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heat

 Definition/Summary Heat is the non-mechanical exchange of internal energy, $U$, between a system and its surroundings as a result of a difference in temperature. By contrast, work, $W$, is the mechanical exchange of energy as a result of force applied across a moving surface (such as the face of a piston). Heat is usually represented by the letter $Q$, and is measured in joules ($J$).

 Equations The first law of thermodynamics $$\Delta U = Q + W$$ Here W is the work done on the system. Alternatively, W is often defined as the work done by the system, in which case the RHS becomes Q-W instead.

 Scientists Sadi Carnot (1796-1832) James Joule (1818-1889)

 Breakdown Physics > Statistical & Thermal >> Heat & Heat Transfer

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 Extended explanation Internal Energy: We define the internal energy as the energy associated with the microscopic energies of a system, that is with the energy associated with the random motion of the molecules within a system. So for a general fluid, the internal energy of a system is the sum of the translational kinetic energies (or thermal energy), the rotational kinetic energies, the vibrational kinetic energies and the potential energies of all the molecules in that system. The internal energy of a system is often erroneously referred to as the heat of a system and we shall see why this is incorrect later. One important point to note here is that the internal energy is a state variable, that is, the change in internal energy between any two states is independent of the path taken. Energy can be external energy (due to macroscopic motion and external fields) or internal energy, $U$ (including relative motion of molecules and dipole moments and stress) Internal energy plus pressure times volume equals enthalpy: $H\ =\ U\ +\ P\,V\text{ , or }H/V\ =\ \rho\,\epsilon\ +\ P$ $\epsilon$ is the internal energy per unit mass, or specific internal energy (s.i.e) Temperature: One useful definition of the temperature of a system, derived from kinetic theory, is kinetic temperature, which is a measure of the average translational kinetic energy associated with the random motion of the molecules with the system. It should be noted that although related to internal energy, temperature is not directly proportional to internal energy since internal energy also involves the rotational and vibrational kinetic energies and the potential energies of the constituent molecules. Work: Well, if you're reading this I assume that you know the definition of work; in thermodynamics work is usually associated with a mechanical transfer of internal energy into or out of a system. Work, outside of thermodynamics, is also associated with mechanical transfer of external energy, due to macroscopic motion (motion of the system as a whole), which is unconnected with temperature. An example of work specific to thermodynamics is the application of a force through a piston, whose movement compresses the gas within a cylinder, thus doing work on the gas. Since work is being done on the gas the W term in the usual expression would be positive. If we assume that the walls of the cylinder are adiabatic (no heat transfer) then all the work done is converted to internal energy. Suppose that after we have compressed the piston, we release it. Intuitively, we expect the piston to recoil back, and this is exactly what happens; the gas expands and does [an equal amount of] work on the piston against atmospheric pressure. In this case, since it is the gas that is doing work, our W term would be negative. Heat: So, we come to the definition of heat. If we examine the first law of thermodynamics, we can see that we can increase the internal energy of a system either by doing work on it, or adding heat to it. Consider a piston and a cylinder filled with gas, we can increase the internal energy of the system by either compressing the gas by applying a force to the piston (work) or by fixing the piston and placing the cylinder in a flame (heat). We can compress and heat the gas in such a way that after the operation all the macroscopic properties (pressure, volume & temperature) are identical, that is the two cylinders are in identical states. Suppose we take two identical cylinders (but not necessarily in identical initial states) filled with a gas at 373K, one of which we compress and the other of which we heat such that both cylinders are at 473K and all their macroscopic quantities are identical, that is the final states of the two cylinders are identical. If we were to now examine the final states of the two cylinders, we have no way of knowing which was compressed and which was heated; the only conclusion we can draw is that their internal energies have increased. In this way we can consider heat as the microscopic analogy of work (macroscopic). I therefore, offer you a formal definition of heat:"Heat is the non-mechanical exchange of energy between the system and surroundings as a result of a difference in temperature" Both work and heat can be considered as methods of transferring energy within or between systems. It should now be apparent why the statement "a body posses heat" is nonsensical. To say that a body possesses heat is analogous to stating that a "body has work", which you must agree is utter rubbish. Rather, one transfers energy to a body by doing work on that body and one transfers energy to a body by heating or adding heat to that body. Similarly, it is incorrect to state that a body's heat has increased, rather its internal energy has increased. A note about Thermal Energy: Some texts make use of the term "thermal energy" to describe the "translational kinetic energy" of the molecules, I personally find that the term "thermal energy" only serves to confuse discussions further. - original written by Hootenany

Commentary

 sanka @ 04:53 PM Apr10-12 Excellent explanation of a fundamental concept. Wish more texts would address such topics from a microscopic point of view.

 AngeliqueN @ 11:52 PM Mar31-12 is there thermodynamics involved with subatomic particles?

 Redbelly98 @ 06:13 PM Mar1-12 Please note, homework questions should be asked by posting in one of the subject areas that you can find here: http://www.physicsforums.com/forumdisplay.php?f=152 The Comments section of this (or any) Library Entry are meant more for pointing out errors or suggesting changes and additions to the entry.

 nilu @ 03:49 AM Mar1-12 A thin rod consists of two parts joined together. One-third of it is sliver and two-thirds is gold. The temperature decreases by 26 C⁰. Determine the fractional decrease in the rod’s length, where L 0 Silver and L 0 Gold are the initial lengths of the silver and gold rods. Can ne1 solve this ques for me....

 jadeturners @ 05:59 AM Feb6-12 This is a very good explanation. Keep it up!

 sanka @ 08:20 AM Jan31-12 Excellent explanation, much more clear and concise than in some textbooks I have read

 Redbelly98 @ 05:30 PM Sep28-11 chirumamilla, that's a good question to ask by posting in the forum. These library entries do not get very much viewership, and are not very conducive to getting questions answered.

 chirumamilla @ 07:12 AM Sep28-11 how can we convert heat into electricity

 Mr. G. Mandal @ 03:20 AM Sep24-11 thanks...

 Vespa71 @ 04:42 AM Mar30-11 All combustion heat comes from radiation. In popular terms I would like to claim that radiation particles holds all most no mass, and all most infinite speed. Quality and quantity of such radiant energy depends of the type of reactions. When the radiation-particles are destroyed on impact with surrounding matter, they loose their miniscule mass and their kinetic energy is entirely converted to heat. The heat-capasity of the surroundings (exhaust gas, superheater-arrays) define the resulting temperature. Am I now being ractionary? I guess this suggestion is close to the heat concept that Carnot and his contemporarys used in their research. (moving of calorific matter) As to the last comment above, I too would like to read some views as to what is "real" radiation, and what is heat-radiation, where's the limit? Is it concievable that perceived heat-radiation derives from the dynamic generation and destruction of a high number of low energy radiation-particles, in the hot matter? Then, the calorific is back!

 LG_FUAD @ 04:58 AM Mar15-11 @ phyeinstein_c Just to answer your question abt Q + W. Technically most books in general say that the first thermodynamic equation is (increase in) U= q(heat gained, mc(temperature change) + w(work done 'on' matter) However, when they say Q - W, they are directly taking W= p x (change in volume), which is work done 'by' the particles of that matter. The definition states that w is the work done 'on' the particles which is the opposite when we say 'by'. So technically w= -W= -( p x (change in volume) ).

 phyeinstein_c @ 08:27 AM Jan30-11 infrared is not heat... anything that absorbs it is heated and then it is called sort of 'deliberate heating..this is caused even by white light but its only 8-25 micrometers... not distinct from the emission of visible light by incandescent objects and ultraviolet by even hotter objects (wiens displacement law) so, in general, all radiations heat up bodies its only how much they heat them

 phyeinstein_c @ 08:11 AM Jan30-11 some books use Q+W where as some use Q-W y is that so??

 anmol singh @ 06:48 AM Aug7-10 In chemistry we use ΔU=Q+W in physics we use Q=ΔU+W

 born2bstar @ 01:42 AM Jun7-10 oh i get it a cylinder filled with gas.. hmm