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Lecture

Tuesday October 23rd, 2012.docx

4 Pages
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Department
Natural Science
Course Code
NATS 1840
Professor
Carol Bigwood

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Science, Technology and the Environment (SC NATS 1840A) - Tuesday October 23rd, 2012 READING: CH 3.4, 3.7 Heat and Electromagnetic Radiation: Intimately connected in a manner crucial to understanding climate and climate change. Example: A rock is exposed to sunlight. Its surface absorbs E.M. energy continuously. It is going to absorb some of the energy from the sun and as it does so, but how does this absorption mechanism work, you have to think about the atoms that make up the rock. Through conduction the temperature increases due to the E.M. energy being absorbed.  Molecules’ average speed increases.  Thermal energy (temperature) increases.  Why does the rock not eventually melt? Motion of molecules causes emission of E.M. Radiation.  Rock loses thermal energy in E.M. form at the same time as it absorbs it.  Frequency of emitted ration determine by object’s temperature. The Object “glows” Radiation Equilibrium Constant temperature (steady state) reached when (Total energy absorbed)/sec = (Total energy radiated)/sec No further net energy build-up in the object. At night no energy is absorbed, but rock continues to “glow” (lose energy) due to thermal content. It cools All objects glow in this way. Average frequency of glow increases with object’s temperature.  “Glow” is distributed over a range (spectrum) of frequencies, with the peak determined by temperature. Units of Energy and Power: Energy: Metric unit is the Joule (J). 1 J = 4.18 calories (cal) 1 Calorie = Energy needed to raise temperature of 1 gram of water 1 degree Celsius. Power: Watt (W) defined as 1 Joule per second. Often, if power is expressed in kilowatts (kW) then energy is given in units of kilowatt-hours (kWh). 1 kWh = 1000 W x 1 hr = 3.6 x 10^6 J = 3.6 MJ Everyday energy scales – Some examples:  Example: Energy used by a 100w light bulb Left on for one hour 100w = 100 J/S Energy used = 100 J/S x 3600sec = 3.6 x 10^5 J OR 360kj OR 0.1kWh  Example: Clothes dryer running for one Hour = 18MJ  Human body (one day; at rest) 6-8MJ (1600 kcal)  Car travelling at 100 km/h = at least 460kJ  Jumbo jet at take-off speed = at least 1.3GJ  Jumbo jet at cruising altitude (11km) = at least 45 GJ Energy Transformations: Energy can be converted form one form to another, Example: E.M. energy converted into thermal energy, and vice versa For tire swim, continuous conversion of potential energy (1) into kinetic energy (2) and back again. At intermediate stages the energy is partly kinetic ad partly potential (3).
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