Heat Exchange/Collection Activity
By
Jim Roberts, Professor of Physics and Material Science
University of North Texas
OBJECTIVE: This activity is designed to show how the basic laws of heat
exchange can be used to show that dark objects and light colored objects collect
heat energy at different rates and radiate at different rates. Data collectors (EA200) and graphing calculators (CFX-9860GC plus) are used to show how the
data can be collected and then displayed for analysis.
Introduction
The basic law of heat exchange is summarized in figure 1. This is an expression
of the conservation of energy law that pervades all of nature. The sun
exchanges its heat with a specific sample of water contained in a glass reservoir.
Two different color containers are used in this experiment to collect heat energy
into reservoirs of water. Energy from the sun falls on the surfaces of the
containers whose surface area is known. The heat energy is allowed to flow into
the volume of water for a given time. When the desired time has passed the
containers are taken from the area where the sun’s energy is falling on them and
placed in a shade. The cooling rate for each container is then monitored by
using the EA-200 to collect the temperature over time for about 1000 sec at
intervals of 10 seconds.
Figure 1. A basic diagram that shows the law for heat exchange. The sun loses
energy and the water in the container collects (gains) some of this energy. The
rate for this exchange is monitored by using the EA-200 data collector. The CFX9850 GC plus graphing calculator is used for display of the data. The data are
transferred from the calculator into a computer and reduced by an Excel routine.
Procedure
Put 200 ml (cm3) in each of the heat collectors. One of the collectors is colored
with white paint sprayed on and allowed to dry. Two coats were applied in this
experiment and glass containers were used to contain the water. A second
container with identical amount of water and the same volume of container is
painted black (flat black) with two coats. The two containers are then placed
such that the direct energy of the sun can fall on them. Both containers are
made identical in every way except for the color of the paint on the surfaces.
This procedure reduces the amount of arithmetic that you have to do to make
any calculations.
Figure 2. A picture of the apparatus needed to collect energy from the sun. This
energy collection rate was used to understand heat exchange with surfaces of
different colors. Data were collected using the EA-200 data collector and
displayed using the CFX 9859 GC plus graphing calculator.
Figure 3. A close up view of the heat collector used to measure the energy flux
of the sun. The black container makes both a good absorber of heat and a good
radiator of heat. The data to show the different rates of absorption and radiation
by black and white surfaces is presented in figures 4 and 5.
Place the two containers in position with a temperature probe in each one with
one probe in channel 1 and the other in channel 2 of the EA-200 Data Collector.
Collect data for about 30 minutes to see how the surfaces are collecting heat
over time.
TEMPERATURE (DEG C)
34
32
30
28
26
24
1
11
21
31
41
51
61
71
81
91 101
ELAPSED TIME(SECx10)
Figure 4. A plot of heat energy being collected in two heat collectors. The one
with the larger slope is the black container. The lower curve is the profile for heat
gain in the white container. The curves follow a linear plot over the range before
heat loss becomes a problem. The two curves show that the heat gained by the
black container is more rapid than heat gain in the white container, The “knee”
observed in the curve at about 600 sec is due to the fact that the heat loss rate
becomes important in the picture.
When the data have been transferred to the CFX 9859 GC plus graphing
calculator for storage, reprogram the EA-200 for 1000 seconds total time at10
second intervals to collect data for the cooling part of the experiment.
This experiment can be used to show how different surfaces absorb and radiate
heat at different rates. The difference in land mass temperatures and water
temperatures plays a role in weather patterns. Rocks absorb and radiate heat at
a different rate than the dirt and sand. All of this plays a role in how our globe
responds to the energy from the sun.
Change the experiment by surrounding the containers with different color paints
and make predictions of what the effect will be on the heat energy collected by
the heat reservoirs.
TEMPERATURE (DEG C)
35
34
33
32
31
30
29
28
1
4
7
10 13 16 19 22 25 28 31 34 37 40 43 46 49
TIME (MINUTES)
Figure 5. A graph showing the plot of heat energy radiating from two heat
collectors. The one with the smaller slope is the white container. The lower
curve is the profile for heat loss from the black container. The curves follow the
exponential law of cooling. The two curves show that the heat lost from the black
container is more rapid than the loss from the white container.
Questions
1. You took only a small sample of the energy from the sun with the heat
collector. Does it seem reasonable that this small amount of heat will
behave the same as if we have a very large lake of water to collect the
heat? Keep in mind that the surface area is very important in collecting or
radiating heat energy.
2. If you live near a lake and a mountain range, what kind of activity can you
expect in the movement of air as the sun goes down and then rises the
next day?
3. Describe what the wind patterns will be for people who live along a sea
shore where the water and land mass cool and absorb heat at different
rates.
4. A group of farmers approach you and ask you to calculate the amount of
solar energy absorbed in their fields if the field’s land is red dirt? and if the
land is black dirt?
5. Will the answer to problem 4 determine what kinds of crops can be
planted in the soil to provide the best growing conditions?
6. In Alaska in some places the sun shines over 20 hours a day part of the
year. During the night time the earth in that area cools by evaporation
(loss of heat by radiation). Do you think the differences in nighttime
temperatures and the daytime temperatures are as extreme as observed
in the more temperate zones where the days and nights are close to
equal?
7. Set up an experiment using the EA-200 and the graphing calculator
whereby you can show that the temperatures in a “concrete city” and in
the outlying countryside change in a much different way during the cycle
of night and day in the area. Use what you have learned in this exercise
to devise your set up.