CHEM 115 Lecture Notes - Lecture 7: Thermodynamic System, Sign Convention, Dirham

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17 Jun 2017
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We use the first law of thermodynamics to help us formalize what happens during a process that generates heat and/or work: e=q+w. We need a sign convention for q and w because heat can flow either way between the system and the surroundings as can work: if heat flows from the surroundings to the system, q is positive. Consider a piston containing a constant amount of gas. If we cause the gas to expand, perhaps by increasing the temperature, there will be pressure-volume work done by the system (the piston) on the surroundings (the atmosphere). The expansion of the volume comes about as a result of the (temporary) increase in pressure in the piston. This manifests itself as a force on the piston head and pushes it through a distance, Force times distance is work therefore work has been done by the piston on the atmosphere.

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