Chillin Out: Designing an Insulator

Transcription

1 SHPE Jr. Chapter May 2015 STEM Activity Instructor Resource Chillin Out: Designing an Insulator Students learn about the three ways heat can be transferred from one object to another. They also learn what makes a material a good insulator. Working in teams, students design a system intended to keep a cup of water cool for an hour. Learning objectives Understand thermal conductivity and the three types of heat transfer Assess materials as conductors or insulators Understand how different materials can facilitate or slow heat transfer Predict which materials in what type of design will best insulate a cup of cold water. Engineering/STEM areas: Thermodynamics, materials science, product design Materials Student Resource Sheets (in lesson) Student Worksheets (in lesson) A pitcher or thermos with water and ice in it (enough water to fill one cup per group of students) Waxed paper cups (Plain paper may leak. One per group of students) Thermometer (one per group of students) Materials to use for insulation, such as foil, cotton balls, felt, moss, cardboard, paper, duct tape, straws, string, yarn, packing foam, styrofoam cups, sand, cloth or clothing such as socks or mittens, coffee filters, aquarium marbles, etc. Plastic wrap to hold the insulating materials around the paper cups Time required 45 mins + 15 mins. Note: the activity requires that the insulated cups sit for an hour. During that time, students can do the life skills portion of the class. Suggested group size: 2-3 depending on number of students

2 Preparation 1. Read through both the student and instructor resources so you have the background information 2. Gather all the insulation materials that students will have access to. Suggestions are listed in the Materials section above. 3. Make enough copies of the Student Resource so that each student has one 4. Make one copy of the Student Worksheet per group, plus a few extras 5. Build your own insulated cup that you can show as a demo 6. Set aside an area where students can set their cups during for an hour after they ve constructed them. If possible, make this spot easily accessible so that students can take a temperature reading after 30 minutes. 7. Collect a few insulating containers that do not rely on a vacuum (i.e. not a Thermos). You could use an insulated lunch bag, stryofoam or regular foam insulators, potholders, etc. Procedure 1. Pass around the insulating materials you ve collected and ask students to consider what they re made of. 2. Point out that these items can be used both to keep something warm and to keep it cool. Ask how that can be. 3. Give students this scenario: It s a warm spring day (hopefully it actually will be!) and they re having a picnic. Unfortunately, they forgot a cooler or ice, and they need to devise a way to keep their drinks cold. Fortunately, there are lots of materials in their backpacks and in the environment around them. Their task is to come up with a way to insulate their drinks. 4. Go over the information in the Student Resource and Worksheet, making sure students understand the three types of heat transfer and the properties of heat conductors and insulators. 5. Go over the materials available to build the insulators. Give each group a cup, and allot about minutes for students to design their cup insulators on paper. If you want students to be able to compare how different materials insulate, assign different sets of materials to different groups. 6. Next, give students about 20 minutes to build their cup insulators. They should wrap their cups in only one or two layers of plastic wrap, to hold the insulating material together. (You may want to ask students to consider whether or not plastic is a good insulator.) 7. When each group finishes their insulated cup, fill the cup with ice water and give the students a thermometer to put in the cup. Tell students to let the thermometer sit for about a minute. When the temp looks like it s stopped going down, they should record the thermometer reading on their student worksheet.

3 8. When all the cups are full, tell students to place them in the area you set aside for them (with the thermometers in them). Tell students the cups will sit for an hour, after which students will compare starting and ending temperatures. 9. If possible, give students time to record a temperature reading halfway through the hour. 10. After an hour, students should record their thermometer readings. Ask each group how much the temperature rose in their cup. Compare how the different groups materials and designs performed as insulators. Assessments Each group should describe the following to other students: Why they think their insulator performed as it did, based on the properties of the materials and the types of heat transfer that occurred. They need to describe specific properties the materials must possess to have been as effective (or ineffective) as they were. Students can compare their chosen materials with their classmates choices or with other materials they decided not to use. What improvements they would make to their design and why Extensions Have students research different types of housing insulation and come up with three different insulation plans suited to three different climates. Discuss the way that vacuums can be used for insulation, and why they can be good insulators. Use examples such as a Thermos, Experiment and calculate the thermal conductivity of different materials. ( has an interesting example with lots of background information.) Resources/Bibliography BBC Keeping Warm lesson plan html Maine Energy Education Curriculum Insulation Materials Investigation uoh_insulation/uoh_insulation_activity1.xml Keep It Cool lesson plan

4 May 2015 SHPE Jr. Chapter STEM Activity Student Resource Chillin Out: Designing an Insulator It s a warm spring day. You and your friends are headed to the park for a picnic, toting bottles of cold water and juice. You ve got a variety of ways of keeping those bottles cold. Some are in foam cozies, others are on ice in a plastic cooler, and others are tucked away puffy bag lined with silver material. On a chillier day, you might have warm foods packed in the puffy bag and be putting those cozies around cups of hot cocoa. How is it that the same insulator can keep things both hot and cold? It s all about heat transfer Insulating something isn t really about keeping that thing hot or cold, it s about maintaining a certain temperature. The way an insulator maintains temperature is by slowing the process of heat moving from one place to another. For example, on a cold day, heat leaves your bare hands and goes into the air around them. When you put your hands in your pockets, two things happen: you slow the transfer of heat from your hands to the air, and some heat from your body is transferred to your hands. There are three different ways that heat can move from one place to another, and you probably see examples of each one every day. In order to understand them, we need to think about the difference between heat and temperature. Temperature is a measure of the kinetic energy of particles in a substance or object. Heat is the movement of that energy from the substance or object to another substance or object. Since this movement occurs in a world ruled by entropy, the energy (heat) will always move from the object with the higher

5 temperature to the object with the lower one. Each of the three modes of heat transfer, therefore, has to involve a transfer of energy. And how that transfer happens is related to the properties of the materials involved. Conduction: transferring energy through collisions The most familiar and visible form of heat transfer is conduction. Conduction happens when two objects of different temperatures are in contact. When you wrap your cold hands around a warm mug of coffee, you re heating your hands through conduction. Conduction transfers energy through collisions between particles. The particles in the warmer substance, in this case, the coffee, have a higher temperature, and therefore more kinetic energy. When they collide with the less-energized particles on the sides of the cooler ceramic coffee mug, some of their energy is transferred to those ceramic particles. And those now-energized ceramic particles collide with other particles in the mug, transferring energy again. As this process happens repeatedly, the kinetic energy of the particles (in other words, the temperature) of the mug goes up and the temperature of the coffee goes down. When you wrap your hands around the mug, the same process happens between the outside of the mug and your hands. Particles in a substance heated by a flame at one end become more energized. When they collide with other particles, they pass that energy along. Sometimes an engineer is tasked with designing something (like an insulating cup, for instance) intended to slow the rate at which heat is transferred. In that case, she wants to choose materials that are good thermal insulators (in other words, materials that aren t good thermal conductors). There are many factors that can affect the thermal conductivity of a material. Here are two of the most important ones: Density of particles: The more tightly packed the particles are in a material, the more readily it will transfer heat. When particles on the inside of the mug get energized by colliding with hot coffee molecules, they ll quickly run into neighboring ceramic particles and pass the energy along to them, too. The more particles there are nearby, the more likely the

6 newly energized particles will run into them. And the more run-ins there are between particles, the faster the energy will be passed along. Air permeability: Materials like foam and Styrofoam are good insulators because they trap air inside them. Gases and liquids can be good insulators: they fill up space using fewer particles than a solid such as ceramic. If you fill a Styrofoam cup with coffee, for example, heat gets trapped in pockets of air inside the styrofoam. That air takes longer to receive and pass long energy than a solid would. Metals conduct heat very readily in a different way. If you were to pour that hot coffee into a metal cup, you d find the metal got much warmer, and heated much more quickly, than the ceramic. That s because metals can conduct heat in a way similar to how they conduct electricity, by using electrons. As the electrons move through the metal, they transfer energy. This thermal conductivity isn t the same as electrical conductivity, so thankfully you just get some hot metal but you don t get a shock! Convection: circulating currents of heat Conduction works well in solids but not so well in liquids and gases, because the particles are much farther from each other. A phenomenon called convection is the way heat moves through liquids and gases. You ve seen convection at work in a pot of boiling water. Heat is initially transferred to the water through conduction, when the pot is put on a hot burner. The water at the bottom of the pot gets heated, the particles gain more energy and move away from each other, making the hot water less dense. The less-dense water rises toward the top of the pot, and the cooler, denser water falls to the bottom. The warmer water at the top cools off as it moves away from the heat source, and the water that s fallen to the bottom of the pot gets warmer and less dense. This change in densities sets up a cycle of water falling and rising in the pot. The cycle creates convection currents, which you can see as the rolling boil of the water. When you heat your room in the wintertime, you re also making use of convection. A radiator heats the air around it, making that air less dense. The warm air rises, pushing the cool air toward the bottom of the room, where it s

7 heated by the radiator and rises. The air moves around in much the same way the boiling water does. Convection may be less visible to us than conduction, but it can be found all around us. Convection currents are constantly moving through the ocean, and convection currents in the atmosphere contribute to our weather. It s important to note in important difference between convection and conduction: In conduction, all the material that s transferring heat stays pretty much where it is. The heat passes through the material by collisions of individual particles. In conduction, the particles and therefore the material itself moves. Radiation: electromagnetic waves We feel warmth from the sun, but we certainly aren t in contact with the sun for conduction to occur. And it s not convection currents that give us sunburn. We feel the sun s warmth because of a third form of heat transfer: radiation. Radiation is the transfer of thermal energy using electromagnetic waves. These waves are just like the electromagnetic waves that we use to broadcast a radio signal, take an x- ray, or see as light. In fact, heat often radiates in wavelengths that make up the Infrared photo of hands, showing how they radiate heat of different temperatures. infrared part of the electromagnetic spectrum, and have just slightly less energy than waves our eyes are sensitive to. With special lenses and filters, we can take photos of objects and see the infrared (or heat) radiation. The hotter the object, the higher the energy of the thermal radiation it s sending out. Take your toaster, for example. When it s not toasting, the coils inside are a plain gray. Those coils probably aren t radiating heat because they re the same temperature as their surroundings. But when you turn the toaster on, those coils heat up fast and furious. The particles in them get so much energy that they radiate heat in wavelengths that have enough energy for us to see them. That s why we can see the red coils: not because they ve turned red, but because they are radiating heat.

8 Like all other electromagnetic waves (such as light), thermal radiation doesn t require any particles at all to make its way through space. Which is a good thing, because there s a lot of empty space between us and the sun. Three s a charm In many everyday situations, more than one type of heat transfer is taking place in a given system. Remember our cup of coffee? It s going through all three forms of heat transfer. Heat is being conducted through the mug. Convection currents are sending coffee upward from the bottom and downward from the top (you can see this movement by pouring a bit of cream in and watching it spread out). And if you hold your hands near the mug, you ll feel how it s radiating heat into the environment. It s sometimes an engineer s job to come up with materials and design objects that control or direct each of these processes. And that s what you ll be doing in this activity: keeping the warmth of the atmosphere from making its way to a cup of ice water. Vocabulary Conduction The transfer of heat as particles with higher energy collide with particles of lower energy. Conduction occurs in solids and liquids, but not gasses. Convection The transfer of heat through a substance as mass motion of the substance away from the source of heat. Convection occurs in gasses and liquids, but not solids. Heat The movement of energy in the form of temperature. Insulator A material that slows the process of heat transfer. Radiation The transfer of heat through electromagnetic waves. Temperature The measure of the average kinetic energy of a particle in a substance.

9 May SHPE Jr. Chapter STEM Activity Student Worksheet Chillin Out: Designing an Insulator Activity Procedure You and your friends are having a picnic on a warm spring day. You ve got lots of cold drinks, but no one remembered to bring a cooler. But not to worry: there are plenty of good insulating materials around, and your instructor has collected many of them for you. Your task is to design an insulator that will keep a cup of cold water chilled for an hour. Here s what you ll need: Waxed paper cup Thermometer Insulating materials How will you slow heat transfer? For starters, think about how you imagine heat might transfer to your cup. How much is through conduction? Convection? Radiation? Then consider the materials you have at hand. Which of them seem best at reducing the different types of heat transfer in your particular situation? Why? In the box below, make a list of materials you plan to use in your insulator. Either describe how they will be placed, or make a drawing of your proposed insulator.

10 Next, put your insulator together. Secure all the pieces in place by wrapping it once or twice with plastic wrap. Do not put more than two layers of plastic wrap around it! Have your instructor fill the cup with ice water. Put your thermometer in the cup just after it s filled. Let the thermometer sit for a minute or two, making sure that the temperature reading has stabilized. Record your temperature in the space below: Recorded temperatures: After 1 minute After 30 minutes After 60 minutes Then set the cup in the area your instructor has set aside for you. If you can, measure the temperature again after 30 minutes, and one more time after 60 minutes, recording the result each time. How many degrees did your water warm up over the hour? Where do you think heat transfer occurred the most? What makes you think so? What would you do to improve your design and why?