in terms of entropy.
The sensation you feel when you enter a sauna has nothing to do with entropy.
of course the heat of the sauna flows into your body.
Yes...
as the heat flows into your body, your body uses some of the heat energy as work...
No, it doesn't.
and heat flows out of the heat accepted part of your body into regions in your body that hasn't been heated...this cycle repeats until your body has done lots of work and moved the heat from the sauna into your inner core parts of your body....
"work", is mechanical energy, not the flow of heat energy.
thus, due to the input of heat energy into your body that allows your body to do some free work...
No, the human body does not absorb heat from the environment to generate work. It doesn't even absorb heat at all except in a very transient situation - otherwise you would overheat rapidly and die. In a sauna, only your skin would absorb heat from the sauna, and then only for a short time until your body increased your sweating to compensate.
saunas are healthy for your body because your body doesn't have to create the heat to do the necessary work.
No..... [and repeat....]
 
  • #8
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i think you're wrong about that russ...

heat flow is necessary for work in any real situation. your body must create and maintain the 98.6 degrees temperature to sustain the amount of work. being in a warmer environment makes you sweat because heat is being absorbed into your body and your body must cool itself off through evaporation.

i hope you know that mechanical work needs heat flow unless you're talking about a hypothetical adiabatic situation...

you're not looking into what entropy really means.
 
  • #9
russ_watters
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i think you're wrong about that russ...
About what? I made a lot of statements....
heat flow is necessary for work in any real situation.
For the purpose of this thread, sure...
your body must create and maintain the 98.6 degrees temperature to sustain the amount of work.
True...
being in a warmer environment makes you sweat because heat is being absorbed into your body and your body must cool itself off through evaporation.
No. Your body produces heat and you sweat even if the temperature is well below body temp. As the ambient temp increases, so does the amount you sweat in order to continue to dissipate the 150w (give or take) that you generate. If you were not able to dissipate this heat, your body temp would rise about 1.7C per hour, which would kill you in about two hours, even without a net heat flow into your body from your surroundings.

What's more, you are neglecting the importance of humidity. It can be quite hot outside without a net heat flow into the human body unless the humidity is also high because evaporation carries away a lot of heat. I live in Pennsylvania and on the hottest of the hottest summer days, when it is 95 F and 50% humidity, it is only 60% as hot as it needs to be for your body to absorb heat from the environment (thermodynamically). At that absolute humidity, it would need to be over 200F for your body to gain heat from its surroundings.
i hope you know that mechanical work needs heat flow unless you're talking about a hypothetical adiabatic situation...
Heat is generated internally via chemical reaction. Like a car engine, fuel (chemical energy) goes in, gets burned, and heat and mechanical work go out.
you're not looking into what entropy really means.
You have yet to say anything relevant to the concept of entropy. In fact, if you apply the concept of entropy to this issue, what you find is that it requires that heat flow out of the human body, not in. It is the heat of rejection, like the heat of the exhaust out of the tailpipe of a car.

I don't know where you are getting these ideas about thermodynamics - or physiology for that matter - but they are quite wrong.
 
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  • #10
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let me get this straight. are you telling me that heat does not flow into your body when you enter a 130 degree room???

humidity?? let's neglect humidity and stick with straight heat flow and energy conservations...

2nd law without talking about microstates and macrostates defines the direction of heat flow and the amount of unusable energy produced after heat flows.

anyways, if you investigate the 2nd law more closely, you may see that heat flow is a cyclic process between heat provider and heat absorber that slowly brings down the internal energy of the heat provider while slowly raises the internal energy of the heat acceptor. This process obeys the 1st law and the 2nd law.

I'm about to start on the statistical aspect of entropy...but i need to learn more about eigenvalues and hilbert spaces and hamiltonian and dirac delta fxns...DAMN!!!! LOTS OF WORK!!!!! CAN ANYBODY PROVIDE ME WITH SOME HEAT FLOW??????
 
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  • #11
JesseM
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JesseM said:
Presumably there'd be some ideal external temperature where you body would have to do the minimal amount of work to maintain that temperature? I think I remember reading that because of the heat generated by metabolic activity the most comfortable external temperature is significantly lower than 98.6, but I don't know if "most comfortable" can be equated to "body needs to expend the minimum energy to maintain correct temperature."

Also, aside from extreme things like blood freezing or boiling, what is it that causes harm if your body fails to maintain the 98.6 temperature? Just various biochemical reactions not happening at the correct rate?
I came across this page which gives some broad answers to my questions:
The master control that directs human thought and action is the brain, a fantastically elaborate circuitry of a hundred billion interconnected nerve cells; a comparable number is housed in the lion's cranium. The complex chemical reactions that activate the transmission and reception of signals are temperature-dependent, as are the various hormonal messages sent to the specialized organs. Since all the circuitry is temperature-dependent, having a constant body temperature—one with a little leeway for special circumstances—is simply the best evolutionary choice for animals as complex as we are. A fluctuating brain temperature would lead to unpredictable reactions, ones that might not occur in the same sequence if the learning had taken place at a different brain temperature. The human brain and that of other mammals and of birds—these extraordinary tools—work as well as they do because of the protected, controlled environment they are housed in. Simpler animals, with far less complex brains, have optimized their survival possibilities in other ways, but constant temperature is best for us.

...

Nor is constant brain temperature the only reason for homeothermy. Chemical reactions generally proceed faster as temperature rises, so a higher body internal thermostat setting affords greater activity—up to a point. When excess heat cannot be shed and information is coming too quickly, the system breaks down. Over the past few million years, we, as well as other mammals and, of course, birds, seem to have found that we function most effectively in the vicinity of 100 degrees Fahrenheit.

A physiologist friend of mine told me to think about sexual behavior when faced with a puzzle about animal behavior. The hormonal reactions that control mating, procreation, and countless other commands work best at a high constant temperature in warm-blooded animals. We can even look to temperature for the answer to such fine-tuning questions as why do males have testicles in external scrotal sacs rather than in the more protected abdominal cavity. Presumably a somewhat lower temperature than 98.6 degrees is favored for sperm production.

Given that the human body works most reliably if kept at a constant temperature, why is that temperature 98.6 degrees? The rough answer is a mixture of evolutionary arguments combined with a simple understanding of how our metabolism works. Most machines are quite inefficient and mammalian bodies are no exception. Typically, more than 70 percent of the energy input into the body is converted into heat. This heat then needs to be dissipated into the environment, or else the body, like any overcharged engine, becomes overheated and stops functioning properly. We feel most comfortable in external temperatures some 20 to 30 degrees below our skin's temperature, because that differential produces a comfortable rate of heat loss; any colder and we lose heat too rapidly, any warmer and we retain too much. We correct for the former by adding clothes, blankets, and muscular activity like shivering; we correct for the latter by sweating, fanning, and, when circumstances allow, just taking it easy.
 
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