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USF EGN 3343 - 02__Thermo_Ch_1

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ThermodynamicsChapter 1Dr. DixonOutline• Laws of Thermodynamics• Classical Thermodynamics vs Statistical Thermodynamics• Systems and Control Volumes• Density, specific gravity• Thermodynamic state• Equilibrium• Zeroth law• Temperature scales• Pressure, variation with depthLaws of Thermodynamics• 0thlaw – T, temperature and equilibrium• 1stlaw – U, energy, conservation of energy• You can get out what you put in.• 2ndlaw – S, entropy, as time moves forward, things unravel• Or that energy not only has a quantity, but also has a quality, or a measure of usefulness.• Or, you can get out what you put in, but only at 0 K• 3rdlaw – numerically defines entropy• You can’t actually ever get to 0 K, so you can’t get out what you put in1stlaw• Energy is conserved in all forms• If you start with 100 units of energy in a hot cup of coffee, and 50 units of energy in the cool air around it, then what happens after 10 minutes?• Some of the thermal energy leaves the coffee and ends up in the air. So you have let’s say 80 units left in the coffee and if energy is conserved, how much do you have in the surrounding air? • 70 units.• Notice that the total amount of energy has stayed fixed – we started with 150 total units of energy and we ended with 150 total units of energyAccording to the first law of ThermodynamicsA. Entropy is conservedB. Energy is conservedC. Enthalpy is conservedD. Mass is conservedE. All of the aboveF. None of the aboveEntropy is conservedEnergy is conservedEnthalpy is conservedMass is conservedAll of the aboveNone of the above17%17%17%17%17%17%2ndlaw• 1stlaw - energy is conserved in all forms• We have 100 units of energy in a hot cup of coffee, and 50 units of energy in the cool air around it• Then we feel a sudden chill come over the room and the coffee starts boiling• Some of the thermal energy leaves the cold air and moves into the hot coffee so you now have 140 units of energy in the coffee and only 10 units of energy in the cool air in the room• According to the first law, this is possible. Energy has been conserved. • But does this happen?• No• Simple observation tells us that the 1stlaw of Thermodynamics is not enough to describe reality• The 2ndlaw captures this observation and tells us that energy not only has a quantity but also a quality• If the above situation happened, the 2ndlaw of Thermodynamics would have been violated• https://vimeo.com/68401939Definitions• System, surroundings, boundary• State8Systems• System – the part of the universe that we are studying• Everything else is the surroundings• The line that separates them is the boundary• It can be real, like a wall, or imaginary, like a box surrounding 10 liters of air in the back of the room• 3 kinds of systems• Open – mass and energy can flow freely through the boundary• Also known as a control volume• Closed – just energy can flow through the boundary• Also known as a control mass• Isolated systems – no mass and no energy can flow through the boundaryWhat kind of system is a water heater?A. Closed systemB. Open systemC. Isolated systemClosed systemOpen systemIsolated system33%33%33%What defines a system?• Turns out that when you’ve got a system in equilibrium you don’t need all that much information to define what we call the state of the system• What do we need to know?• A few macroscopic properties – temperature, pressure, density, etc.• What components/substances are present in the system – air, water, refrigerant, carbon dioxide, etc• Whether the system is homogeneous or heterogeneous• Whether the system is in equilibriumCoffee mixed with cream isA. HomogenousB. HeterogeneousC. NeitherHomogenousHeterogeneousNeither33%33%33%Water with ice cubes isA. HomogeneousB. HeterogeneousC. NeitherHomogeneousHeterogeneousNeither33%33%33%What does it mean for a system to be in equilibrium?• The system properties (density, temperature, pressure, etc.) don’t vary across the system and don’t change in time.A hot cup of coffee sitting in a cold room is in thermodynamic equilibrium.A. TrueB. FalseTrueFalse50%50%After several hours, a cup of coffee left sitting on the counter will be in thermodynamic equilibriumA. TrueB. FalseTrueFalse50%50%Properties of a system• Temperature, pressure, volume, mass, density, enthalpy, entropy, internal energy, etc.• Properties can be categorized as either Intensive or Extensive properties• Extensive properties scale with the size of the system • Intensive properties do not scale with the size of the system• Properties can also be “specific” which indicates that it has units that are on a per unit mass basis (example – specific volume is m3/kg)• Categorize the following properties as either intensive or extensive• Temperature, pressure, volume, specific volume, mass, density, internal energyDefining the state of the system• What defines the state of a system?• Only 2 independent, intensive properties• Whether it is in equilibrium• Whether it is hetero or homogeneous• The components of the systemEquilibrium• But is any system really in equilibrium?• Probably not.• Does that mean that Thermodynamics can’t help us?• Probably not.• What about a system, like the piston-cylinder device in an engine? It is constantly changing, so is it in equilibrium?• Sort of.• Depending on the speed of the piston, the system is most likely in a state of “quasi-equilibrium”. In this state, everything is changing, but at any given instant, the properties are “pretty much” constant across the entire system.• So most thermodynamic processes involve constantly changing systems that are moving through states of quasi-equilibriumProcess/Path• During a process, such as the compression of a cylinder or the combustion of a gas, the system properties change• As the properties change, the state of the system follows a path. This path can be shown on various diagrams, such as the Pressure-Volume diagram to the right• A series of processes can be combined, and they become a cyclewhen the system returns to the initial stateJet engine• Is this an open or a closed system?• If this is in an airplane at cruising altitude, is the engine in equilibrium?• This is a steady-flow steady state system (SFSS system) and is very common in the field of Thermodynamics0thlaw• If body A and body B are in thermal equilibrium and


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