29. Heat and temperature#
29.1. Hand as thermometer#
Some classrooms have hot and cold water. If yours does not have, then send a student to quickly to get 3 beakers of water, one hot, one lukewarm, and one cold. Then let some (or all!) students put one finger in hot water and one in cold and then transfer both fingers to lukewarm. What do you feel?
29.2. Adding temperatures or not? Intensive versus extensive variables#
I have 5 glasses of water from the faucet, each at a temperature of 20 \(^o\)C. When I put two glasses together, what will the temperature be? A. 20 \(^o\)C,
B. 40 \(^o\)C, or
C. just a bit less than 40 \(^o\)C?
Nobody will get that wrong? Well, take a blind vote (students close their eyes and raise their hands with one finger for A, two for B, and 3 for C). You can check by having a student put the finger in a single glass and in the container with the contents of two glasses together.
Now another question. We have a metal bar and cut it into a piece X and Y and the volume of X is twice the volume of Y (figure X). The relationship between the densities \(ρ_x\) of \(X\) and \(ρ_y\) of \(Y\) is:
A. \(ρ_x = 2ρ_y\)
B. \(ρ_x = ρ_y\)
C. \(ρ_x = ½ ρ_y\)
Here quite a few students will go wrong. One could ask similar questions about other materials properties such as specific heat, specific resistance. Which physical properties can one add? Mass, weight, volume. Which cannot be added (temperature, density, material properties)?
29.3. Heat and friction#
Rub your hands, what do you feel? Stretch a rubber band several times and hold it on your upper lip to feel the rise in temperature.
29.4. Conduction#
Let students feel different materials, for example the metal of chairs, wood, plastics, textiles. How warm does it feel? Is it possible that these materials in the same classroom have a different temperature? If the temperature is really the same, the classroom environmental temperature, how come materials feel differently, metals feel cold, textile materials feel warm? The answer is conduction!
29.5. Convection#
What are the hot spots of our body and what are the cold spots, for example when we are outside in winter? How do we explain that? Blood circulation, distance to the main arteries, convection!
29.6. Roleplay: Melting-boiling-condensing-solidifying#
Get a group of 15 students (actors) up front and make 3 rows of 5 facing the audience. First arrange them neatly into a “crystal”. Start at the absolute zero, they are moving a little bit (there is movement at the absolute zero, remember Heisenberg!). Then crank up the temperature (the teacher uses his/her hand to indicate temperature, the hand moves up then temperature goes up) and the actors start oscillating around their fixed spot more and more wildly. Pass the melting point and actors leave their fixed location but stay in a bunch. Pass the boiling point and the actors fly out all over the room. Call them back before they are too far and start the opposite process of condensation and solidifying into a neat crystal. Throughout the activity ask the audience what the actors should do. Pass the melting point … what should the actors do? Keep asking questions which force students to think back-and-forth between atoms/molecules and this people model. What is the difference between boiling and evaporation in this model? Evaporation can only happen on the outside, at the surface. So only an actor on the surface can “evaporate”. With boiling, gas bubbles can form inside the liquid. The danger of this activity is a reinforcement anthropomorphic thinking: assigning human characteristics to atoms. So conclude with a discussion of differences between atoms/molecules and this people model. And then go to another model, for example, a PhET applet. Can atoms escape from the solid phase? Yes, sublimation. It can be played in the roleplay, but in reality it is only one in zillions of atoms that can escape. Some solids have a smell, that clearly is an indication of sublimation. A toilet refreshener is based on sublimation and there it is so much that in the end most of the solid is evaporated. But take iron, rightly or wrongly I think I can smell it but an iron bar will not evaporate away so it is only one atom in zillions that escapes. Our noses are very sensitive.
29.7. Melting and plate tectonics#
Anybody’s got some chocolate in his bag? Take the wrapper off and hold it in your hand while all of you work on this little assignment (give them a task). After a few minutes: show your hand to the class, what happened? Yes, the chocolate melted. This is what melting looks like. What do you think the approximate melting temperature of chocolate is? Chocolate happens to melt somewhere between 33 and 37\(^o\)C, just a bit below body temperature. So do not put chocolate in your pockets. The gradual process of melting chocolate is nice to observe. Think of the asthenosphere, the earth’s 3000 km thick layer of hot semi-fluid rock under the tectonic plates of rock. I always think of it like butter or partly molten chocolate. So it can flow and take the tectonic plates along with it, several cm per year.
29.8. Cooling effect of narrow nozzles#
Exhaling with mouth wide open (warm), exhaling through small opening in lips (cold []). Let your students do and feel this themselves . Expanding air cools. Compressed air heats up (feel the bottom of your bicycle pump). Or would evaporation also play a role in cooling here? Several simple follow-up experiments are possible to further investigate this.
29.9. Evaporation and cooling#
Let a little bottle of alcohol or acetone go around with an eye dropper. Students put a drop on the back of their hand and experience the cooling effect. Sorry, this experiment requires something to be taken in from outside the lecture room, but there is a good chance one of the girls can produce some nail polish remover. If so, borrow it, and pay something for replacement.
29.10. Evaporation/condensation#
In winter in cold countries look at the windows, why is there water on the inside of the window? The increasingly common double glass has decreased the effect. Better yet, do take a glass with cold water or a can of cold drink to class! If you forgot, take a glass of water from the faucet and breathe on it. It will get foggy on the outside as long as the water temperature of the glass is lower than your breath which should be near 37 \(^o\)C when exhaling with the mouth wide open.
29.11. Evaporation and diffusion#
There must be a student with a bottle of perfume in her bag. Open it, and after a while we can smell it from a distance. Or promise the girl a new bottle and put a drop on the hand of a few students and let them go around and let all students smell.
29.12. Sublimation#
The above experiment will also work with a piece of solid soap, there should be a piece in the school somewhere. If not, see whether students have any solids in their bags that smells (food?). Sublimation! Or is it a solid material that contains some gas that we smell and is it not sublimation?
29.13. Energy transport: conduction, convection, evaporation, radiation#
Remember that “heat transport” can take place through conduction, convection, radiation, and evaporation (which is a special form of convection). Actually, heat transport is a wrong term as heat is defined as energy moving between systems, thus energy in transport. A better term might be transport of thermal energy. Anyway, a trivial experiment, bring your hot cup of coffee from the teachers’ room to class. How does the coffee cool? What is conduction? How can I feel that? Touch the sides of the cup. What is convection? How can I feel that? Hold your hand above the cup. What about evaporation? How can I show that? Put a cold object (like a saucer) right above the cup, drops will form under it from evaporated coffee which condenses on the colder object. What is radiation? How can I feel that? Put your hand a bit away from the cup, but perhaps better, borrow a water cooker from the teacher room.
29.14. Keeping water warm#
Take two beakers of hot water, or better, take a full thermos and two beakers and some additional material such as saucers, a towel, old newspaper. I am going to fill the two beakers with hot water. How can I keep it warm longer? How do I prevent conduction, how convection and evaporation, how radiation? Take student suggestions, then insulate one beaker and use the other as control. Having two thermometers would be a great help, otherwise use the finger of one of the students. While waiting for the cooling, give students some questions or problem solving to chew on. I used to do this as a lab activity for all students but then with thermometers. In a first round they would just measure cooling of two open glasses to practice taking temperature-time measurements. In a 2\(^{\text{nd}}\) round they would insulate one glass in whichever way and compare with the uninsulated glass. In a third round they learned first about heat transfer through conduction, convection, and radiation and then redesigned the insulation. That third round resulted then in quite good Joule meters, good enough to do specific heat measurements [].
29.15. Cooling water quickly#
One could also do the opposite. Bring a hot coffee from the teachers’ room. It is still too hot to drink, what can I do to cool it fast? Stirring? Blowing across the top of the cup or beaker? How does this relate to conduction, convection and evaporation? (Stirring stimulates cooling at the surface through convection/evaporation. Blowing across keeps removing the evaporated and saturated coffee/water vapor thus accelerates cooling through evaporation).
29.16. Conduction, convection, radiation with a match or lighter#
Now these may not be common objects in a bare classroom anymore but should be standard in the pocket of a science teacher. With the matches one can demonstrate conduction (hold a metal object in the flame), convection (hand above the flame, but not too close), and radiation (hand to the side at some distance).
The following candle experiments can be done by the teacher (with a big candle in a darkened room and preferably with a webcam), or synchronous by teacher and students on their desks, or as student lab. Obviously when students have candles on their desks, more candles and matches are needed and it is no longer a “pocket” demo.
29.17. Describing candles and flames and formulating questions#
A candle should be a standard object in the bag of a physics teacher, just like a balloon, a ruler, and matches. Let students describe what they see: a. unlighted candle, b. lighted candle, c. just extinguished candle. Let them then think of explanations and follow-up experiments to test their explanations. For example, the unlighted candle consists of wax and a wick. Can the wax itself be made to burn with a match? (No) What is the function of the wick? Can liquid wax be ignited by a match? (No) Draw a flame, put in the colors. There seem to be three regions: blue, grayish-yellow, and bright yellow. What questions can be asked? See Faraday’s 1860 beautiful and very readable description of candle experiments in [].
29.18. Candle, what is it that burns?#
Try lighting the solid wax, it does not burn. Look at the molten wax, as if it does not burn. Turn the candle upside down, the flame gets extinguished. The liquid wax kills the flame! What is it then that burns? Light the candle, then extinguish it, and then quickly hold a lighted match at some 5 - 10 cm from the candle in the resulting whitish smoke. Wow, a flame again. It was the wax vapor that can easily be ignited. When the wick is lighted, it burns, it melts the wax, it heats the wax until vapor creeps up the wick through capillarity and the vapor is what burns. There is a small distance between liquid wax and the bottom of the flame. So the vapor burns at a few millimeters above the liquid wax. .
29.19. Exploring the temperature pattern of the flame 1#
The tips of unlighted matches can be used as temperature sensors. When approaching the flame slowly, at a certain distance it will ignite, that is where the temperature equals the ignition temperature of the material at the tip of the match. Now try this around the flame and that will produce an “isotherm” of the ignition temperature around the flame. Near the bottom of the flame one can come quite close with the match. Above the flame the match ignites at much greater distance from the flame. Explain! [] (p205).
29.20. Exploring the temperature pattern of the flame 2#
Exploration can also be done with a piece of paper by the teacher only. Hold a piece of paper horizontally above the flame and look at scorching pattern. Obviously strips of paper can also be used instead of the matches of the previous demo. The typical ignition temperature of paper is usually somewhere between 200 and 300 \(^o\)C.
29.21. Products of candle flames#
What are the products of candle flames and how can you see them? Burning hydrocarbons should produce water and CO\(_2\). Water vapor can be easily verified. Use a metal or glass object, wipe it dry, and hold it near or above the flame. Drops of water should form. See: https://engineerguy.com/faraday/pdf/faraday-chemical-history-complete.pdf See https://www.candles.org/candle-science for interesting candle science.
29.22. Preventing oxygen to reach the candle flame#
Light the candle, put a big glass upside down over the flame. What will happen? The flame goes out, no more oxygen. If available, use 3 identical candles with three glasses of different size. Which flame will go out first? So there is something in the air which is needed by the flame. More air, then also more of that stuff (oxygen).
29.23. Rising water#
Let the candle float on water in a bowl, light the flame and invert a glass over it. See the set-up in figure X. When the flame extinguishes, the water rises into the glass. There are two major reasons for the rising water. One is the oxygen that is used up but partly replaced by CO\(_2\). The other is the expansion of air while the flame is on, some air even escapes from under the glass. When the flame stops, the air contracts and water vapor produced by the flame condenses. Result low pressure under the glass so the air pressure outside the glass pushes the water inside.
29.24. Convection around candle flames#
What does a flame need? Oxygen and fuel. Now what would happen to a flame when there would be no convection around the flame? The flame would go out. How can we make the flow of air visible to our students? One could say the flame is its own windvane, just look at the shape of the flame. Another indicator is the movement of the smoke when the flame is extinguished. The smoke goes up. One could also try to obstruct the flow outside the flame in different ways and see how it affects the flame.
29.25. Convection: Tea bag rocket#
Take a tea bag from the teacher room, open it on two sides, take the tea out, and make the teabag into a cylinder and set it upright. Make sure there is nothing flammable nearby and then light the cylinder from the top. Hot air will start moving upward (convection). At a certain point the yet unburned part of the tea bag will be light enough to move upward with the moving air. For good questions and explanations see [](p208). There are many video clips.
29.26. Money surviving the flames#
Post-pandemic more people have small bottles of alcohol in their bags. I saw this especially in the Philippines, even in special small bottles which dangle from women’s bags. That makes the famous money burning experiment possible in a bare classroom. Take a glass or cup, add a little water (before students enter the room), add a similar amount of alcohol (in the presence of students). Mix the two. A little bit of salt would be nice to color the flame. Who has a dollar or Peso bill or whatever currency? (Or use your own). Soak the dollar bill into the mixture. Use a big tweezers, a laundry clip, or an improvised tweezer (two pencils with the paper money in between) to hold the bill and then ignite it with a match. In spite of the flame, the money does not burn. It is the alcohol that burns while the water keeps the temperature of the paper below 100 \(^o\) C until all the water has evaporated. The 100 \(^o\)C is way below the ignition temperature of paper which is typically above 200 \(^o\)C. In a 50-50 mixture of alcohol and water, the ignition temperature will not be reached. Theatrics can greatly increase the suspense, borrowing money from a student can make it more fun. Just a bit of salt turns the alcohol blue flame into bright orange through the presence of sodium.