Come to think of it, the work done on a body is converted into some form of energy.But why is it that it ultimately tends to produce heat? In physics we all talk about energy dissipation in the form of heat,but why not electricity, or even light(somehow or the other it tends to form heat,exceptions barred).Why is thermal death,so prevalent a term for non usable energy, and not, say 'electrical death'?What specific mechanism, if any, exists to see to it that all energy is wasted as heat,and not as some other non usable form ?

  • $\begingroup$ All forms of energy conversion are limited by thermodynamics, we just don't talk about that at the high school level of physics education because the necessary level of knowledge of the second and third law of thermodynamics can not be taught. For the conversion of sunlight to electricity, for instance, we can calculate a maximal thermodynamic efficiency of 95% based on a radiation temperature of sunlight of 5800K and a solar panel temperature of 300K. In the general case one always has to take the temperatures of the systems into account that store the different forms of energy. $\endgroup$ – CuriousOne Mar 2 '16 at 19:34
  • $\begingroup$ Heat can be converted to do other work. en.wikipedia.org/wiki/Steam_engine $\endgroup$ – Keep these mind Mar 2 '16 at 20:03
  • $\begingroup$ Sure it can be,but my question is that why is heat a major form of relased energy in so many processes $\endgroup$ – Abhinav Mar 2 '16 at 21:45

1) Heat means collisions.

2) Energy can be dissipated by radiation (light or photons). The coined term is "radiative losses". It is a very important source of losses for circular beam accelerators (or any sort of device accelerating charged particles, say an antenna) and for spacecraft re-entering the atmosphere before crashing/landing on Earth. A very import notion regarding radiatives losses it that of black bodies. A human body dissipates more energy through radiation as a black body (to keep its temperature at 37°C) than by doing work (moving, talking, thinking, etc.).

3) A key notion for your further reading: entropy.

Entropy is studied through statistical physics (from which can be derived thermodynamics as a limit case). Transfer of momentum from particles can be considered as a random process with an uncertainty about how much momentum is transferred from one particle to another. The transfer of momentum depends on the angle of collision, the momentum of particles colliding and the cross section of the collision. The cross section contains information about the physical interaction (hard spheres, coulomb force, etc.) that we call a collision. Also the collisions between particles happens at such a small scale that quantum mechanical Heisenberg uncertainty theorem must be taken into account, meaning that we can't be sure how two particles will exactly exchange momentum. All of this results in a spreading of the input energy given to a physical system among its many many components (1 mole = $10^{23}$ particles). That's what we call energy dissipation. To describe the state of a system we then use distributions of momentum, speed and energy (see Maxwell-Boltzmann distribution). Entropy is the quantity that monitors the behavior of those distributions. Heat then describe a transformation of the distribution (displacement of the distribution mean, flattening of the distribution) that is allowed by how the entropy should evolve (2nd principle of Thermodynamics).

To sum up:

Heat <=> collision <=> random momentum transfer process + uncertainty about quantity of momentum exchanged => change in entropy + modification of the energy distribution <=> dissipation (or waste) of energy

  • $\begingroup$ A small note: "heat means collisions" is certainly true when the only interactions are contact interactions, but if there are also long-range interactions present in the material, then heat can also be transferred without colliding, per se. $\endgroup$ – probably_someone May 23 '18 at 23:06
  • $\begingroup$ Do you mean radiation by long range? Because then it is photon collisions (elastic or inelastic). One particular aspect of this sort of heat transfer is that they don't need a solid media to propagate and that makes them long-range in that sense. But eventually, if there is interaction, there is collision of photons with matter. And I have already mentioned this type of heat transfer in my answer... $\endgroup$ – A.G. May 29 '18 at 13:17

This can be attributed to the Second Law of Thermodynamics which talks of Entropy. Entropy is disorder. What is heat? Heat is in fact used to quantify entropy. For any process to occur, the entropy of the whole system must increase. That means the process should result in an ultimate increase in disorder - which can be thought of as an increase in temperature since this increase in disorder is the 'heat production' you talk of.

Heat is an internal form of energy. It is energy in transit, or in fact a means for transfer of energy. Don't get me wrong here - heat definitely is energy but I would prefer it being thought of as a vehicle rather than a store. The other vehicle being work.

Heat is transfer of energy in a very crude and randomised manner. Providing heat, increases the temperature and increases random motion of the molecules. Hence you can attribute the heat released - unusable form of energy - to an increase in entropy.

  • $\begingroup$ The first law of thermodynamics clearly states that heat is a form of internal energy and that the energy conservation law applies. At the microscopic level one can identify heat with kinetic and potential energy of the atoms or molecules of the system. $\endgroup$ – CuriousOne Mar 2 '16 at 19:57
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    $\begingroup$ Heat definitely is energy. I meant to stress on the 'form' part. It is energy in transit. It is comparable to work. Would you call work a form of energy? It is just a mode of delivering energy. What the 1st Law says is that this energy can be delivered to the system either as heat or work. Its just a matter of semantics. guess i should clarify this up there :) $\endgroup$ – Polisetty Mar 2 '16 at 20:08
  • $\begingroup$ "Heat is not a form of energy" is not semantics but simply a false statement and what you "prefer" is completely irrelevant. We are trying to explain physics correctly around here. You have your own workable interpretation of physical quantities, but you should not be using language that is plain false when a student would use it in a test. $\endgroup$ – CuriousOne Mar 2 '16 at 20:10
  • $\begingroup$ Right.. I guess I should be deleting it. By calling it energy, it implies its a form of energy. But then how do i convey the point? should i be calling it a 'special' form?? $\endgroup$ – Polisetty Mar 2 '16 at 20:14
  • $\begingroup$ Heat is an "internal form of energy". That is how physics defines it and it works perfectly that way. In statistical mechanics we make the connection to ordinary kinetic and potential energy. $\endgroup$ – CuriousOne Mar 2 '16 at 20:16

There are two main types of energy, potential and kinetic. Potential energy is energy that has the potential to perform work. Kinetic energy is the energy associated with motion. Kinetic energy can take macroscopic forms such as translation or rotation of a solid body, large vibrations within a system of particles, flow of a fluid, or wave motion. Kinetic energy can also take microscopic forms such as molecular vibration, incoherent molecular motion, microscopic vortexes, microscopic waves, or electromagnetic radiation, which I will call thermal energy. All accessible forms of energy can be inter-converted to other forms of energy in one or more steps with the exception of thermal energy at equilibrium, which cannot be converted to any macroscopic form of energy. Note: In many cases, the effective equilibrium temperature of the Earth may be taken as the ambient air temperature, but when considering the thermal radiation from space the night sky the equilibrium temperature is at 2.7 K, so that some useful work can be recovered from the thermal radiation of Earth.)

The number of states of motion or molecules that move in a simple coherent macroscopic fashion are minuscule compared to the number of states moving in some complex "disordered" way. As a result, systems of molecules are much more likely to be in a state of "disorder" than a state of macroscopic order, meaning that ordered kinetic energy naturally becomes thermal energy (disordered kinetic energy). As the temperature naturally settles to equilibrium (ambient temperature), it becomes impossible to convert thermal energy to another form. Consequently, thermal energy at thermodynamic equilibrium is the local end state of all energy.

Energy flows from sun to Earth and eventually to space. A small amount of this energy is captured by photosynthetic organisms, convective thermodynamic engines (air and water convection), and solar panels for useful work, fluid flow, growth, and reproduction.

  • $\begingroup$ Well note that the 2.7 K temperature is the temperature of the cosmic microwave background, so way out in the middle of nowhere, that would be the temperature. But within the solar system, the temperature will vary greatly depending on factors such as in the sun light, bodies radiating heat, etc. $\endgroup$ – Kyle Kanos Aug 2 '17 at 1:54

Thermodynamics, and reversibility of processes.

Most of ways energy can be stored are reversible. Kinetic energy to potential gravitational energy and vice versa. Electric to magnetic and vice versa. Chemical to electric, and vice versa. Energy stored one way will eventually be recovered and transformed into another.

Transforming thermal energy into any other form takes a lot of effort though - you need a heat engine like peltier cell or stirling engine to extract other forms of energy from thermal - and if it's randomly distributed in objects all around collecting it is simply not viable. So, thermal energy tends to stay thermal, occasionally transforming to electromagnetic (infrared photons) which are sent in random directions and end up transformed into thermal energy elsewhere. Simply, very lousy reversibility of transformations of other forms into thermal energy means most of "free energy" end up trapped there - and stays there.


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