Objectives Learn the formula for calculating Heat Energy Evaluate social, economic, and environmental issues related to thermal energy Learn the differences in heat transfer. Conduction, Convection, Radiation
Energy (Thermal): Main Ideas Work and thermal energy are often related. When mechanical, fluid or electrical work is done, part of the work turns into heat energy. Heat energy can also be used to do work (steam engines and internal combustion engines). There is a direct relationship between heat energy and mechanical energy. This relationship is called “the mechanical equivalent of heat”. Heat energy always moves from hot objects to cold objects. The heat gained or lost by an object can be calculated using the formula, H = mcΔT Heat moves from hot objects to cold objects in three processes: conduction, convection and radiation. Heat energy losses prevent 100% efficiency in useful energy conversions. When mechanical work is done (measured in Joules), some of this work is used to overcome resistance. This produces heat energy which is measured in BTUs or Calories. There is a direct conversion from mechanical work to heat energy which is referred to as the mechanical equivalent of heat. Mechanical Energy Expended Mechanical Equivalent of Heat = Heat Energy Produced when all mechanical energy goes into heat energy
Processes for transferring Heat Energy Conduction: Heat energy is transferred from a hot region to a cooler region by vibrating molecules or atoms. Convection: An air mass or volume of fluid is used as a medium by which heat energy is transferred. Radiation: Movement of heat energy by electromagnetic waves. Radiation may be visible (red hot piece of steel), or invisible (infrared radiation from the sun).
Using Solar Energy for HeatSocietal Context:Millions of people get sick every yearfrom drinking contaminated water.An estimated 1.5 billion people getdiarrhea because of bad water and fromthat 2 million deaths occur. Billions ofpeople worldwide don’t have access toclean water that we enjoy, andinfrastructure doesn’t exist to allowaccess to clean water.
Making use of Solar EnergyThese populations dohowever have plenty ofsun, that can be usedfor free energy. Solarovens can not onlycook food, but can killharmful microbes thatcause illness anddeath.
Water PasteurizationLouis Pasteurdiscovered the scienceof killing harmfulpathogens and weknow it now aspasteurization; thekilling of disease-causing contaminantsin food.
Practice ProblemHow much heat energy is required to pasteurize1 Liter of water that is originally 10˚C? specific heat of water (c) = 4.186 J/g•˚C water pasteurization = 65˚C formula: H=mc∆TH=(1000g)(4.186J/g•˚C)(55˚C)H=230,230 Joules
Practice ProblemHow much heat energy is required to heat acan of chili (3.06kg) from 21˚C to boiling point(100˚C) c of chili is 5.128 J/g•˚C H=mc∆T H=(3060g)(5.128 J/g•˚C)(79˚C) H=1,239,642.72Joules H=1.239MJ
Solar Oven Challenge Engage in cooperative learning to create a solar oven that can pasteurize water and cook food Create an oven that is inexpensive to build and that can be easily transported and stored Conduct experiments that test the effectiveness of your constructed oven
Divide into teams Get into teams of 3-4 people Read the instruction packet Timeline: 2 full class days to construct. Should be finished by next weekend. Use RTI if more time is needed School-wide Physics Class Potluck April 16 during lunch You can conduct research to help you find a good oven design, so you don’t have to start from scratch