Unit IV: Heat
Introduction
General Instructional Objectives (and corresponding Specific Learning Outcomes) | Unit Content |
Knows terms
Understands the meaning of terms |
temperature, absolute zero, heat, internal energy, conduction, convection, radiation, evaporation, condensation, boiling, regelation, entropy |
Understands scientific concepts | specific heat capacity, Newton’s law of cooling, solar constant, solar power, the greenhouse effect, the thermos bottle, geysers, changes of state, absolute zero, Laws of Thermodynamics, adiabatic process |
Knows all laboratory procedures
Prepares a plan for an experiment Participates in classroom activities Respects the scientific process Displays a scientific attitude Demonstrates skills in laboratory work |
All content |
Instructional Foci
The second section explores heat transfer. The exploration begins with conduction. To demonstrate how conduction differs according to the material being heated, place a tile and a piece of wood on the same burner and note the differences in temperature. Then discuss convection. Possible topics to discuss related to convection are why warm air rises and why expanding air cools. Then study the third form of heat transfer, radiation (radiant energy). Examples of heat radiation are infrared waves from a fireplace and infrared waves from the sun. But we also reflect some of the radiation that hits us as well as radiate our own energy, therefore maintaining some balance. To demonstrate what happens when all radiation incident on an object is absorbed paint the inside of a box white and place a small hole on one side. When the lid is on the box it appears black inside the hole because all radiation is absorbed. But, when the lid is removed students will see that it is actually white inside now that radiation is allowed to be reflected and absorbed. This same concept explains why the pupils in our eyes appear black unless they are illuminated directly—like with a camera flash, in which case they appear pink. Also discuss the imbalance of radiation flux at night that we see in the form of frost or just cooler temperatures. Then study Newton’s law of cooling, which states that an object at a different temperature than its surroundings will eventually reach thermal equilibrium with the surroundings. On an environmental note, students should be taught about the greenhouse effect, the excess heat problem, and solar power. A likely conclusion to this unit is a demonstration of the thermos bottle, which inhibits all three methods of heat transfer to some degree. An obvious demonstration is placing a hot or cold liquid of a known temperature into a thermos and into a cup that is not insulated and measuring the temperature of each at the end of class time.
Change of state is the central topic of the third section. The teacher should explore each of the changes of state, including evaporation, condensation, and boiling, melting and freezing. Water is an easy source to illustrate each of the different states (geysers are excellent examples of boiling). Also, pressure induced freezing (regelation) as well as energy induced freezing should be discussed. The changes in energy associated with the changes in states should be studied.
In the final section of the unit on heat, thermodynamics
will be studied. Thermodynamics is the study of heat and its transformation
to mechanical energy. First, the teacher should discuss the lower limit
of temperature, absolute zero on the Kelvin scale (as opposed to no known
upper limit on the temperature scale). Then the law of conservation of
energy can be applied to thermal systems as the first law of thermodynamics.
The concept of adiabatic processes, in which mechanical work is done to
increase the internal energy of a system—following the first law of thermodynamics,
should be discussed. Two concrete examples of an adiabatic process which
students should be familiar with are the bicycle pump and the compression
and expansion of the gases in cylinders of an automobile engine. The teacher
may also want to discuss changes in weather and the first law (change in
temperature ~ change in pressure). For the second law of thermodynamics,
which states that heat will only flow from hot to cold, not the other direction,
can be applied to heat engines, which change internal energy into mechanical
energy. The second law of thermodynamics, based on the heat engine, can
be restated as follows: natural systems tend to proceed toward a state
of greater disorder. For example, a closed jar full of gas molecules, when
opened will diffuse the gas molecules into the air and not vice versa.
The teacher should then conclude this unit with a discussion of the definition
of this amount of disorder, entropy.