Heat Measure

A Heat Measure is an energy measure associated with changes of internal energy, work, entropy or temperature.

• Context:
  • It can be defined as [math]\displaystyle{ Q=Cm\delta T }[/math], where C is the specific heat, [math]\displaystyle{ m }[/math] is the mass and \Delta T the changes of temperature.
  • It can be determined through the heat differencial [math]\displaystyle{ \delta Q }[/math] expression and using the First and Second Law of Thermodynamics as:
    • as a function of the changes of internal energy ([math]\displaystyle{ \delta U }[/math]) and of changes of the work done ([math]\displaystyle{ \delta W }[/math]) on the system ⇒ [math]\displaystyle{ \delta Q=\delta U-\delta W }[/math];
    • as a function of the temperature (T) and of changes the entropy ([math]\displaystyle{ \delta S }[/math]) ⇒ [math]\displaystyle{ \delta Q= T \delta S }[/math] in the case of a reversible process
    • as a function of the pressure (P), changes of volume ([math]\displaystyle{ \delta V }[/math]) and internal energy ([math]\displaystyle{ \delta U }[/math]) ⇒ [math]\displaystyle{ \delta Q=\delta U - P\delta V }[/math] in case of an isothermic process and monatomic ideal gas.
  • …
• Example(s):
  • Specific Heat.
  • Volumetric Heat Capacity.
• Counter-Example(s):
  • Temperature.
  • Entropy.
  • Pressure

See: Thermodynamics, Temperature, Thermal Energy, Entropy, First Law of Thermodynamics, Second Law of Thermodynamics, Heat flow, Heat Transfer.

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References

2009

• (Wikipedia, 2009) ⇒ http://en.wikipedia.org/wiki/Heat
  • In Physics and Thermodynamics, heat is the process of Energy transfer from one body or system to another due to a difference in Temperature. In Thermodynamics, the quantity TdS is used as a representative measure of the (inexact) heat differential δQ, which is the absolute temperature of an object multiplied by the differential quantity of a system's entropy measured at the boundary of the object.
  • A related term is Thermal Energy, loosely defined as the energy of a body that increases with its Temperature. Heat is also loosely referred to as thermal energy, although many definitions require this thermal energy to actually be in the process of movement between one body and another to be technically called heat (otherwise, many sources prefer to continue to refer to the static quantity as "thermal energy"). Heat is also known as "Energy".
  • Energy transfer by heat can occur between objects by radiation, conduction and Convection. Temperature is used as a measure of the internal energy or Enthalpy, that is the level of elementary motion giving rise to heat transfer. Energy can only be transferred by heat between objects - or areas within an object - with different temperatures (as given by the Zeroth Law of Thermodynamics). This transfer happens spontaneously only in the direction of the colder body (as per the Second Law of Thermodynamics). The transfer of energy by heat from one object to another object with an equal or higher temperature can happen only with the aid of a Heat Pump, which does work.

2005

• (Hyperphysics Encyclopedia, 2005) ⇒ http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heat.html#c1
  • QUOTE: Heat may be defined as energy in transit from a high temperature object to a lower temperature object. An object does not possess "heat"; the appropriate term for the microscopic energy in an object is internal energy. The internal energy may be increased by transferring energy to the object from a higher temperature (hotter) object - this is properly called heating.

It is related to changes of internal energy or work through the First Law of Thermodynamics, [math]\displaystyle{ \Delta U= Q - W }[/math]

1963

• (Feynman et al., 1963) ⇒ Richard P. Feynman, Robert B. Leighton and Matthew Sands (1963, 1977, 2006, 2010, 2013) "The Feynman Lectures on Physics": New Millennium Edition is now available online by the California Institute of Technology, Michael A. Gottlieb, and Rudolf Pfeiffer ⇒ http://www.feynmanlectures.caltech.edu/
  • QUOTE: First law: Heat put into a system+Work done on a system=Increase in internal energy of the system:

[math]\displaystyle{ dQ+dW=dU }[/math]

Second law: A process whose only net result is to take heat from a reservoir and convert it to work is impossible. No heat engine taking heat [math]\displaystyle{ Q_1 }[/math] from [math]\displaystyle{ T_1 }[/math] and delivering heat [math]\displaystyle{ Q_2 }[/math] at [math]\displaystyle{ T2 }[/math] can do more work than a reversible engine, for which:

[math]\displaystyle{ W=Q1−Q2=Q1(\frac{T1−T2}{T1}) }[/math].

The entropy of a system is defined this way:

(a) If heat [math]\displaystyle{ \Delta Q }[/math] is added reversibly to a system at temperature [math]\displaystyle{ T }[/math], the increase in entropy of the system is:

[math]\displaystyle{ \Delta S=\Delta Q/T }[/math] 

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