Lecture07_10 - Physics19 GreatIdeasofPhysics Lecture7 HeatandEnergy ConservationLaws ,nomatter what ,wesayit isconserved tosymmetries(lastCh

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Physics 19: Great Ideas of Physics Lecture 7: Heat and Energy
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Conservation Laws A “conservation law” is a statement that a  particular quantity never changes, no matter  what When a quantity remains constant, we say it  is “conserved” Somehow, nature keeps an exactly balanced  “account” of the quantity! Conservation laws have a deep connection  to symmetries  (last Chapter of S&A)
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Isolated Systems An isolated system is  one which is closed –  nothing can get in or  out An approximation! In  practice, there are no  truly closed systems! A very important  concept, and key to  understanding  conservation laws
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Conserved Quantities Mass Conserved in chemical reactions Not a fundamentally conserved quantity Momentum Always conserved in closed systems Mechanical Energy (?) Kinetic energy (energy of motion) plus Potential energy (energy of interaction) Mechanical energy can be transferred into our out of  a system by a Force which does Work
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Isolated and Non-Isolated Systems A falling book can be considered as a system Is the book an isolated system? In that case, the (external) force of gravity does  work on the book to increase its kinetic energy and  speed Or we can include the book AND the Earth in  our system Now (to a good approximation) we have an isolated  system In that case, we say some of the system’s potential  energy due to the (internal) force of gravity is  transformed into kinetic energy
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Inclined Plane Galileo’s experiments  demonstrate  conservation of energy The ball ends up at the  same height as it starts Initial energy: Kinetic = 0 (at rest) Potential = mgh Final energy (= Initial): Kinetic = 0 (at rest) Potential = mgh h
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The Lever The lever is another  example of energy  conservation The work done on the long  end is a  small  force  multiplied by a  large   displacement On the short end, that  amount of work applies a  larger  force over a  shorter   displacement Small Force f Large Displacement D Large Force F Small Displacement d Conservation of energy: f × D = F × d
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Dissipative Systems Mechanical energy is not always conserved though When we slide a block over a table, applying a  force equal to that of friction, when the force stops,  the object stops too Force F Displacement x Force of Friction = -F
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When work is done against forces like  friction, the mechanical energy of a system  decreases Is energy truly conserved?
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This note was uploaded on 11/26/2010 for the course PHYS Physics 19 taught by Professor Davidcasper during the Spring '10 term at UC Irvine.

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Lecture07_10 - Physics19 GreatIdeasofPhysics Lecture7 HeatandEnergy ConservationLaws ,nomatter what ,wesayit isconserved tosymmetries(lastCh

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