ME
Le21

# Expansion of a gas or liquid to lower pressures 3

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expansion of a gas or liquid to lower pressures 3. Chemical reactions: H 2 +0.5O 2 =H 2 O (Water) 4. Mixing of matters of different composition- Milk in coffee 5. Friction and drag 6. Electric current flowing through a resistance 7. Magnetization or polarization with hysteresis 8. Inelastic deformation of materials.

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10/15/2010 21 Introducing property “Entropy” and Entropy Statement of the 2 nd Law 1 st Law for an open system (Conservation of Energy): It is impossible for any system to operate in a way that entropy is destroyed. Increase in Entropy Principle ME200 Therm I Lecture 21, Prof. Mongia 2 2 1 1 [ ] [ ] 2 2 cv cv cv i i i i e e e e i e dE Q W m h V gz m h V gz dt cv cv i i e e i e dS S m s m s dt
10/15/2010 ME200 Therm I Lecture 21, Prof. Mongia 22 Eng. Model as shown on the next slide: 1. Closed system with interaction with two heat reservoirs 2. Steady-state approximation

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10/15/2010 W out =Q H -Q C >0 Hot Reservoir Cold Reservoir W out =Q H -Q C Device #2 Q H’ Q C’ Device Q H Q C W out Proposed System Hot Reservoir Cold Reservoir Combined system Q C Q C’ Q H Q H’ W out =0 Violates Clausius statement
Hot Reservoir Cold Reservoir Device Q H Q C W out =Q H -Q C Device #2 Q H’ Q C’ W out Hot Reservoir Cold Reservoir Combined system Q C Q C’ Q H Q H’ W outC >0 Consider: Q C’ =-Q C so that Q Ccombined =0 ' ' ' ' ' ' ' ' 0 out H C C C H out out H H out out out H H W Q Q But Q Q Q W W Q Q W W W Q Q Violates Kevin-Planck statement

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10/15/2010 ME200 Therm I Lecture 21, Prof. Mongia 25 (a) False; (b) False; (c) False; (d) True
10/15/2010 ME200 Therm I Lecture 21, Prof. Mongia 26 (a) False: The two laws are independent principles. Therefore, there can be situations where you may not be violating the both simultaneously. (b) False: Energy conservation law for a closed system: E=Q-W. Since Q and W are independent means of changing E, only under the condition of E=0, we can make the statement Q=W.

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10/15/2010 ME200 Therm I Lecture 21, Prof. Mongia 27 (c) False: Entropy change is always positive even when there is no transfer across the boundary. (d) True: Entropy is a property. It is defined by the end states and not the process or path. cv cv i i e e i e dS S m s m s dt
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