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, ΣQ, 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." Heat is difficult to predict and hard to measure even in well-controlled laboratory circumstances.
Thermodynamics and physics texts (often 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.
TEMPERATURE:
Cold, hot and heat are relative ideas for humans. Below are listed some peculiarities of heat and temperature.
- Test Your Nose:
The tip of your nose is a temperature gage, of sorts. Can your nose tell you what's hot, what's not? In the kitchen, select a metal spoon and a wooden spoon. Place the metal spoon against your nose. Phew! This spoon is cold! Next place the wooden spoon against your nose. So is the wooden spoon warmer? Both items, having been in the kitchen for many hours will have attained the same temperature (probably colder than your nose). The sensation, "cold" as in "cold metal spoon" has to do with heat. The metal has a higher heat capacity than porous wood. Your nose experiences greater heat to the metal spoon hence the spoon feels colder.
- Locke's Parlor Game (1690):
As a parlor game, Locke puzzled his guests about hot and cold. He arranged four tanks of water in pairs, two on the left and two on the right so his guests could walk between them (blind-folded for more excitement) and immerse their hands in each. The first pair of tanks, the left to receive the left hand and the other tank received the right hand. As set up, this pair of tanks contained decidedly hot and cold water respectively. Naturally when hands were immersed, all guests exclaimed, "left is hot" and "right is cold." Next, a step or so forward, their left and right hands were immersed again in tanks both of which contained water at room temperature. The temperatures now, the guests agreed unanimously, were "left is cold" and "right is hot."
- 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.
- Hot Potato: A baked potato on the rack in an oven has a temperature of 350°F well above the boiling temperature of anyone's fingers (212°F). However, one can remove the potato bare-handed provided one shouts, "HOT POTATO," and remembers to bounce the tuber from one hand to the other - not allowing it time to burn your fingers. A physics professor also uses the fact, "heat takes time," when he stabs his hand into a pot of molten lead then quickly snatches it out and shouts, "Ah HA!".
- Does a Hammer Blow Heat a Penny? Take a copper penny from your pocket. Place it against your nose as a "measurement" of its temperature. Initially it will seem cold but warmer with time. Place the penny on a steel anvil. Get a heavy steel hammer and bludgeon the penny forcefully. Promptly touch the penny to you nose again. It will be noticeably hotter. So did the hammer blow heat the coin? No. The hammer did destructive compression work on the copper. The blow increased the internal energy of the penny.
Car Door Jimmy Teenage twin sisters are locked out of their jeep (duh?). They need a long wire, bent to have a hook, to retrieve the keys which are inside on the seat. Each girl finds a length of wire; one is coat-hanger wire, the other is spring steel. "Let me do it," the girl with the spring steel says. So she bends her wire but it straightens - no hook. She bends it again and it straightens - still no hook. "Outa the way," her sister pushes. She bends a hook in the coat hanger wire that she decides is too small. The next hook she bends seems too large. By the third bend, the wire is hot, on the fourth bend, the coat-hanger snapped; so they called a cab.
Spring steel is a special combination of iron and carbon. When the spring steel wire was bent (deformed) by work, its internal energy increased in an organized way with negligible temperature increase. When the spring was released nearly all (springs also break - eventually) of that energy left the spring as work returning to the bending fingers. For the coat-hanger wire the "bend" was not organized and the temperature of this wire increased. The event proceeded promptly; there was no heat. Bent, bent, then once more; it snapped. Had there been heat, the wire might have required more bends before failure occurred.