2053Lecture15 - Laws of Thermodynamics Laws Topics of...

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Unformatted text preview: Laws of Thermodynamics Laws Topics of Discussion Topics Heat, Work & Internal Energy Conventions Work & Gases The Laws of Thermodynamics Thermal Processes Heat, Work & Internal Energy Heat, The energy transferred, called the heat Q, iis the s The result of a temperature difference between a thermodynamic system and its surroundings. thermodynamic Mechanical work, or work, W is the product of a or work force F applied over a distance d. The internal energy U is the energy associated The with the microscopic components, atoms and molecules, of the system which include the translational kinetic and potential energies, the rotational energies, the vibrational energies and intermolecular potential energies of the system. intermolecular Sign Conventions for Heat & Work Sign Heat Q is positive when a thermodynamic Heat system gains energy and negative when it losses energy. losses Work is positive when a thermodynamic Work system does work on its surroundings and negative when work is done on a thermodynamic system. thermodynamic The Thermodynamic System for a Gas System Recall that a thermodynamic system for a gas is Recall thermodynamic described in terms of pressure P, temperature T temperature and volume V. The variables P, T and V are The called the state variables of the thermodynamic state system. Surroundings system. Q Q Thermodynamic S ystem P, V, T Work Done on or by a Gas Work The amount of work done on or by a gas The can be determined from the PV–graph, PV graph, where the area under the PV–graph is PV graph equal to the work. equal P W ∆P V ∆V The Laws of Thermodynamics The Zeroth Law: Two systems individually in thermal equilibrium with a third system are in thermal equilibrium with each other. equilibrium First Law: ΔU = Uf – Ui = Q – W Second Law: Heat flows spontaneously from a substance of higher temperature to a substance of lower temperature, and NOT in the reverse direction. direction. Third Law: It is not possible to lower the temperature of any system to absolute zero in a finite number of steps. finite Types of Thermal Processes An isobaric process is a thermodynamic process An isobaric that occurs at a constant pressure. that An isochoric or isovolumetric process is a An or thermodynamic process that occurs at a constant volume. constant An isothermal process is a thermodynamic An isothermal process that occurs at a constant temperature. process An adiabatic process is a thermodynamic An adiabatic process that occurs without the transfer for heat. process Other thermal processes. Thermal Processes Thermal Process Isobaric Isochoric Work Done First Law W = P(Vf – Vi) W=0 ΔU = Q – W ΔU = Q ΔU = 0, W=Q Q = 0, ΔU = – W Isothermal W=nRTln(Vf /Vi) Adiabatic W = 3nR(Tf – Ti)/2 Adiabatic ...
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