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 Plastic cup and cover (not insulated, not foam)
Temperature probe Hot and warm water
Universal Laboratory Interface (ULI) 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. 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 set 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 / System / Temperature (oC) / Difference from room temperature (oC) / Pulses for 5 minutesI-1 / Room Air Temperature
I-2 / Hot Water (~40oC above room temperature)
I-3 / Warm Water (~20oC above room temperature)
I-4 / Cool Water (~5oC above room temperature)
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.
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.
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 80oC 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. 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 / ExperimentalConditions / Final Temp
(oC) / Initial Temp
(oC) / Temp change for
5 minutes (oC)
II-1 / Covered foam cup
Uncovered foam cup
II-2 / Covered foam cup
Covered metal can
II-3 / Covered metal can
Covered black painted can
II-4 / 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.
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 80oC, 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?
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) covering the container,