Lesson Plan: What Makes Something
Feel Warm? Modeling Energy Transfer:
A Macroscopic and Particulate View
FOR THE TEACHER
Background Information for Teachers
Although most students have a common sense idea about what energy
is, they may find it difficult to give a precise definition. Teaching
students about chemical energy can be even more difficult. This may be
due in part to the fact that we often postpone discussion of energy until
late in the year, when we cover topics such as thermochemistry and
thermodynamics; despite the fact that energy is a cross-cutting concept
that accompanies every change in chemistry.
Submitted by
ACS High School
Professional
Development Team
Washington, D.C.
As Richard Feynman said, “It is important to realize that in physics today, we have no
knowledge of what energy is!” Our main experience with energy comes when we recognize
energy changes. In this session we will explore changes in thermal energy, such as how we feel
when we forget to put on a sweater during cold weather, or when we wear too many clothes
when it’s hot outside. Students’ understanding of heat, temperature and chemical systems
needs to be carefully constructed.
Objects/systems do not contain “heat.” The motion of molecules is not “heat.” The term “heat
content” is historic and archaic (even though it unfortunately continues in use) and comes from
a time when heat was visualized as a fluid called “caloric.” As a corollary, heat does not “flow.”
Heat is a measure of the energy that is transferred from one object to another
due to a difference in their temperatures. The energy transferred is often called “thermal
energy,” since it is dependent on there being a temperature difference. The form of the transfer
is the standard stuff—conduction and radiation. It’s OK to call the amount of energy transferred
“heat” as long as we are clear that “heat” is a number (with units) that is not within the objects
themselves, but is the measure of a process.
Heat is an artifact of the transfer of energy. For thermal energy transfer, molecules must come
into contact with each other. The transfer of energy occurs continuously on a
particulate/molecular level. On the macroscopic scale, however, we will notice that “Thermal
Equilibrium” is reached when the temperatures of the objects are equal.
The idea is that energy can move from one particle to another in a given system
randomly. There are more distinguishable states (therefore more probable) when the energy is
evenly distributed-hence equal temperatures.
Whether something feels hot or cold depends on the rate of thermal energy transfer, which
depends on specific heat capacity, thermal conductivity, mass and density. The material
scientists have a term called “Thermal Effusivity, which relates all of the variables in the
equation. Thermal effusivity is the square root of the product of thermal conductivity ( λ )
and heat capacity ( c ) and density, (ρ):
Thermal Effusivity = √𝝀 ⋅ 𝒄 ⋅ 𝝆
For more on Thermal Effusivity, see Dr. Jack Josefowicz’s explanation.
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Temperature is defined as the measure of the average kinetic energy (KE) of the particles.
Though students have been exposed to temperature measurements, they often assume that
temperature is a measure of how hot or cold an object is. This is clearly not the case, and the tile
experiment shows this well. However, it is difficult for students to have a clear understanding
about WHY we define temperature in this way. Many textbooks will claim that a postulate of the
Kinetic Molecular Theory is that temperature is proportional to the average KE of the particles,
yet this idea was actually derived from the Ideal Gas Law, and the derivation is sometimes
outside of the scope of middle and even high school level courses. Here is a screencast of the
derivation for your information. You may choose to include this in your instruction, depending
on your students. One must have knowledge some physics and a working knowledge of algebra
to fully understand the derivation; therefore, it may be enough for your students to see the
correlation between temperature and kinetic energy by looking at the rate of diffusion of dye in
water with different temperatures. See Supplementary Materials Demonstration #1: What is
Temperature?
The initial question about moms and sweaters is designed to get students thinking about the
idea of thermal energy transfer. This question serves as an informal cognitive assessment of
student ideas about hot and cold.
The second question is to have students think about materials that will help the frozen chicken
defrost more quickly. This question elicits student thinking about materials and what might
help in defrosting. This serves as an engagement to the Designing a Fast Defroster for the
Forgetful Chef Lesson. If you choose not to do this lesson, you may consider skipping the
second question.
Students are typically surprised by the results of the first activity. This provides a great
opportunity to construct accurate ideas about energy transfer, “heat”, temperature and
perception. Hopefully the class will agree that whether something feels hot or cold doesn’t
predict temperature.
Students initially think the aluminum tile is cold because it feels cold. But the aluminum tile is
at room temperature. Their fingers, however, are warmer (~98० F), so there is thermal energy
transfer from the hotter finger to the ‘colder’ tile. When this happens, our finger feels ‘cold’. The
same thing happens with the other tile, but the big difference is the rate of energy
transfer. Aluminum is remarkably conductive, 237 W/m-K. On the other hand, the plastic tile
has a thermal conductivity, 0.033 W/m-K.
It is important to let students work through these ideas by sharing their ideas and hearing what
others think. Teachers can guide these discussions to help students construct accurate
understanding.
You might also find the opportunity to extend this activity when discussing specific heat. Many
students are confused by the idea of specific heat, and think that it is how much heat something
can hold. To battle that idea, one must be clear that the definition is the amount of energy
needed to change the temperature of a certain amount of a substance. If we think about
particles, it is the amount of energy needed to be transferred so that the particles increase their
average KE. There are two supplementary demonstrations that provide observations and data
for students to build a better understanding of heat capacity. See Supplementary Materials:
Demonstration #2: Measuring Heat and Demonstration #3: Dramatic Demonstration of
Thermal Conductivity and Specific Heat Capacity.
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