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Unformatted text preview: 1 LECTURE 2 THERMODYNAMICS An Engineering Approach The 1 ST Law of Thermodynamics / Properties of Pure Substances Ideal Gases / Energy Analysis of Closed System Dr. MinJun Kim Department of Mechanical Engineering & Mechanics Drexel University The 1 st Law of Thermodynamics This law is often called the conservation of energy law !!! When a system changes from one state to another along an adiabatic path, the amount of work done is the same , whatever the means employed. The cyclic integral of the heat transfer (the net heat transfer during the cycle) The cyclic integral of the work (the net work during the cycle) In any cyclic transformation the work done by a system on its surroundings is equal to the heat withdrawn from the surroundings. 2 The 1 st Law of Thermodynamics For a change in state of a control mass: Here we are concerned with a process rather than a cycle. We begin by introducing a new property, the energy E. Consider a system that undergoes a cycle in which it changes from state 1 to state 2 by process A and returns from state 2 to state 1 by process B. Considering the two separate processes. From the 1 st law of thermodynamics, we have Considering the two separate processes, we have For this cycle we can write Subtracting the second of these equation from the first, we obtain ! Q ! W d e p e n d s o n ly o n t h e in it ia l a n d f in a l s t a t e s a n d n o t o n t h e p a t h f o llo w e d b e t w e e n t h e t w o s t a t e s . The 1 st Law of Thermodynamics We conclude that this is a point function, and therefore it is the differential of a property of the mass. This property is the energy of the mass . Thus we can write dE = ! Q ! W when integrated from an initial state 1 to a final state 2, we have E 2 E 1 = 1 Q 2 1 W 2 Energy Balance: The net change in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process E in E out = " E system The first law of thermodynamics for a change of state may be written 3 Practice (I) Consider 5 kg of steam (water vapor) contained within a pistoncylinder assembly. The steam undergoes an expansion from state 1, where the specific internal energy (the internal energy per unit mass) is u 1 = 2709.9 kJ/kg, to state 2, where u 2 = 2659.6 kJ/kg. During the process, there is a heat transfer of energy to the steam with a magnitude of 80 kJ. Also, a paddle wheel transfers energy to the system by work in the amount of 18.5 kJ. There is no significant change in the kinetic or work in the amount of 18....
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 Winter '08
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