Unit IV: Heat

 

Introduction

Heat is not the same as temperature. Heat is the energy that flows due to a difference in temperature. If I put my cold hands in front of the warm fire, heat flows from the fire and is transferred to my hands. Temperature is related to the average kinetic energy of the molecules within the material. The molecules in ice have a very small kinetic energy, whereas the molecules in boiling water have a rather large kinetic energy. This unit will further explore the difference between heat and temperature in the first section. The next section will discuss different ways to transfer heat, followed by a section on the changes of states in matter we just learned due to heat (or lack thereof). In the final section the laws of thermodynamics will be explored. About two and a half weeks should be devoted to this unit. Intended Learning Outcomes
 
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

Experiments: specific heat, coefficient of linear expansion of metals, thermometer fixed points; demonstrations: dented Ping-Pong ball in boiling water, heated tile vs. heated wood (at same temperature), black box, thermos bottle, paper wrapped around an iron bar exposed to flame; homework, quizzes, class discussion, questions for thought from the Flying Circus of Physics Teaching Strategies The first section of this unit should begin with a discussion of temperature as compared to heat (including specific heat). Then discuss using heat to increase the temperature of a material that, in most cases, will then expand. A thermostat is an everyday example of this concept in use. Place a dented Ping-Pong ball in boiling water and watch it release its dent due to expansion. Discuss the anomaly, water, when its temperature is increased from ice to above 4° C (should expand with an increase in temperature, but contracts slightly before expanding due to collapsing of ice crystals).

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.
 
 

Special Notes
  1. All labs should be preceded by an in class demonstration by the teacher of the proper procedures. After viewing the demonstration, students’ homework should be to prepare a plan for the experiment. Due to the time constraints of high school classes it is imperative that students have thought through the experiment before class. You, the teacher, need to know that they are aware of proper equipment set-up and safety precautions as well as appropriate hypothesis development, variable controls, and observation and measurement procedures.
  1. Labs can be found in Laboratory Manual in Conceptual Physics by Bill Tillery.
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