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Transcript
Raffles Institution Science Performance Task 2011 Done by: Terence Lee (27) Chua E Cher (3) Ryan Chua (4) Class: 1H 1 Methodology Firstly, we placed an ice cube into containers of different materials. These materials are: plastic, ceramic, and cardboard. Next, we let the ice melt for 30 minutes and see how much ice was left. The results are shown in the table below. After plotting the results, we chose our materials, namely cardboard and cork. Then, we took about 2 weeks to create the box. We obtained the cork by purchase from an art store and we constructed the box. We split the components of the report into equal portions to do and then we combined them. The report was finalized after compiling our various works. 2 Three Principles of Heat Transfer Heat can be transferred by three methods: conduction, convections and radiation. These methods affected our design of the prototype as we need to have our knowledge of heat transfer to put them to practical use by creating a box that reduces transfer of thermal energy. Conduction Firstly, summaries of principles of conduction are: it is the transfer of thermal energy between regions of matter due to a difference in temperature. Heat flows from a region of higher temperature to a region of lower temperature and until reached equilibrium. On a particulate scale, conduction occurs because vibrating atoms and molecules interact with neighbouring particles, transferring some of their kinetic energy. Hence, heat is transferred as adjacent atoms vibrate rapidly. Convection Secondly, we shall define and summarise the principles of convection. Convection is the transfer of heat from one part of a fluid to another by the circulating movement of the fluid itself. Generally, convection happens when liquid/air expand after being heated and becomes less dense, thus it will rise up. Cold liquid/air is denser and will sink to replace the hot liquid/air. Radiation Lastly, we shall summarise the principal of radiation. Radiation is the transfer of heat through electromagnetic waves and does not require a medium. Reasons for Choosing This Design Next we shall explain how these mechanisms affect our prototype. Considering the principles of conduction, we decided to make a box with two layers of different materials and leaving a gap between them. As conduction (as stated above) transfers by physical contact due to the fact that particles transfer kinetic energy by interacting with neighbor particles, we decided to leave a gap so as to prevent conduction of heat from the surrounding to the ice cube. The decrease in conduction of heat happens because the gap created separates vibrating particles of the outer layer from the inner layer, preventing them from interacting with one another. The prototype is also affected by principles of convection. As stated above, convection happens when liquid/air expand after being heated and becomes less dense, hence raising up. To reduce convection, we decided to place the inner layer of the box closer to each other, so as to break the convection current flow and thus able to reduce the transfer of heat by convection. We also decided to make outer layers of the box at same thickness so as to allow equal amount of heat to enter and heat up the air equally. This is to slow down the process of convection as there will be less difference in the density of the air. Considering the principles of radiation, we decided to make the outer layer as shiny as possible. That is because radiation as stated travels by electromagnetic waves and thus shiny surface can help to reflect a lot of radiation from the surrounding, reducing heat gain by radiation. Choice of Materials Air For the gap in the two layers, we decided to use air as it is poorest conductor of heat on earth beside Silica Aerogel. The reason why it is a poor conductor is that it has low density due to the fact that it is in gaseous state and thus the distance between the airs particles are large so conduction of heat is reduced as particles have to collide randomly to transfer energy. The next reason is that it has higher thermal resistance to heat and thus little heat is transferred through air. The last reason is that 3 unlike metals, it does not have free electrons to transfer kinetic energy to particles. Due to the above reasons, we choose air as one of the layer. Cardboard We chose corrugated cardboard for the inner layer due to its insulating properties. Corrugated cardboard is a good insulator because of its double walls and its molecular makeup. Double walls are used to insulate many things, such as refrigerators and coolers. Double walls work well as insulation because air is trapped between the walls. The air trapped between the walls retains its temperature for a long period and thus can keep the interior of the container either hot or cold. Since corrugated cardboard is made up of two layers of paper with fluting in between, air is trapped within the cardboard, thus providing good insulation, due to the properties of air as said above. Cork Our group chose cork as our main outer material because it is a poor medium for conduction of heat and it can hold on to its free electrons to minimize transfer of heat as much as possible. Cork is also a poor medium for convection as it is a solid which means that its particles are packed tightly together so the principle of density does not work on it. However, as radiation, a method of heat transfer does not require any medium including cork; cork is unable to prevent heat transfer from occurring. Therefore, our group used a silver spray paint to coat on its outer layer to reduce radiation so as to insulate the ice even further. Table 1.1 Results of experiment with similarly-sized container of different materials. Material chosen for experiment Percentage of ice that melted after 35 min. Time taken for ice to melt completely (min) Plastic cup 20% 82 Ceramic Cup Cork Cardboard 70% 15% 30% 41 90 72 BONUS QUESTION Cork Classified As: Miscellaneous Carbon Footprint: 1kg of Cork ≡ 0.19kg CO2e The mass of the cork is approximately 50 grams, so it produces 0.0095kg of CO2e, or 9.5grams. Cardboard, Recycled Classified As: Miscellaneous Carbon Footprint: 1kg of Cardboard, Recycled ≡ 0.42kg CO2e Corrugated cardboard 100 percent recycled The mass of the corrugated cardboard is approximately 20 grams, so it produces 8.4 grams of CO2e. Total: 17.9g Yes, the carbon footprint of the materials affects our choice of materials. We decided to choose materials that produced less carbon dioxide when they were manufactured so as to save Earth from global warming. In this century, global warming is an important issue and we hope to cut down our carbon emission and thus we decided to choose materials that produced less carbon dioxide. 4 References http://www.ehow.com/about_6469087_corrugated-cardboard-good-insulator_.html http://furtech.typepad.com/furtech/2006/11/heat_loss_blizz.html http://www.fao.org/docrep/006/Y5013E/y5013e08.htm www.newton.dep.anl.gov/askasci/phy00/phy00781.htm www.engineeringtoolbox.com/radiation-heat-transfer-d_431.html www.engineeringtoolbox.com/convective-heat-transfer-d_430.html http://www.vtaide.com/png/heat2.htm http://wiki.answers.com/Q/Why_is_cork_a_good_insulator 5