Transcript Chapter 12
Chapter 12 Laws of Thermodynamics Chapter 12 Objectives • • • • • • • • • • Internal energy vs heat Work done on or by a system Adiabatic process 1st Law of Thermodynamics 2nd Law of Thermodynamics Isobaric, Isovolumetric, Isothermal Heat engines Efficiency of a heat engine Carnot engine Entropy Internal Energy • Internal energy can be thought of as all the energy in a system that is not being transferred as heat. – This could include nuclear energy, chemical energy, elastic energy as well as heat that has not been transferred yet. • Temperature can often be thought of as a measure of internal energy. • This is any amount of energy that cannot be included as mechanical energy. – Potential – Kinetic Work W = PV • Internal energy can be transferred between systems without transferring heat. – That would mean that the temperature would not change. • So the internal energy could be transferred as mechanical energy in the form of work. • Recall that work required some displacement to exist, we also need that fluid to create a displacement. • So work can only be done when there is a change in volume. – The pressure should remain constant. – If not, then the equation above should be broken down parts of constant pressure. Work On or By the System • Work can be positive or negative, depending “who” is doing the work. • The gas does work on the system when the volume is expanding. – That means that V is positive, so work is positive. • When work is being done by the system, the volume is decreasing. – So V should be negative, so work will be negative. Isobaric, Isovolumetric, Isothermal • A system can be isobaric when the pressure is held constant in that system. – So cross out P in the equation • A system can be isovolumetric when the volume is held constant in that system. – So cross out V in the equation • A system can be isothermal when the temperature is held constant in that system. – So cross out T in the equation Adiabatic • An adiabatic process is one in which no heat is transferred between the system and the environment while work is being done. • Which means the gas has the ability to freely expand in the container while the container is completely insulated from its environment. • Usually involves filling a container with more gas molecules. – Such as filling a balloon with air. 1st Law of Thermodynamics • This is generally known as the Law of Conservation of Energy. – So the internal energy of the system cannot be created or destroyed. • So the change in internal energy needs to account for the heat in the gas and whatever work is done by the gas. Work done by gas U = Q - W Change in Internal Energy Heat released or absorbed by gas Isolated System • An isolated system is one in which the system does not interact with its surroundings. • No interaction means – No pressure to change the gas pressure – No volume change of the container – No temperature due to no transfer of energy. • No pressure = no work! • No temperature difference = no heat! Cyclic Process • A cyclic process is a process that starts and finishes at the same state. – Heat engines are a good example of a cyclic process. • Air conditioner • Since the initial and final state of the system is constant, the internal energy remains the same. – So Q = W U = Q - W Isobaric Process • An isobaric process remember maintains constant pressure. • Since pressure is constant, that allows work to be done whenever there is a volume change. • Temperature can also change, so that means heat can be transferred. U = Q - W W = PV Isovolumetric Process • An isovolumetric system is one in which the volume does not change. • No change in volume means that there is no work being done. • So any change in the internal energy is directly due to the heat being released or absorbed by the gas. U = Q - W W = PV Isothermal Process • An isothermal process is one in which the temperature is kept constant. – This would mean that the internal energy of the gas must be kept constant. • So Uf = Ui – U = 0 • So any heat released or absorbed by the gas is a result of work being done. U = Q - W Q=W Adiabatic Process • Recall that an adiabatic process is one in which there is no heat transfer and yet there is work being done. – This process is one in which the number of particles are being increased. • Like blowing up a balloon. • An increase of particles would require the system to do work to bring those particles in. – That would use up internal energy to do that work. • The opposite would be true also when the particles are released. – Here the gas would do work on the system by adding gas molecules to it. U = Q - W U = - W Heat Engine • A heat engine is any device that converts heat energy into useful forms of energy. – Mechanical energy – Electrical energy • A heat engine carries some working material (fluid) that transfers energy from a cold to hot reservoir. – – – – Steam engine Internal combustion engine Refrigerator Air Conditioner/Furnace • The net work done by a heat engine is equal to the difference of the hot and cold reservoirs. – Hot reservoir can also be thought of as input energy. – Cold reservoir can also be thought of as output energy. W = Qh - Qc 2nd Law of Thermodynamics • It is impossible to construct a heat engine that is 100% efficient. • Efficiency is found by the ratio of net work done to the heat absorbed by the hot reservoir (input energy). Qh - Qc W = e= = Qh Qh Qc 1Qh Carnot Engine • The Carnot engine is often thought to be the most efficient heat engine possible. – Sadi Carnot • French – 1796-1832 • The Carnot engine is an engine operating in an ideal reversible cycle. – A reversible cycle is one in which at every stage of the cycle the system is in thermal equilibrium. • Also called a Carnot cycle. • Carnot’s Theorem – No real engine operating between two heat reservoirs can be more efficient than a Carnot engine, operating between the same two heat reservoirs. Efficiency of a Carnot Engine • The efficiency of a Carnot engine is based purely off of the temperature difference between heat reservoirs of the system. – Temperature measured in Kelvin. e= Th - Tc Th Tc = 1Th Based on the Carnot cycle, the highest efficiency of a gasoline internal combustion engine is 30%! Entropy and Disorder • Entropy is a measure of the disorder found in a thermodynamic system. – Larger the entropy, the more disorder of the molecules and their behavior. • Based on probability, systems with high disorder are much more likely to happen in nature. – With that said, the entropy of the Universe is always increasing.