III. Developing Central Ideas Based on Evidence
Emily van Zee and Elizabeth Gire
What does it mean to develop an idea based on evidence? This is a practice central to doing science, to observe phenomena closely, record findings, and make sense of these results in ways that explain what is happening. Why, for example, would a child seek as a “good seat” a chair made out of wood rather than metal?
A. Developing central ideas about thermal phenomena
Some things feel cool when you touch them; other things not so much. How do their temperatures compare when measured with a thermometer?
Question 2.2 How would you rank different materials in order of temperature?
- Without touching, consider different kinds of materials often found in kitchens:
- Rank these materials in order of temperature.
- Explain the reasoning for your predicted ranking.
When all of your group members are ready:
- Touch the materials. Rank them in order of temperature.
- Explain the reasoning for your ranking.
- Talk with your group members about your rankings and reasoning.
If you want to record some of their ideas or change yours, leave your initial responses unchanged on the front of your paper and write instead on the back
- Try to come to consensus for ranking these materials in order of their temperatures.
How do you as a group explain these observations based on touching the blocks?
- Measure the temperature of all of the items with a thermometer.
There are several ways to measure the temperature of objects. One simple way is to place a thermometer on each object in turn. One also can use a set of objects made out of different materials with a hole drilled in each to hold a thermometer as shown in Fig. 2.2
In using a regular glass bulb and tube thermometer, it is important to hold on to the thermometer near the top so one’s hand does not affect the reading. Also keep holding the thermometer while waiting for the reading to stabilize so that the thermometer does not fall over and break.
- What is your ranking in order of temperature reading?
- Explain the reasoning for your ranking.
- Talk with your group members about your rankings and reasonings.
If you want to record some of their ideas or change yours, leave your initial responses unchanged on the front of your paper and continue writing instead on the back.
- Try to come to consensus for ranking these materials in the order of their temperatures.
How do you as a group explain these observations based on measurement with a thermometer?
- Write a summary of your findings and explanation.
After completing your own summary, look at an example of student work
1. Example of student work about how different materials feel to the touch
Many students report feeling confident that the Styrofoam feels warmest and the metal feels coolest, based on prior experience with these materials as well with actually touching them now. There usually are some differences of opinion about how to rank the Styrofoam and wooden blocks and in what order to rank the steel and aluminum metal blocks.
One student described her experience in ranking the temperature of the four blocks after she touched them as follows:
Four blocks were laid out in front of me. I touched all four of the blocks and I noticed that they all felt like they were different temperatures. The two metal blocks felt the coldest, the wood block felt warmer than the metal blocks, and the Styrofoam block felt the warmest. Since these different materials all felt like they were different temperatures when I touched them, this is evidence that materials differ in how hot or cold they feel to the touch.
Physics Student, Spring 2016
Another student described what happened when her group used a thermometer to measure the temperatures of the four blocks. The scale of the thermometer was marked in degrees Celsius. This temperature scale is named after Anders Celsius (1701-1744), a Swedish scientist who defined a scale of a hundred degrees between the freezing and boiling points of water. On the current Celsius scale used world-wide, the freezing point of water is 0ºC, room temperature is in the neighborhood of 20ºC, human body temperature is about 37ºC, and the boiling point of water is at 100ºC at standard atmospheric pressure. For a summary of the history of measuring temperatures, see https://www.britannica.com/technology/thermometer#ref227799.
The student reported the following experience:
Students were then asked to flip over the plates and found that each one had a small hole drilled into it. Each table was then given a thermometer and asked to measure the temperature of the plates. Our table measured first the wood and Styrofoam and found both to be 24 oC. I thought that potentially there was something wrong with my thermometer, so I warmed it up to 30 oC and then placed it into a hole on one of the metal plates. The temperature went down and read 24 oC. I could not believe it. So I tried again with the thicker plate of metal and once again the temperature was measured to be 24 oC.
Physics Student, Fall 2015
Most groups are surprised to find that all the materials are at about the same temperature. The metal feels cool and the Styrofoam feels warm but the thermometer readings for the temperature of the metals, wood and Styrofoam are nearly or exactly the same! How can this be!
One possibility is that the thermometer is broken. That is readily tested by holding the thermometer’s bulb in one’s hand. The liquid in the thermometer’s tube usually rises quickly with a warm hand on the bulb. Another possibility is that the thermometer is working properly and that the blocks are actually at or nearly at the same temperature.
Question 2.3 Why do some materials feel warmer or cooler than others?
- Talk with your group members about some possibilities for why the materials are all at the same temperature even though some feel warmer or cooler than others.
- Share your ideas with other groups. Listen closely to ideas that other groups are proposing.
- Talk with your group members about any suggestions from the other groups that seem helpful. Refine your group’s ideas or pursue some new possibilities for explaining what was happening when you touched the different materials.
- Share your current ideas with the other groups. Listen closely to ideas that other groups are proposing now.
- Keep talking and refining ideas until your group and the other groups reach a consensus on some central ideas about what must be happening in order for the materials to have the same temperature but to feel so different when touched.
- Write a summary of the central ideas based on evidence that emerged from the small group conversations and whole group discussions.
After completing your own summary, look at an example of student work about thermal phenomena, nuances in exploring how hot or cold different materials feel to the touch, and some thoughts about the nature of science in this context.
1. Example of student work about developing central ideas based on evidence about thermal phenomena
A student interpreted the surprising finding that materials can feel different even if at the same temperature as follows:
…the thermometer did not read a different temperature for any of the four plates. This made students realize that since each of the plates was found to have the same temperature, that they were all at room temperature. Essentially each of the plates had been left for a long time untouched in the room and had not had a heat source or sink that would have affected them. This means that they all were the same temperature.
Physics Student, Fall 2015
Usually while discussing why the blocks are all at the same temperature, someone eventually utters the phrase room temperature. Occasionally someone will use the phrase ambient temperature. This has a more general meaning referring to the temperature of surroundings, sometimes used to refer to the temperature of the air surrounding a big computer. Once uttered, the phrase room temperature shifts most students from puzzlement to acceptance of the finding that the four blocks are all at the same temperature.
What remains is puzzlement about why the blocks feel so different even though they are all at room temperature.
The small group reports of current thinking may range from “I have no idea” to thoughts that hint at the next step. Someone, for example, may say something about hands being warmer than all of the blocks and someone else may struggle to express an idea about the materials of the different blocks being different in some way, hinting at the idea of a particular property of the materials that is making a difference.
Having heard these still-emerging ideas hesitatingly expressed, the small groups may make some progress if given another opportunity to talk with one another. Another round of reporting out may yield a well-articulated explanation that the class as a whole then adopts. This student, for example, continued with such an explanation:
Students were surprised to find that all of the plates were the same temperature, because when they placed their hands upon them, they could feel that the plates were colder or warmer than one another. What students came to realize is that what they were feeling was energy transfer by conduction. Essentially the metal objects are conductors of heat, which means that the energy from the students’ hands was flowing from their palm into the metal…which left our hand feeling colder as the energy was leaving.
In this experiment, the students found that metal was a conductor, meaning that the metal transfers energy more quickly and has a high thermal conductivity. However, the Styrofoam, a substance often used for coolers, is an insulator, meaning that it has a low thermal conductivity, meaning that it transfers energy more slowly so it can keep hot things hot and cold things cold.
Physics Student, Fall 2015
The rounds of conversations that result in such a clear statement of what is happening take time, but they seem to help students make sense of what may have been a very puzzling experience. A goal of this course is to raise issues but to have students resolve these issues through small group conversations and whole group discussions. The intent is to create opportunities for students to experience science in ways similar to the ways that scientists experience science, as both interesting and comprehensible.
2. Nuances in exploring how hot or cold different materials feel to the touch
Students who held the bulb of their thermometer in their hands noted that the temperature indicated by the thermometer was higher than the temperature that the thermometer indicated for the four materials. They observed that there was a temperature difference between their hands and the blocks and inferred that energy flowed from their hotter hands into the metal blocks, which lowered the temperature of their hands.
The inference that energy flows only from hot to cold can be hard to accept because prior experiences may suggest a different direction. When one is sitting on a metal bleacher at a late fall soccer game, for example, the perception likely is that cold is seeping into one’s body. However, the inference here is that what is actually happening is that one’s body heat is flowing out into that entire metal bleacher!
The metal blocks felt cold; the Styrofoam felt warm. The inference is that the metal blocks differed from the Styrofoam in the property ofhow easily the metal blocks conducted energy away from the students’ hands, the property of thermal conductivity.
The inference is that more energy flowed out of the students’ hands into the metal blocks than into the Styrofoam block. The energy flowing into the metal blocks spread rapidly throughout the blocks; the metal blocks had a higher thermal conductivity. The students’ hands lost more energy to the metal blocks and therefore felt cold when touching the metal blocks.
Another inference is that the energy flowing into the Styrofoam blocks did not spread throughout the Styrofoam blocks but stayed near where the hands were touching the blocks. The students’ hands lost very little energy to the Styrofoam blocks and therefore their hands continued to feel warm
This introduces central ideas about the transfer of energy from hot to cold objects and the ease with which such energy transfer occurs. The inference is that energy flows quickly through materials that are conductors, which have a high thermal conductivity; energy flows slowly through materials that are insulators, which have low thermal conductivities.
Note that both judgments about the four blocks were based on evidence. Students produced rankings that differed based on observations made by touching the blocks. Students produced rankings that were the same based on observations made with a thermometer.
The usefulness of the rankings would depend upon the purpose. If one is choosing a material on which to sit on a cold day, rankings in terms of thermal conductivities would be helpful. If one is interested in the temperature of a room (without a heat source or sink), one could choose to use a thermometer to measure the temperature of an object made with any of these materials, the number obtained would be the same, or close to the same, for all. (Holding the object for a long time while making the measurement might change its temperature if the object has a high thermal conductivity.)
3. Some thoughts about the nature of science in this context
Science involves making judgments based on evidence. One needs to be aware, however, both of the nature of the evidence and its appropriateness for answering a question. When puzzlements occur, one may need to clarify ambiguities in formulating the question as well as in designing an exploration. Asking How would you rank different materials in order of temperature? turns out not be answerable by ranking materials by how they feel to the touch. Underlying the mismatch of that question with the suggested procedure is a conceptual distinction between heat and temperature.
B. Clarifying distinctions between closely related ideas
If something seems puzzling, one way to seek a better understanding is to ponder ideas that seem closely related, are they the same or different?
Question 2.4 What is the difference between the concepts of heat and temperature?
A useful way to organize outcomes is to review the set up, evidence, and relevant vocabulary for central ideas that emerge from explorations and discussions, as in Table II.1
- Clarify for yourself the difference in the meaning of the words heat and temperature in the context of physics by completing the following table:
- Make a sketch of the set up with the four different materials
- Note the evidence of how the materials felt when touched and what their temperatures were as measured by a thermometer
- Define any relevant vocabulary.
- Then write a summary of the central ideas about thermal phenomena developed so far in this unit.
|TABLE II.1 Explorations of Thermal phenomena|
|Sketch of set up||Evidence||Central Ideas||Relevant Vocabulary|
|Materials differ in how hot or cold they feel to the touch|
|Temperature is measured by a thermometer|
|Materials left for a long time without a heat source or sink in the room come to the same temperature, room temperature||Room temperature|
|A temperature difference implies a flow of energy from hotter objects to colder objects. When the objects are touching, this process is called energy transfer by conduction.||Conduction|
|Materials differ in their thermal properties such as how well they conduct energy throughout the material, their thermal conductivity. Conductors have high thermal conductivities. Insulators have low thermal conductivities.||Thermal Conductivity
|Heat and temperature are different ideas|
After completing the table above and summarizing your understanding of each central idea, look at an example of student work.
1. Example of student work clarifying the meaning of the words heat and temperature
Figure 2.3 shows one student’s notes for the table above. Also presented is this student’s summary of the central ideas about thermal phenomena developed so far in this unit.
For “sketch of the set up” in the first row, this student drew the four blocks in order from “feels coolest” on the left to “feels warmest” on the right and labeled the blocks “light metal, dark metal, wood, Styrofoam.” For evidence, the student wrote “I touched the four plates and they physically felt different.”
In the second row, the student drew four blocks with four thermometers and wrote “We put the thermometer in a hole in each plate and found their temperatures.”
In the third row, the student drew the four blocks and two wall clocks showing the times of 1 and 2 o’clock. The student wrote, “The materials have been sitting out for an hour and they are the same temperature.”
In the fourth row, the student drew a picture of a block and hand, with a representation of energy flowing out of three of the fingers of the hand into the block. The student wrote, “The plate felt cold because the energy left my hand and went into the plate.”
In the fifth row, the student drew a picture of two blocks, each with a hand touching the block. The hand touching the metal block has a representation of energy flowing out of four fingers; the hand touching the Styrofoam block has a representation of energy flowing out of only one finger. The student wrote, “The metal felt colder than the Styrofoam because it conducts energy better.”
In the sixth row, the student drew a picture of a thermometer and labeled it “temperature” and a picture of a block with a hand on it and labeled “heat”. The student wrote : “The temperature is a number measured by a thermometer and heat is the energy that is flowing or not flowing from us into the metal.”
The student wrote the following rationales for the central ideas claimed in the third column of the table:
Materials differ in how hot or cold they feel to the touch. Four blocks were laid out in front of me. I touched all four of the blocks and I noticed that they all felt like they were different temperatures. The two metal blocks felt the coldest, the wood block felt warmer than the metal blocks, and the Styrofoam block felt the warmest. Since these different materials all felt like they were different temperatures when I touched them, this is evidence that materials differ in how hot or cold they feel to the touch.
Temperature is measured by a thermometer. The thermometer that we used to measure the temperatures of the four blocks was long and narrow and had rounded edges. The thermometer was made out of glass and it measured the temperatures in degrees Celsius.
Materials left for a long time without a heat source or sink in the room come to the same temperature, room temperature. The four blocks of different materials were laid out for a long time without being touched at all. There were no heat sources or sinks in the room. This allowed the blocks time to sit in room temperature.
Each of the four blocks had a small hole in it that allowed the thermometer to sit inside of the material. To measure the temperature of each of the blocks, I held the thermometer in the hole for a minute by holding it at the top so that my hand did not affect the temperature. After waiting for a minute for the thermometer to finish reading the temperature of the material, I recorded the temperature of each of the blocks.
The light-colored metal block was 18°C, the dark-colored metal block was 18.25-18.5°C, the wood block was 18.5-18.75°C, and the Styrofoam was 19°C. Even though the blocks differed in how hot or cold they felt to the touch, the readings are all within 1°C of each other. Because the readings on a thermometer for the four blocks of different materials were all relatively the same temperature, room temperature, this is evidence that materials left for a long time without a heat source or sink in the room come to the same temperature, room temperature.
A temperature difference implies energy is flowing from hot objects to colder objects.Four blocks that are room temperature were laid out in front of me. We know that the blocks are room temperature because they were left for a long time without any heat source or sink in the room.
When students touched the blocks, they noticed that some felt colder or warmer than others. Human body temperature is warmer than room temperature, so my hand is warmer than the blocks. When I touched the metal block it felt cold. There is a difference in temperature between my hand and the metal block, which is what allowed energy to flow from the warmer object, my hand, to the colder object, the metal block. Energy leaving from my hand to the metal block made my hand feel cold. Metal has high thermal conductivity, so energy is quickly transferred from my hand and I feel the temperature difference right away.
The wood and the Styrofoam are insulators, so they have lower thermal conductivity. Because the transfer of energy takes longer for these materials, there is less energy flowing, and as a result my hand does not feel as much of a temperature difference.
Energy transfer by conduction, in this context, refers to the rate that energy is transferred by touch between two objects, such as a hand and a metal block. Energy is transferring from my hand to the blocks, because they are different temperatures. My body temperature is warmer than the metal block and when I touch the metal block my hand feels cold, which is evidence that a temperature difference implies energy is flowing from hot objects to colder objects.
Materials differ in their thermal properties such as how well they conduct energy throughout the material. Thermal conductivity is referring to the rate or speed that energy is transferring between two objects when they touch. First of all, metal is a conductor. When I felt the metal, it felt the coldest out of the four blocks. Since the metal has high thermal conductivity, it rapidly transfers energy. So, when I touch the metal, energy leaves my hand quickly, so there is a lot of energy flowing, therefore leaving me to feel a big temperature difference between the object and my hand.
Next, the wood and the Styrofoam are insulators. The wood and the Styrofoam have low thermal conductivity, so they slowly transfer energy. Styrofoam transfers energy slowly so that it can keep hot things hot and cold things cold, which is why it is often used for coolers. Since it takes a while for all of the energy to transfer from my hand to those materials, there is less energy flowing and I feel less of a temperature difference between the object and my hand.
So, because metal which is a conductor and wood and Styrofoam which are insulators feel like they are different temperatures to the touch, this tells us that they conduct energy at different rates. This difference in rates for conducting energy is evidence that materials differ in their thermal properties such as how well they conduct energy throughout the material.
Heat and temperature are different ideas.…Temperature is a number measured in degrees. In class we used a thermometer to measure the temperature of the different blocks. According to the thermometer, the temperature of the light- colored metal block was 18°C, the dark colored metal block was 18.25-18.5°C, the wood block was 18.5-18.75°C, and the Styrofoam was 19°C. So, the numbers measured by the thermometer were the temperatures of the blocks.
Heat can be thought of as a feeling, for instance the metal blocks felt colder than the other blocks, however what the students are feeling is the transfer of energy. Heat is the energy that is flowing or not flowing from us into the metal. So, when I touched the metal with my hand, the energy from my hand rapidly transferred to the metal because it is a conductor. The heat is what one feels because of the transfer of energy from one thing to another. Less energy was flowing from my hand to the wood, and even less to the Styrofoam, because they are insulators and are slower at conducting energy.
Because the different materials felt like they were different temperatures, the rate of the energy flow of each of the materials is different. Even though the rate of the energy flow of each material is different, the temperatures on the thermometer are all within 1°C of each other. So, since the thermal conductivity of the materials are different while the temperatures of the materials are about the same; this is evidence that heat and temperature are different ideas.
Physics student, Spring 2016
This student recognized that even though the measured temperatures were in the expected direction, with the Styrofoam block’s temperature slightly higher than the light-colored metal block’s temperature, a measured difference within 1°C could not explain the very large difference felt when touching these materials. The inference is that the large difference in how these materials felt to the touch was due to a large difference in a property of the materials, in how well they conducted energy from warm hands, their thermal conductivities.
Styrofoam is an insulator; it has a low thermal conductivity. Energy flowing from a warm hand to the Styrofoam stayed where the hand was touching the Styrofoam; therefore, energy stopped flowing from the hand to the Styrofoam when the hand and the small place where the hand was touching the Styrofoam became the same warm temperature. The rest of the Styrofoam remained near room temperature.
The metals, however, are conductors; they have high thermal conductivities. Energy flowing from a hand to the metal continued flowing and spreading throughout the metal block. Energy continued to flow from the hand, noticeably cooling the hand so the metal felt cool.