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Note 2 - Topic 2 Thermal Energy and Heat Thermal Energy and...

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1 1 Topic 2: Thermal Energy and Heat Thermal Energy and Power Transforming Energy Energy in Humans • Temperature • Heat Heat Transfer Home Heating Penguins in a huddle during a snow-storm Penguin Huddle 2 Thermal Energy and Power ; Transforming Energy; Energy in Humans; Temperature; Heat; Heat Transfer; Home Heating 3 Thermal Energy and Power ; Transforming Energy; Energy in Humans; Temperature; Heat; Heat Transfer; Home Heating Atoms are bound in a solid but they vibrate near their equilibrium position. Molecules in a liquid only weakly bound. Thermal energy in a liquid is mostly in (linear) motion of molecules but also in vibrations and rotations of molecules. Thermal energy in gas is mostly equal to the total kinetic energy of the atoms or molecules
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2 4 Thermal Energy and Power ; Transforming Energy; Energy in Humans; Temperature; Heat; Heat Transfer; Home Heating Power = Energy flow per unit time Unit: 1 watt. 1W = 1J/s Energy unit used by power companies: 1kWh 1 kilowatt = 1000J/s so 1000J = 1kWs How much energy (in J) is 1 kilowatt-hour? 1 Calorie (food) = 1000 calories = 4200 J (1 Cal = 1kcal = 4.2 kJ) t E P Δ Δ = 5 i>Clicker Question A kWh in B.C. costs currently ~ 6.5 cents. The total energy consumption in a 1000 ft 2 large apartment is approximately 200 MJ/day. If the apartment is heated electrically, what’s the energy cost each day? A. $ 11 B. $ 3.60 C. $ 2.30 D. 71 c E. 48 c 6 Thermal Energy and Power ; Transforming Energy; Energy in Humans; Temperature; Heat; Heat Transfer; Home Heating Power: Rate of Energy Transfer Computer: ~ 200 W TV on standby: a few W Electrical stove: 3 – 5 kW Refrigerator: 1.2 kWh/day (What is this in watts?) (50 W)
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3 7 Thermal Energy and Power; Transforming Energy ; Energy in Humans; Temperature; Heat; Heat Transfer; Home Heating Energy can be transformed from one form to another (e.g., potential into kinetic). In addition not all of the available energy can be transformed: Wind or water going through a turbine must still have some speed going out. These limitations are expressed as efficiency . It describes how much of the available energy can be transformed into a useful form: e = useful/available . 8 A car engine has typically an efficiency of 20%. How much gasoline is used just to accelerate a 1500 kg car from rest to a speed of 15 m/s? The available chemical energy from gasoline is 34.6 MJ per liter (Wikipedia). A. 0.010L B. 0.024L C. 0.17L D. 0.84L E. 1.42L i>Clicker Question 9 Thermal Energy and Power; Transforming Energy ; Energy in Humans; Temperature; Heat; Heat Transfer; Home Heating Kinetic energy of a solid object can be transformed into thermal energy or heat. Example: Book sliding on a table top. While the book slows down, temperature of the book and the table surface increases due to friction.( Friction Simulation).
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