Physics 202 Book Notes CH.11. Sections 1-5
Physics 202 Book Notes CH.11. Sections 1-5 PHYS 202
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This 5 page Class Notes was uploaded by Melissa on Sunday January 10, 2016. The Class Notes belongs to PHYS 202 at University of Oregon taught by Jenkins T in Fall 2015. Since its upload, it has received 45 views. For similar materials see General Physics >4 in Physics 2 at University of Oregon.
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Date Created: 01/10/16
1. Transforming energy A. Energy is not lost it is just converted to less important forms B. Work into a system: positive C. Work out of a system: negative D. Work includes electric energy into and out of motors and generators E. Energy output= energy input F. Thermal energy is irreversible a. Energy isn't lost but it is less eﬃcient 2. Eﬃciency A. A. Larger the loss of energy in a system, the lower the eﬃciency a. Comes from 1. Process limitations 2. Fundamental limitations 1. 1. Energy in the body A. Energy inputs a. ATP b. Burning food transforms all of its chemical energy into thermal energy which can be measured in calories 1. • Energy outputs Body uses energy at rate of 100W at rest ‣ Converted to thermal energy and then transformed as heat to the environment • Eﬃciency of the human body Climbing up stairs ‣ Use equation ABOVE • Thermal energy and GPE are increasing so they will be positive and the chemical energy is being used so it will be negative. Therefore... In the ﬁnal position you are at a greater height and your body is warmer Eﬃciency for stair climbing is 25% Find the what you are getting energy wise ‣ the change in potential energy • Find what you had to pay ‣ cost is the metabolic energy your body uses in completing the task which here has been researched to be 7200J Find eﬃciency • • Energy you use per second while running is proportional to your speed Running twice as fast takes twice as much power ‣ also takes you twice as far but the energy you use to run a certain distance does not depend on how fast you run Walking at constant speed on level ground means KE and PE are constant but the KE is converted to thermal energy • 11.3 temperature, thermal energy, and heat Thermal energy of an ideal gas is eaual to the total kinetic energy of the moving atoms in the gas ‣ heating the gas causes th3e atoms to move faster and increases the thermal energy of the gas and increases the tTemperature ‣ Temperature is related to the kinetic energy of the gas atoms • Temperature does not depend on size of system ‣ Larger volume so there will be more total thermal energy but atoms are still moving at the same speed as before so average kinetic energy per atom is unchanged Temperature of an ideal gas is a measure of the average KE of the atoms that make up the gas Temperture Scales ‣ ‣ Zero degrees is the point at which the KE of atoms is zero What is heat ‣ Heat is energy transferred between two objects because of a temperature diﬀerence between them • ﬂows from hot to cold • Q is symbol for heat ‣ The transfer of energy occurs at the atomic level in a perfectly elastic collision where the faster atom loses energy while the slower one gains energy • thermal energy is transferred from the faster moving atoms on warmer side to the slower moving atoms on the cooler side continues until thermal equilibrium is reached ‣ occurs when atoms on both sides have the same average KE • • Two systems placed in thermal contact will transfer thermal energy from hot to cold until their ﬁnal temperature are the same • First Law of Thermodynamics For systems in which the thermal energy changes, the change in thermal energy is equal to the energy transferred into or out of the system as work(W) or heat (Q) or both ‣ In a insulated container in which a piston is used to compress the gas, the temperature of the gas increases because there is work being done on the gas and thermal energy is being transferred into the system • Heat engines thermal energy is transferred from a hot to a cold reservoir and a heat engine takes some of the heat being transferred and converts it to other forms ‣ The engine;s thermal energy does not change ‣ To compute the heat engine's eﬃciency • • • Heat engines will always exhaust some fraction of the heat into a cold reservoir • Max possible eﬃciency of a heat engine is ﬁxed by the second law of thermodynamics • • • • To increase the deﬁciency of a heat engine you should either increase the temperature of the hot reservoir or decrease temperature of the cold reservoir • Work done is always less than heat input • Heat Pumps, Reﬁgerators and Air Conditionerrs Job a heat pump is to transfer heat energy from a cold reservoir to a hot one which is the opposite of a heat engine Reﬁgerator ‣ heat is transferred from the cold air inside to the warmer air in the room Air conditioner • Transfers heat from the cold air in the house to the warm air outside Requires work to be done and energy must be conserved ‣ heat on hot side must equal heat removed from the cold side and work input • Coeﬃcient of Performance ‣ Heat pump for cooling • Energy removed from the cold reservoir divided by the work required to perform the transfer ‣ Has a maximum which relates the temperatures of hot and cold reservoirs • ‣ Larger coeﬃcient of performance means a more eﬃcient heat pump • usually greater than 1
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