Close
THERMO Spoken Here! ~ J. Pohl © ~ 2017 www.ThermoSpokenHere.com (C5700-160)

3.16 About Heat

Science students are physically aware of temperature and the associated event, heat. When heat and temperature are being considered, the model BODY or point mass is inadequate. The new model is substance or material (metal, solid and so). Also a new energy form is introduced: internal energy.

HEAT:   Energy transfer between a system and surroundings as a consequence of temperature difference is called heat. The term of our energy equation, Q, represents the sum of all heats of a system event. The sign convention of heat is:

In the absence of all other effects, heat attendant with an
increase of system energy is defined to be positive.

In beginning level thermodynamic analysis, virtually all thermodynamic systems are assumed initially to be in thermal equilibrium with the surroundings. That is, initially 0 = ΣQ. With an event it is common for a substance to experience temperature change. For the case that "some location of the system becomes hotter" (than some temperature of the surroundings) then in time the system temperature (and system energy associated with that temperature) will decrease via heat to the surroundings. That system event is said to be (or to have been) "heat transfer with the surroundings" (or simply "heat".) Heat is difficult to predict and hard to measure even in well-controlled laboratory circumstances.

Thermodynamics and physics texts (sometimes without statement) avoid confrontation with heat by use of one of three stratagems.

  • Heat is the Unknown: If the system and all aspects of its change are specified, the heat of the event can be calculated from the energy equation applied to the event. Texts supply all other information required and ask the student to add them to obtain: the heat Q or the heat rate Q.

  • Heat is Specified: A second academic treatment of heat is to specify the heat as a known magnitude and direction as though heat were controlled and applied precisely to the event. When "precise heat" (stated as a signed number) is entered into the event equation then some other term (the unknown) is calculated.

  • Heat is Zero: A system event for which there is no heat (not ΣQ = 0, but all Q = 0) is termed adiabatic. Two means of minimizing heat are system insulation and quickness of the event. A perfectly insulated system would be adiabatic; but perfect insulators do not exist. Heat requires time. Thus, heat being zero, is a reasonable assumption for system events known to be quick or very fast.

We close with a thought puzzle: Does a car overheat or does it under-heat?

Beware of Under-Heating:  Automobile operators must be cautious of the level of thermal energy of their operating engines. The machines have schemes to steadily move thermal energy to the engine boundary and dump it, as heat, to surroundings. While heat leaves, all is well. But thermal energy elimination schemes do fail, whereupon the required or normal heat (dump, elimination, or expulsion) becomes less. As the system under-heats, the machine begins to retains thermal energy and engine temperatures sky-rocket. The thermodynamically ignorant see temperature go up and claim the effect to be overheating or heat added to the engine. A destroyed engine is a sadness. If more people knew just a little thermo, there would be fewer under-heating catastrophes.

Tags: None