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B Confusions about thermodynamics

  1. Sep 16, 2016 #1
    1. Can I say Carnot efficiency of a heat engine is always smaller than 100 because the temperature of cold reservoir is always greater than 0 Kelvin?
    2. Is the following analogy correct to describe the relation between temperature and heat energy?
    "Heat energy affects internal energy which affects temperature. If heat energy doesn't affect internal energy, it won't affect the temperature."
    3. What is the difference between thermal, internal and heat energy?
    Last edited: Sep 16, 2016
  2. jcsd
  3. Sep 16, 2016 #2
    1. Yes
    2. Yes...temp is an indication of internal energy
    3......wait to see what comes
  4. Sep 16, 2016 #3
    1.Yes. The carnot efficiency will always be below 100 percent.And a realistic efficiency will always be smaller than the carnot efficiency.
    But that seems a bit trivial, to be honest.
    Your second and third question can probably be answered if you read the articles of the terms on hyperphysics.

    A quick explanation(although not entirely correct) would be:
    Thermal energy is all of the kinetic energy of the constituent atoms/molecules in an object/system.
    (obviously the disordered kinetic energy not any collective movement like a ball flying through the air)
    Internal energy is thermal energy but also considers potential energies associated with intermolecular forces and such.For example the potential energy of the objects state(as in liquid, solid, gaseous)
    Heat is the energy that is transferred in a spontaneous process(conduction/radiation/convection) from one system to another.
    I have also used heat as any energy added to a system as thermal energy(like when "heating" up a wire through electrical resistance).
    But I don't know whether that is actually an incorrect use of the word.
    Temperature is a quantity that describes how heat will flow.
    If two objects have different temperatures heat will flow from the object with higher temperature to the object with lower temperature.
    Obviously, temperature is connected to internal energy but not really in a simple way.
    Higher temperature does not necessarily indicate higher internal energy.
    Last edited: Sep 16, 2016
  5. Sep 17, 2016 #4
    "Obviously, temperature is connected to internal energy but not really in a simple way."
    The derivation of the ideal gas equation, PV =nRT, makes a direct link between temperature, T, and average translational KE of gas molecules. In fact you could say that the ideal gas equation gives a physical meaning to what is meant by temperature (absolute temperature T)...It is a measure of the KE of gas molecules. For ideal gases temperature (absolute temp T) and internal energy are one and the same thing, higher temperature means higher KE of molecules means higher internal energy.
  6. Sep 20, 2016 #5
    In an ideal monoatomic gas, temperature and kinetic energy are proportional.That is true.
    But that is one specific case. And such a neat relationship is (to my knowledge) only true for very specific cases.
    Once you introduce rotational and vibrational motions of the gas molecules you get a change in heat capacity with temperatures.
    As shown on in this diagram : http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/imgkin/shmh.gif
    When we talk about solids it gets worse.The relationship follows a curve that looks somewhat like logistical growth and tends to one maximum heat capacity for all substances.But the curve isn't the same for each solid.Some substances' heat capacity increases slower or faster than the heat capacity of others.
    And as far as I know we cannot even generally predict the molar heat capacity of liquids in one formula.
    If you were to introduce phase transitions, things would become even worse.
    Objects at the same temperature can have vastly different densities of thermal/internal energy.
    That is why I would advise against treating thermal energy similar to temperature.

    Temperature is a quantity that very closely relates to the flow of heat.That is the most important or almost defining property in my opinion.
  7. Sep 22, 2016 #6
    'Temperature is a quantity that very closely relates to the flow of heat.That is the most important or almost defining property in my opinion.'
    closely related to heat transfer but does not tell you what Temperature IS !!
    I am in great agreement with you but I prefer the results of kinetic theory which gives a meaning to temperature....KE of molecules.
    I have enjoyed this post !!
  8. Sep 23, 2016 #7


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    Basically, temperature tells you which direction heat will spontaneously flow if you put two systems in contact. Heat will flow from the system with lower (reciprocal of temperature) to higher (reciprocal of temperature). For positive temperatures, that means heat will flow from higher temperature to lower temperature.
    A more quantitative definition is given by the equation:
    ##\frac{1}{T} = \frac{\partial S}{\partial U}##
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