1. Chapter 12 Laws of Thermodynamics

2. Chapter 12 Objectives Internal energy vs heat Work done on or by a system Adiabatic process 1 st Law of Thermodynamics 2 nd Law of Thermodynamics Isobaric, Isovolumetric, Isothermal Heat engines Efficiency of a heat engine Carnot engine Entropy

3. 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

4. Work 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. W = P  V

5. 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.

6. 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

7. 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.

8. 1 st 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.  U = Q - W Change in Internal Energy Work done by gas Heat released or absorbed by gas

9. 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!

10. 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

11. 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 = P  V

12. 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 = P  V

13. 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 U f = U i –  U = 0 So any heat released or absorbed by the gas is a result of work being done.  U = Q - W Q = W

14. 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

15. 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 = Q h - Q c

16. 2 nd 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). W e = QhQh Q h - Q c QhQh = = 1 - QcQc QhQh

17. 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.

18. 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 = T h - T c ThTh = 1 - TcTc ThTh Based on the Carnot cycle, the highest efficiency of a gasoline internal combustion engine is 30%!

19. 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.