lecture24

# lecture24 - Phys 2101 Gabriela Gonzlez A force acting on a...

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Phys 2101 Gabriela González

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2 A force acting on a system can heat it up (or cool it down), by working on it. A change in temperature produced in a system can be used to produce mechanical work. At any point in the process, the system (gas) will have temperature T, pressure p and volume V. dW = Fds = pAds = pdV W = pdV V i V f
3 Changing the system’s pressure and volume from an initial to a final state can be done using different amounts of work (and heat transferred): Going back to the original state does not mean that no work was done! W = pdV V i V f

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4 The work W done by the system during a transformation from an initial state to a final state depends on the path taken. The heat Q absorbed by the system during a transformation from an initial state to a final state depends on the path taken. However, the difference Q – W does not depend on the path taken! We define this quantity as the change in “internal energy”: Δ E int = Q – W The internal energy of a system increases if energy is added as heat, and decreases if energy is lost as work done by the system.
5 Figure 19-39 a shows a cylinder containing gas and closed by a movable piston. The cylinder is kept submerged in an ice water mixture. The piston is quickly pushed down from position 1 to position 2 and then held at position 2 until the gas is again at the temperature of the ice water mixture; it then is slowly raised back to position 1. The figure shows a p - V diagram for the process. If 140 g of ice is melted during the cycle, how much work has been done on the gas? [46600] J

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6 At the microscopic level, temperature is a measure of the kinetic energy in the elemental units (atoms or molecules); pressure exerted by a gas is produced by molecular collisions on the walls of its container; the fact that gases fill up the volume of the container they’re in is due to the freedom of the molecules to move around. Useful unit for number of atoms:
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## This note was uploaded on 12/01/2011 for the course PHYS 2101 taught by Professor Grouptest during the Fall '07 term at LSU.

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lecture24 - Phys 2101 Gabriela Gonzlez A force acting on a...

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