Laws of Thermodynamics, Schemes and Mind Maps of Thermodynamics

Download Laws of Thermodynamics and more Schemes and Mind Maps Thermodynamics in PDF only on Docsity! Laws of Thermodynamics Thermodynamics • Thermodynamics is the study of the effects of work, heat, and energy on a system • Thermodynamics is only concerned with macroscopic (large-scale) changes and observations Introduction According to British scientist C. P. Snow, the three laws of thermodynamics can be (humorously) summarized as 1. You can’t win 2. You can’t even break even 3. You can’t get out of the game 1.0 You can’t win (1st law) • The first law of thermodynamics is an extension of the law of conservation of energy • The change in internal energy of a system is equal to the heat added to the system minus the work done by the system ΔU = Q - W Glenn First Law of Thermodynamics Research Heat Transfer W Work E = Intemal Energy State 2 E,-E, =Q-W Any thermodynamic system in an equilibrium state possesses a state variable called the intemal energy (E). Between any two equilibrium states, the change in intemal energy is equal to the difference of the heat transfer into the system and work done by the system. State 1 1.1.2 Isothermal Process • An isothermal process is a constant temperature process. Any heat flow into or out of the system must be slow enough to maintain thermal equilibrium • For ideal gases, if ΔT is zero, ΔU = 0 • Therefore, Q = W – Any energy entering the system (Q) must leave as work (W) 1.1.3 Isobaric Process • An isobaric process is a constant pressure process. ΔU, W, and Q are generally non- zero, but calculating the work done by an ideal gas is straightforward W = P·ΔV • Water boiling in a saucepan is an example of an isobar process 1.1.4 Isochoric Process • An isochoric process is a constant volume process. When the volume of a system doesn’t change, it will do no work on its surroundings. W = 0 ΔU = Q • Heating gas in a closed container is an isochoric process 2.0 You can’t break even (2nd Law) • Think about what it means to not “break even”. Every effort you put forth, no matter how efficient you are, will have a tiny bit of waste. • The 2nd Law can also be stated that heat flows spontaneously from a hot object to a cold object (spontaneously means without the assistance of external work) Glenn Second Law of Thermodynamicspresearch Center Heat Transfer AS = Entropy = AQ T There exists a useful thermodynamic variable called entropy (5). A natural process that starts in one equilibrium state and ends in another will go inthe direction that causes the entropy of the system plus the environment to increase for an irreversible process and to remain constant for a reversible process. S,= 5, (reversible) S,> S, (reversible) 2.1 Concerning the 2nd Law • The second law of thermodynamics introduces the notion of entropy (S), a measure of system disorder (messiness) • U is the quantity of a system’s energy, S is the quality of a system’s energy. • Another C.P. Snow expression: – not knowing the 2nd law of thermodynamics is the cultural equivalent to never having read Shakespeare 2.4 Heat Engine • A device which transforms heat into work is called a heat engine • This happens in a cyclic process • Heat engines require a hot reservoir to supply energy (QH) and a cold reservoir to take in the excess energy (QC) – QH is defined as positive, QC is negative 2.4.1 Cycles • It is beyond the scope of this presentation, but here would be a good place to elaborate on: – Otto Cycle – Diesel Cycle – Carnot Cycle • Avoid all irreversible processes while adhering to the 2nd Law (isothermal and adiabatic only) 2.4.2 The Carnot Cycle be [a ei Ni ol oi) 2.5 Engine Efficiency • The thermal efficiency of a heat engine is e = 1 + QC/QH • The “engine” statement of the 2nd Law: – it is impossible for any system to have an efficiency of 100% (e = 1) [Kelvin’s statement] • Another statement of the 2nd Law: – It is impossible for any process to have as its sole result the transfer of heat from a cooler object to a warmer object [Clausius’s statement] 2.6 Practical Uses • Automobile engines, refrigerators, and air conditioners all work on the principles laid out by the 2nd Law of Thermodynamics • Ever wonder why you can’t cool your kitchen in the hot summer by leaving the refrigerator door open? – Feel the air coming off the back - you heat the air outside to cool the air inside – See, you can’t break even! 3.0 You can’t get out (3rd Law) • No system can reach absolute zero • This is one reason we use the Kelvin temperature scale. Not only is the internal energy proportional to temperature, but you never have to worry about dividing by zero in an equation! • There is no formula associated with the 3rd Law of Thermodynamics Thermodynamic Equilibrium 22°, (Zeroth Law) Center Object #1 (Thermometer) * <—_> When two objects are separately in thermodynamic equilibrium with a third object, they are in equilibrium with each other. 4.1 Temperature Standards • See Heat versus Temperature slides for a discussion of these two concepts, and the misconceptions surrounding them – Heat is energy transfer – Temperature is proportional to internal energy – Fahrenheit, Celsius, and Kelvin temp scales