L08 - 8.1 Chapter 8 Thermodynamic Principles Motivation All...

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Chapter 8: Thermodynamic Principles – Motivation All of our technology is based on forms of energy conversions which are never perfectly conservative, in the sense that some of the energy is lost in unrecoverable forms during the process of conversion 8.1 Ex : When an electric engine converts electrical energy into kinetic energy, some of the energy is a) lost along the wires (they warm up due to energy dissipated when the electrons drift), and b) to counterbalance the friction with axle (which warms up due to the transfer of energy to the atoms of its material). The respective loss of energy can be minimized by a) reducing the resistance of the wires, and b) reducing the friction by using a lubricant This fact is built into the nature of the respective processes as described by the science of thermodynamics which explains how the loss can be reduced but not completely removed So, in order to learn how we can minimize the loss of energy while we produce or use it (that is, how to increase efficiency), we have to introduce some principles of thermodynamics Moreover, these principles will allow us to delineate ways to dispose of the lost energy in clean, environmentally responsible ways
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Basic Thermodynamics – Conversion of thermal energy In most cases, both the production and use of energy is based on the expansion of gas: in both cases a thermal energy is converted into mechanical energy 8.2 Gas in The conversion happens because the expanding gas does work on the blades of a turbine or on a piston Ex : For instance, if the gas pushes a piston with a force F perpendicular on its surface a linear distance d , the work will be and, by the Work-Energy theorem, a kinetic energy will be produced Turbine: W Fd = KE W Δ = Expanding gas Piston: We can relate this transfer of energy to the particular state of the gas by introducing the concepts of pressure and temperature Thus, pressure will tell us how the gas acts on surfaces and temperature will give us a reason why the gas tries to expand, or increase its volume , since these three quantities are in a relationship that will define a thermodynamic state of the gas
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Basic Thermodynamics – Pressure When a force is exerted onto a wall, its effects will be different depending on how it is distributed on the surface: if a strong enough force is applied to one point, it will prick through; if the same force is imparted on many points on the surface wall, its effects will be diminished 8.2 Ex : Pushing a balloon against a nail is not a good idea, since the reaction to your push will be acted on a small surface. However, pressing the balloon against many nails is fine since the same force is distributed on a large surface. The difference between the two cases is not in force but in pressure Def : The pressure P associated with a force F exerted against a surface S is S F P = 2 2 newtons meters N m Pascal Pa = = = A popular alternative to Pascals as units for pressure is the pound per square inch
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