1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Apr 13, 2009 #1
    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?????
  2. jcsd
  3. Apr 13, 2009 #2


    User Avatar

    Staff: Mentor

    Re: entropy

    Doesn't make a whole lot of sense to me...
    Heat goes into the "heat provider"? So it is also a "heat acceptor?"
    Ok, so your "heat provider" isn't a single device, it is an entire carnot heat engine?
    Isentropic, not isothermal compression, but yeah...
    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
    Minus the rejected heat.
    Last edited: Apr 14, 2009
  4. Apr 13, 2009 #3
    Re: entropy

    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????
  5. Apr 14, 2009 #4
    Re: entropy


    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???????
  6. Apr 14, 2009 #5
    Re: entropy

  7. Apr 14, 2009 #6


    User Avatar

    Staff: Mentor

    Re: entropy

    Could you tell me what the four processes are there, because I only see 1 and it is an isothermal phase change...?
    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...
    Last edited: Apr 14, 2009
  8. Apr 14, 2009 #7


    User Avatar

    Staff: Mentor

    Re: entropy

    The sensation you feel when you enter a sauna has nothing to do with entropy.
    No, it doesn't.
    "work", is mechanical energy, not the flow of heat energy.
    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.
    No..... [and repeat....]
  9. Apr 14, 2009 #8
    Re: entropy

    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.
  10. Apr 14, 2009 #9


    User Avatar

    Staff: Mentor

    Re: entropy

    About what? I made a lot of statements....
    For the purpose of this thread, sure...
    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.
    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 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.
    Last edited: Apr 14, 2009
  11. Apr 14, 2009 #10
    Re: entropy

    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??????
    Last edited: Apr 14, 2009
  12. Apr 14, 2009 #11


    User Avatar
    Science Advisor

    Re: entropy

    I came across this page which gives some broad answers to my questions:
    Last edited: Apr 15, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook