Physics 23 Laboratory
Heat Energy Flow
In this laboratory exercise we will perform three investigations:



Find the amount of heat supplied to maintain the temperature of water at different fixed values,
and deduce the dependence of the rate of heat loss to temperatures and temperature differences.
Find the heat loss due to convection, conduction, and radiation, and deduce by what method the
rate of heat loss is greatest.
Design a system to minimize heat loss.
Investigation I. Maintaining water at a fixed temperature.
In this investigation, you are going to keep a cup of water at a constant temperature by replacing the heat
that flows from the water to the surroundings. This can be done by transferring pulses of heat to the
water using a heat pulser. In this investigation you will examine how the rate of heat lost depends on the
temperature of the water.
Materials:
PC-Temp software
Temperature probe
Universal Laboratory Interface (ULI)
Plastic cup and cover (not insulated, not foam)
Hot and warm water
Heat pulser (relay box and heater)
Prediction: If you transfer in heat to water at the same rate as it is leaving, what will happen to the
temperature of the water? What will happen if you transferred in more heat than is leaving? Less
than is leaving?
Prediction: How does the amount of heat you must transfer to keep the temperature constant
depend on the temperature of the water? For example, suppose that room temperature is 20°C
and you have two cups of water, one at 30°C and one at 60°C. Do you think you need to
transfer different amounts of heat to keep these cups at 30°C and 60°C for 5 minutes? If so,
which one will require more? Roughly, how much more heat transfer? Twice as much, three
times as much, or some other ratio?
Experiment I-1: Keeping hot water hot
You are going to try to keep hot water at a constant temperature by transferring heat with the
pulser. During the 5 minutes the experiment lasts, someone must stir the water continuously to keep
all of it at the same temperature (in thermal equilibrium).
Before you start, measure room air temperature: ___________°C.
1
1.
Prepare to graph with PC-Temp. Connect the temperature probe to DIN 1 of the ULI, and the
phone cord from the heater relay box to the Port 1. Plug the power cord into an outlet.
Double click on Logger Pro 2.2.1 and open the folder Tools for Scientific Thinking. Open the
Heat and Temperature folder and click on Direct-Current Temp Probe. Open lab04in01 DCT file.
Note that there is a Collect button and a Heat button. The Collect button starts the timing
and collects the temperature data. The Heat button turns on the immersion heater for 2
seconds each time the button is pushed. Be sure that the time axis is set to 5 minutes and se t
the temperature axis set to between 50° to 70°C.
WARNING!! DO NOT PLUG IN THE HEATER OR CLICK ON THE HEAT BUTTON
UNLESS THE HEATER IS IN THE WATER.
2.
Set up the hot water in the cup. Start with 100 ml of hot water 40°C above room temperature
(the water should be about 60°C) in an uninsulated plastic (not foam) cup. Small holes are cut
into the cover for the heater cord and temperature probe. Cover the cup and be sure that the
temperature probe and heater are in the hot water. If necessary, use the heat pulser to warm
the water before you start.
Room temperature _______+ 40°C =________°C (desired initial temperature of water)
Actual water temperature at the start: ________°C (should be about the same as above)
3.
Keep the hot water hot. When you are ready to start graphing, click on Collect. Try to keep
the water temperature constant (within a half degree). Click Heat with the mouse when you
need heat. Note: You must wait for the Heat button to come back on before you can pulse the
heater again.
The computer will make a small square on the graph when you press the button and will
keep track of the number of pulses you use. Be sure to stir the water during the heating process.
Keep stirring!
4.
In the table below, record the temperature and the number of heat pulses transferred to keep
the hot water at a constant temperature for 5 minutes.
Experiment
Temperature
(oC)
System
I-1
Room Air Temperature
I-2
Hot Water (~40 oC above
room temperature)
I-3
Warm Water (~20 oC above
room temperature)
I-4
Cool Water (~5 oC above
room temperature)
2
Difference from
room temperature
(oC)
Pulses for 5
minutes
Prediction: How many pulses do you think it will take to keep the same amount of warm water
(20°C warmer than room temperature) at a constant temperature compared to the number you
needed for hot water (40°C warmer than room temperature)?
Experiment I-2: Keeping warm water warm
Repeat Experiment I-1 you just did, but use warm water this time.
1.
Set up the water. The water should be 20°C warmer than room temperature (about 40°C). Use
about 100 ml and cover the cup again. Set the temperature axis to 30°C to 50°C.
Room air temperature ________°C + 20 °C = ________ oC (desired temperature)
Actual water starting temperature: ________°C
If necessary, use the heat pulser to heat water to the starting temperature before you start
graphing. Remember to stir.
2.
Keep the warm water warm. Start graphing. Try to keep the water temperature constant
(within a half degree). Keep stirring!
3.
Record your results in the table on page 2.
Prediction: How many pulses do you think it will take to keep the same amount of cool (almost room
temperature) water (just 5°C warmer than room temperature) at a constant temperature?
Experiment 3: Keeping cool (almost room temperature) water at the same temperature.
1.
Repeat Experiment 1 with water at almost room temperature (5°C above room temperature).
Use 100 ml again and cover. Set the temperature axis to 10°C to 30°C.
Room air temperature ________°C + 5 °C = ________°C (desired temperature)
Actual water starting temperature: ________°C
2.
Record your results in the table on page 2.
3
Questions:
1.
When you heat water in a teakettle on the stove, the temperature of the water rises. How is it
possible to transfer heat pulses to the water in the cup and not have the temperature rise? Explain
where the heat is going.
2.
About how many heat pulses would you need to keep the same amount of water at the same
temperature for 10 minutes? Explain your answer.
3.
Based on your experimental observations, write down a law that describes how the amount of heat
that must be transferred to keep water at a constant temperature depends on the temperature of the
water. Does it depend on just the temperature of the water, the difference between the water
temperature and room temperature or something else? Justify your answer using your results in the
table on page 2.
4.
Using your answer in Question 3 and your results, how many heat pulses would be needed to keep
the same amount of water in the same cup you used in this experiment 30° above room temperature
for 5 minutes? Explain your answer.
4
Investigation II. Slowing down heat flow
In this investigation we will find out how to minimize the heat flow from a container, and find out how
the heat energy flow depends on the properties of the container.
Materials:
PC-Temp software
tightly fitting covers for the cups and cans
two temperature probes
heat pulser (optional, for heating water)
Universal Laboratory Interface (ULI) 2 foam plates
3 foam cups
pieces of foam and metal at room temperature
2 metal cans, one shiny and one painted outside with flat black paint
Introduction: You have seen that when hot water is placed in an uninsulated cup on a table in a
room, heat is transferred to the surroundings. This transfer takes place by conduction through the cup
into the table and surrounding air, by convection in the hot air and evaporating water rising from the
cup, and by radiation of electromagnetic waves (mostly infrared) emitted from the hot sides of the
cup. In this investigation you will see what can be done to slow down the heat flow from the cup.
You will examine different methods and see which are more effective. The techniques for slowing
heat flow that you learn here can be applied to conserving heat energy in houses.
Prediction: Which cup do you think will cool faster, a covered cup or an uncovered cup? Do you
think that covering a cup will make much difference in the rate of cooling? Why?
Experiment II-1: To cover or not to cover?
1.
Prepare software to graph. Open the file lab04in2 DCT. Set the time axis to 5 minutes, and
set the temperature axis to 60° to 90°C.
Connect two temperature probes to DIN 1 and DIN 2 of the ULI, and the phone cable from
the heat pulser relay box to the Port 1. Set up to display and graph both probes
(Temperatures 1 & 2).
2.
Set up your foam cups. Place the two foam cups on the table on top of the foam plates, and
be ready to cover one. Cut a small hole in the cover so that the temperature probe fits tightly.
Be sure that the cover fits tightly, and that there are no other holes in it.
Pour the same amount of water (about 100 ml) at 80°C into each cup. This can most easily be
done quickly by making a 100 ml mark on both cups beforehand, and then filling to this
mark.
Now, put the cover (with probe) on one cup. Put a probe in the other cup. Both cups should be
at the same temperature, 80°C, when you start graphing. If they are not, use the heat pulser to
heat the water.
3.
Graph the temperatures. Wait until the temperature of the water in both cups is 80 oC and not
rising, then start graphing and continue for the full 5 minutes. Remember to stir!
4.
Sketch the temperature/time graphs for both cups on the axes below. Label each graph. Put
in the time and temperature scales.
5
Temp
(oC)
Time (minutes)
5.
Summarize your results. Fill in the first two rows in the following table. (You may use
Analyze on the Data menu to read the numbers from your graphs.)
Experiment
II-1
II-2
II-3
II-4
Experimental
Conditions
Final Temp
(oC)
Initial Temp
(oC)
Temp change for
5 minutes (oC)
Covered foam cup
Uncovered foam cup
Covered foam cup
Covered metal can
Covered metal can
Covered black painted can
Covered foam cup
Covered double foam cup
Questions:
1.
Which cup, the covered or uncovered foam cup, had the smaller change in temperature?
2.
Which cup lost the most heat during the 5 minutes?
3.
If you had to keep these cups at the same constant temperature using a heat pulser, which cup
would need the most pulses?
4.
Did the cover slow down the heat flow very effectively? Explain your answer, using your
experimental results.
6
Prediction: You found that a cover slowed down the heat flow.
Which cup do you think will cool faster, a covered metal can or a covered foam cup? Do you
think that one will be very different in its rate of cooling? Why?
Test your prediction.
Experiment II-2: Heat flow through a covered metal can and a covered foam cup.
Compare the rate of temperature decrease and the heat loss through a covered metal can and
through a covered foam cup. Place the can and cup on top of the foam plates as before.
Use 80°C water and the method of Experiment I-1. Be sure that both starting temperatures are 80 oC,
and that the covers fit snugly. Also, remember to start graphing after the temperature stops rising. Keep
stirring!
Before you carry out the experiment, fill up the metal can and foam cup with hot water, and
touch the side of each.
Record your data in the above table on page 6.
5.
Which cup had the smaller change in temperature?
Prediction: Which cup do you think will cool faster, a covered unpainted metal can or a covered dull
black painted metal can? Why?
Test your prediction.
Experiment II-3: Heat flow through a covered metal can and covered dull black painted can
Compare the rate of temperature decrease and the heat loss through a covered metal can and
through a covered dull black painted can. Place the cans on top of the foam plates as before. Use
80°C water and the method of Experiments 1 and 2. Be sure that both starting temperatures are the
same, and that the covers fit snugly. Also, remember to start graphing after the temperature stops rising.
Keep stirring!
Record your data in the table on page 6.
Question:
6.
Which cup had the smaller change in temperature?
7
Prediction: Which cup do you think will cool faster, a covered double foam cup or a covered single
foam cup? Do you think that one will be very different in its rate of cooling?
Test your prediction.
Experiment II-4: Heat flow through covered double and covered single foam cups
Compare the rate of temperature decrease and the heat loss through a covered double foam cup
and through a covered single foam cup. Place the cups on top of the foam plates as before. Use
80°C water and the method of Experiments 1, 2 and 3. Be sure that both starting temperatures are the
same, and that the covers fit snugly. Also, remember to start graphing after the temperature stops rising.
Keep stirring!
Record your data in the table on page 6
Questions: Use your data in the table on page 6 to answer the following questions.
7.
Which cup had the smaller change in temperature?
8.
Which seemed most effective in decreasing heat flow to the surroundings
(1)
(2)
(3)
(4)
covering the container,
substituting foam for metal,
doubling the thickness of the foam,
painting the surface of the container dull black?
List these by number in order of effectiveness from most effective in decreasing heat flow to
least effective.
9.
Why was it important to cover all of the cups after Experiment 1?
10. Which of the modes of heat flow--conduction, convection or radiation--is most affected by each of
the methods explored in this investigation?
1) covering the container:
____________________
2) substituting foam for metal:
____________________
3) painting the surface dull black:
____________________
4) doubling the thickness of the foam:
____________________
8
Investigation III. Keep it hot contest (super insulated cup caper)
In this investigation you will design a system to minimize the heat energy flow from water.
Materials:
PC-Temp software
temperature probe
Universal Laboratory Interface (ULI)
heat pulser (optional, for heating water)
Introduction: This activity is a contest to see which group can keep 100 ml of 80°C water hottest for
ten minutes.
The winning group will be the one whose water has the smallest temperature decrease over ten
minutes.
Using what you have learned about heat flow in Investigation II-2, construct a system using the
following materials to keep the rate of heat flow as small as possible.
4 styrofoam cups
2 plastic lids
30 cm x 30 cm sheet of aluminum foil
30 cm x 30 cm sheet of plastic wrap
You are not required to use all of the materials. You may cut some of the cups if you wish. You must figure
out a way to have the temperature probe in the water.
Experiment:
1.
Prepare software to graph. Open the file lab04in3 DCT. Set the time axis to 10 minutes, set
the temperature axis to 20° to 100°C, and graph only the probe in Port 1 (Temperature 1).
2.
Figure out the best arrangement to keep the water hot. Using all or some of the materials,
design a way to slow the rate of heat flow as much as possible. You must have the
temperature probe in the water.
3.
Start graphing. Put 100 ml of water at a temperature of 80°C into your arrangement, close it
up quickly, and begin graphing. (Use the heat pulser to heat the water to 80°C before
starting if necessary.)
Fill in the following as you do the experiment:
Beginning temperature:
____________(must be 80°C)
Final temperature:
____________
Change in temperature:
____________
Report your values to the instructor.
9
Questions:
1.
Sketch a diagram of your arrangement.
2.
Explain what part or parts of your arrangement were designed to minimize heat energy flow by
Convection:
Conduction
Radiation:
3.
In what ways would you improve your arrangement if you did it again? Why would you make
these changes?
10