Lecture 5 Heat - Thermal and Fluids Engineering I Prof....

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Thermal and Fluids Engineering I Prof. Deborah A. Kaminski Lecture 5 – Heat The First Law as a Rate Equation The first law for a closed system is K EP EUQW ∆+ ∆=− The terms on the left depend on the difference between two states, while the terms on the right do not. For example 22 21 mm KE ∆= V 2 1 K EK E d K −= E 1 The change in kinetic energy depends only on the end states and not on any intermediate values of velocity. Kinetic energy can be represented by an exact differential. Potential energy may similarly be represented by exact differentials as 2 2 1 P E d P Em g zm g z = − The change in potential energy also depends only on the end states, specifically the elevation levels, z 1 and z 2 . Internal energy, like kinetic and potential energy, is an exact differential represented by 2 1 UU d U In the case of an ideal solid or liquid, internal energy change is Lecture 5 Page 1
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Thermal and Fluids Engineering I Prof. Deborah A. Kaminski () 21 UUm c TT −= For this case, the change in internal energy is only a function of temperature. Work, however, is an inexact differential, which depends on the path. There are no state variables corresponding to work and no meaning to . For example, for boundary movement work (expansion or compression) W WP d = V A B WW > From the first law K EP EUQW ∆+ ∆=− All energy terms on the left are independent of path, so A B QQ > Like work, heat is path-dependent and heat is an inexact differential. The differential form of the first law becomes Lecture 5 Page 2
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Lecture 5 Heat - Thermal and Fluids Engineering I Prof....

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