Chapter 2 - Chapter 2 Energy, First Law and Cycles MAE 221...

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Chapter 2 Energy, First Law and Cycles MAE 221 Thermodynamics Notes
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14 Sep '08 MAE 221 Chapter 2 2 Notes which follow are largely based on: Fundamentals of Engineering Thermodynamics* , 6 th ed, M Moran & H Shapiro *Figure and section (§) numbers refer to text
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14 Sep '08 MAE 221 Chapter 2 3 Topics Energy Work Heat First Law Cycles
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14 Sep '08 MAE 221 Chapter 2 4 First Law of Thermodynamics For closed systems E = Q – W Change in Energy is balanced by addition of work & heat Energy E Can be stored in various forms (macro/microscopically) Can be transformed Can be transferred (by work or heat transfer) Total amount is always conserved
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14 Sep '08 MAE 221 Chapter 2 5 Units of Energy 1 Joule (J) = 1 N m = 1 kg·m 2 /s 2 1 Thermal calorie = 4.184 J 1 “diet” calorie = 1000 “thermal” calories 1 BTU = 1055.1 J 1 BTU = 778 ft·lbf
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14 Sep '08 MAE 221 Chapter 2 6 Contributions to Energy E = KE + PE + U macroscopic microscopic where KE = kinetic energy: ½ mv 2 (zero at zero velocity) PE = potential energy: mgz (zero at ref. Height on earth) U = internal energy Internal Energy, U, includes Electronic energy Magnetic energy Microscopic Kinetic Energy (translational, rotational and vibrational motion) Potential Energy (from intramolecular forces) Values of U are generally obtained from tables
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14 Sep '08 MAE 221 Chapter 2 7 Heat, Work & Power Heat and Work Means by which energy can be transferred across the boundary of a closed system Q and W, as stated previously, are not properties Work W: organized macroscopic energy transfer Heat Q: disorganized microscopic energy transfer Sign Convention (unless otherwise noted) W > 0 work done by system on surroundings W < 0 work done by surroundings on system Q > 0 heat transferred by surroundings to system Q < 0 heat transferred by system to surroundings Power is the time rate of energy transfer Power where the overdot denotes time rate of change d/dt W P & =
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14 Sep '08 MAE 221 Chapter 2 8 Differential Forms of State Variables Exact differentials (e.g. dv) of state variables Inexact differentials of heat and work: δ Q, δ W However, dE = δ Q - δ W Important to note: The differential of every property is exact because it does not depend on the details of the process between two states The differential of non-properties, such as work, are inexact because it depends on the details of the process between two states 1 2 2 1 V V dv = 1 2 2 1 W W δ W
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14 Sep '08 MAE 221 Chapter 2 9 Definitions and Forms of Work Thermodynamic definition of work Work is done by a system on its surroundings if the sole effect on everything external to the system could have been the raising of a weight Work can be thought of as a “generalized force” multiplied by a “generalized displacement” where E is the electric field strength, P is the electric dipole moment/vol A simple compressible substance has only pdv work associated with it ) d(V δ W ) d(V δ W dv p ) d p( d p δ W d δ W 0 M H - p E - x A x A x F = = = = =
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Chapter 2 - Chapter 2 Energy, First Law and Cycles MAE 221...

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