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**Reversible and irreversible Processes**

intuitive approach to reversible and irreversible processes later introduce entropy and the 2nd law foundation of thermodynamics Reversible process: can be defined as one whose “direction” can be reversed by an infinitesimal small change in some property of the system. “Gedankenexperiment” to picture a reversible process: Make a video recording of a process 1 Observable process reversible Run the recording backwards 2 Process impossible to observe irreversible

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Examples: Process is possible reversible Backward recording reversible Backward recording

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**but x x Small changes can be reversed reversible V1 ,Ts V2 ,Tf gas**

You never observe reversed process of free expansion irreversible

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**Reversibility is an idealization**

(in strictest sense, almost all real processes are irreversible) Reversibility requires equilibrium processes but Not every equilibrium process is reversible Almost perfect insulation Example Tg gas in equilibrium at any time > T0 Qout Although system in equilibrium, no small change of the system will reverse the heat flow

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**Reversibility is an idealization**

Dry friction between 2 objects You never observe the reversed process: object starts to move without assistance

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**> T1 T2 1 to 2 x Friction between piston and cylinder**

irreversibility Heat Conductivity in Solids (an example for irreversibility) Remember: Heat is an energy transferred from one system to another because of temperature difference T1 > T2 System 2 System 1 Heat Q flows from 1 to 2

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**> T1 T2 *(in the textbook T2>T1) Heat reservoir 2**

L T(x) T1 T2 L x A Heat transfer per time interval through homogeneous solid object: K: thermal conductivity of the rod L where A: cross-section of the rod

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**Electric Systems (examples for reversibility and irreversibility)**

#1 + battery - : work done against electrical forces per unit charge Work: Current

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**Resistor network with total resistance R**

Irreversible case #1 #2 Resistor network with total resistance R In the steady state: Internal energy of black box unchanged 1. law: Heat leaving the system Application as heater #1 #2 reversible case Capacitor network with total Capacity C - Charging an uncharged capacitor - Discharge of the capacitor done by the capacitor No heat transferred (Q=0)

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