Thermodynamic Processes Chapter First Law of Thermodynamics Imagine a roller coaster that operates without friction. The car is raised against the force of gravity by work. Once the car is freely moving, it will have a certain kinetic energy (KE) and a certain potential energy (PE). 2 First Law of Thermodynamics The coaster will move slower at the top of the rise and faster at the low points in the track. However the mechanical energy, KE + PE, remains constant throughout the rides duration. 3 4 First Law of Thermodynamics If friction is taken into account, mechanical energy is not conserved. 5 First Law of Thermodynamics A steady decrease in the cars total mechanical energy occurs because of work being done against the friction between the cars axles and its bearings and between the cars wheels and the coaster track. 6 First Law of Thermodynamics Mechanical energy is transferred to the atoms and molecules throughout (a system) the entire roller coaster. Thus, the roller coasters internal energy increases by an amount equal to the decrease in the mechanical energy. 7 First Law of Thermodynamics Most of this energy is then gradually dissipated to the air surrounding the roller coaster as heat and sound. 8 First Law of Thermodynamics If the internal energy for the roller coaster and the energy dissipated total energy will be constant. 9 First Law of Thermodynamics The principle of energy conservation that takes into account a systems internal energy as well as work and heat is called the First law of thermodynamics. 10 First Law of Thermodynamics In all the thermodynamic processes described so far, energy has been conserved. To describe the overall change in the systems internal energy, one must account for the transfer of energy to or from the system as heat and work. 11 First Law of Thermodynamics The total change in the internal energy is the difference between the final internal energy value (U f ) and the initial internal energy value (U i ). U = U f - U i 12 First Law of Thermodynamics Energy conservation requires that the total change in internal energy from its initial to its final equilibrium conditions be equal to the transfer of energy as both heat and work. 13 First Law of Thermodynamics U =Q W U - internal energy Q heat energy W work energy SI Units : Joules 14 First Law of Thermodynamics According to the first law of thermodynamics, a systems internal energy can be changed by transferring energy as either work, heat, or a combination of the two. 15 Cyclic Processes If a systems properties at the end of the process are identical to the systems properties before the process took place, the final and initial values of internal energy are the same and the change in the internal energy is zero. 16 Cyclic Processes U net = 0 Q net = W net 17 Cyclic Processes This process resembles an isothermal process in that all energy is transferred as work and heat. But now the process is repeated without no net change in the systems internal energy. This is a cyclic process. 18 Cyclic Processes In a refrigerator, the net amount of work done during each complete cycle must equal the net amount of energy transferred as heat to and from the refrigerant. 19 Cyclic Processes Energy is transferred as heat from the cold interior of the refrigerator to the even colder evaporating refrigerant (Q cold or Q c ). 20 Cyclic Processes Energy is also transferred as heat from the hot condensing refrigerant to the relatively colder air outside the refrigerator (Q hot or Q h ) 21 Cyclic Processes Therefore, the difference between Q h and Q c equals the net energy transferred as heat-thus the net work done-during one cycle of the refrigeration process. W net = Q h Q c Where Q h > Q c 22 Cyclic Processes The colder you want the inside of a refrigerator to be the greater the net energy transferred as heat (Q h -Q c ) must be. The net energy transferred as heat can be increased only if the refrigerator does more work. 23 Cyclic Processes A refrigerator uses mechanical work to create a difference in temperature and thus transfers energy as heat. 24 Cyclic Processes A heat engine is a device that does the opposite. It uses heat to do mechanical work. 25 Cyclic Processes Instead of using the difference in potential energy to do work, heat engines do work by transferring energy from a high temperature substance to a lower temperature substance. 26 Cyclic Processes For each complete cycle of the heat engine, the net work done will equal the difference between the energy transferred as heat from high temperature substance to the engine (Q h ) and the energy transferred as heat from the engine to a lower temperature substance. 27 Cyclic Processes The larger the difference between the amount of energy transferred as heat into the engine and out of the engine, the more work the engine can do. 28 Cyclic Processes The internal combustion engine found in most vehicles is an example of a heat engine. Internal combustion engines burn fuel within a closed chamber. 29 Cyclic Processes The potential energy of the chemical bonds in the reactant gases is converted to kinetic energy of the particle products of the reaction. These gaseous products push against a piston and thus do work on the environment. 30 Cyclic Processes Although the basic operation of any internal combustion engine resembles that of an ideal heat engine certain steps do not fit in the model. 31 Cyclic Processes When gas is taken in or removed from the cylinder, matter enters and leaves the system so that the matter in the system is not isolated. 32 Cyclic Processes No heat engine operates perfectly. Only part of the available internal energy leaves the engine as work done on the environment; most of the energy is removed as heat. 33