PHY7A Lecture3

# PHY7A Lecture3 - W A Relation between Open Systems and...

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The two energy transfers, Q and W Energy is one of our ways of describing the results of an interaction (i.e. What quantities changed in the interaction? What quantities were transferred from one system to another in the interaction?) There are only 2 ways we use to describe all possible energy transfers. 1) Heat (Q) is the name for the energy transfer that happens when a high T object is put in thermal contact with a low T object. 2) Work (W) is the name for an energy transfer happening at the macroscopic level and involving a force (this turns out to be every energy transfer except heat). We use a force, F, to represent an interaction between two objects A and B. Note: F by A on B is equal in magnitude and opposite in direction to F by B on A . Suppose object A moves by ! x A and object B moves by ! x B during the interaction (i.e. while the force is applied). Then the work done (energy transferred to A) on A is W A = F on A|| ! x A and the work done on B (energy transferred to B) is W B = F on B|| ! x B . E object " ! E object > 0

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Unformatted text preview: W A Relation between Open Systems and Closed Systems This only works for some interactions (e.g. gravitational, springs, electrical, and molecular) but not for friction, air resistance, or other “dissipation” interactions. Suppose the earth exerts a force on an object (this is the force of gravity) and we take the positive y-direction to be up (this is the usual choice), then the “weight” of the object is F by earth on object = -(m object g) This force on the object points straight down (g = 9.8 N/kg is the gravitational field of the earth) So W to object = F by earth on object ! y object = -m object g ! y object Open system: Energy is transferred from the earth to increase the kinetic energy of the object ! KE = W so ! KE = -m g ! y Closed system: Loss of potential energy is equal to gain in kinetic energy ! KE + ! PE = 0 so ! KE + m g ! y = 0 (same answer) KE object " ! KE object > 0 PE object-earth # ! PE object-earth < 0 KE object " ! KE object > 0 W...
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PHY7A Lecture3 - W A Relation between Open Systems and...

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