ME235Lecture_7-8 - Thermodynamics Vm235 Lecture 7 & 8...

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1 Thermodynamics Vm235 Lecture 7 & 8 Chapter 7 – The 2 nd Law of Thermodynamics 1 What we have done so far: Developed a methodology of defining a system, C.V., C.S. etc. Learned how to find the state of the system. Learned about processes that occur between two states. Learned that heat and work things we are interested in – are the ways by which the system interacts with its surroundings. These are not 2 interacts with its surroundings. These are not unique properties like u, h, P, T, v . The amount of work done or heat added depends on the path . Ensured that energy is conserved when a process occurs – The First Law of Thermodynamics.
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2 Review of Process & Cycle State: Points 1, 2, 3, 4 on the P-v diagram correspond to various states of the to various states of the system. Process: Identifies the way in which the system goes from one state to another. Cycle: Refers to a group of 3 processes that bring the system back to its original state. Observations Hot Body Cold Body Thermal Equilibrium (same T) YES NO Consider the following systems (say isolated) with well-defined initial states: High Pressure Vacuum Uniform pressure throughout YES NO Likewise, the 1 st Law is not violated if the following changes occur in any direction: While we have never observed the reverse to occur, it is OK by the 1 st Law. 4 Oxygen Nitrogen Uniform mixture throughout YES NO You can make the reverse happen, but it does not occur SPONTANEOUSLY
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3 More observations The direction of spontaneous change for a ball bouncing on a floor. On each bounce some of its potential energy is degraded into thermal motion of the atoms of the ball & the floor. This energy disperses into other atoms of the ball and the floor. 5 The reverse has never been observed to take place. The reverse, if it occurs, is OK by the 1 st Law as long as the energy is conserved. Recall also that only a small amount of thermal energy is required to make the ball jump very high. Molecular Interpretation A ball resting on a warm surface; the atoms are undergoing thermal motion (chaotic vibration, in this instance), as indicated by the arrows. For the ball to fly upwards, some of the random vibrational motion would have to change into 6 to change into coordinated, directed motion. Such a conversion is highly improbable.
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4 So, the 1 st Law is not enough. Something is missing! What is missing? A Law that will tell us the: DIRECTION OF SPONTANEOUS CHANGE This is the 2 nd Law of Thermodynamics 7 We need it to ensure that systems we design will work. As we will see later ENTROPY is a property that we have invented (like internal energy) that will allow us to apply the 2 nd Law quantitatively. The second law of thermodynamics, a fundamental scientific principle stating that entropy increases over ti i d f d i t 2 nd Law has wide Law has wide-ranging implications & is often misunderstood. ranging implications & is often misunderstood.
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ME235Lecture_7-8 - Thermodynamics Vm235 Lecture 7 & 8...

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