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L 8,9,10

# L 8,9,10 - "The First Law of Thermodynamics Prof Gianluigi...

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“The First Law of Thermodynamics” Prof. GianluigiVeglia

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We are now ready to begin studying the subject of thermodynamics. What is thermodynamics? Definition: We can define thermodynamics as the study of the energy in all its various forms, and how it is distributed and moves around the universe. Why do we care? Well, thermodynamics tells us how much fuel will be required to heat the house in the winter, helps design air conditioners, etc. It tells us why it is not possible to construct a perpetual motion machine.
If A is in thermal equilibrium with B and B is in thermal equilibrium with C, then C is also in thermal equilibrium with A. This law justifies the concept of temperature and the use of thermometer.

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We will begin with the first law of thermodynamics or law of energy conservation. This leads naturally to the concept of internal energy and enthalpy. As usual we will start from theory and move on to the practical applications. We will start from some definitions.
System : We define a system as that part of the universe we are studying. Examples: A liquid water in a beaker; All of the water-liquid and gaseous phases in a closed jar; The reactants enclosed within a vessel. Surroundings : Everything in the universe but the system. Systems are separated from their surroundings by walls, which may be permeable or impermeable. That is, matter may or may not be able to pass between the system and the surroundings. If the walls are permeable, the system is open . If they are impermeable, the system is closed . Systems that are unable to interact in any way with their surroundings are said to be isolated.

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Walls or boundaries are also classified as adiabatic or diabatic . Diabatic walls allow energy to pass as heat between the system and the surroundings; adiabatic walls do not. Walls can also be rigid or non-rigid ( movable and non- movable ). We will see that the rigidity of walls determines whether energy can pass between the system and the surroundings as work.
We can define energy as the capacity of a system to do work on its surroundings. For instance, energy is the amount of work necessary to lift a weight over a certain distance. If the energy is given by E we can partition E into different kind of energy. For instance, suppose our system is the water inside a closed jar, which is on a train travelling uphill: Where U is the internal energy and CM subscript is referred to the center of mass. U E E E U U U U U U U elec magn mgh E mv E p k tot er nuclear rot vib trans elec p CM k + + = + + + + + = + + + = = int 2 ... 2 1

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In Chemistry, we are almost always concerned with changes in objects at rest and not subject to varying fields. Note that it is difficult to give absolute values to the state variables. So, for the energy we will consider changes in the energy rather than the absolute value of the energy: U E =
Energy, like T, P and is a state function. That is, when I change a system from one state to another, the energy change

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L 8,9,10 - "The First Law of Thermodynamics Prof Gianluigi...

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