Presentation on theme: "Thermodynamics II The First Law of Thermodynamics"— Presentation transcript:
1 Thermodynamics II The First Law of Thermodynamics Heat and Work. First Law of ThermodynamicsHeat and Work on Quasi-Static Processes for a Gas.The Second Law of ThermodynamicsHeat Engines and the Second Law of ThermodynamicsRefrigerators and the Second Law of ThermodynamicsThe Carnot EngineHeat PumpsIrreversibility and disorder. EntropyReferences: Tipler; wikipedia,…
2 Surroundings System The First Law of Thermodynamics Energy exists in many forms, such as mechanical energy, heat, light, chemical energy, and electrical energy. Energy is the ability to bring about change or to do work Thermodynamics is the study of energy.SurroundingsThe system can exchange mass and energy through the boundary with the environment.An example of “closed system” - no mass flow- is the gas confined in a cylinder. The boundary –in this case real wall- is made by the cylinder and the piston walls.SystemThe boundary of the system is arbitrarily chosen
3 First Law of Thermodynamics → Conservation of Energy: The First Law of ThermodynamicsFirst Law of Thermodynamics → Conservation of Energy:Energy can be changed from one form to another, but it cannot be created or destroyed. The total amount of energy and matter in the Universe remains constant, merely changing from one form to another.The First Law of Thermodynamics (Conservation) states that energy is always conserved, it cannot be created or destroyed. In essence, energy can be converted from one form into another.The energy balance of a system –as a consequence of FLT- is a powerful tool to analyze the exchanges of energy between the system and its environment.We need to define the concept of internal energy of the system, Eint as an energy stored in the system.Warning: It is not correct to say that a system has a large amount of heat or a great amount of work
4 The First Law of Thermodynamics. Heat, Work and Internal Energy Joule’s Experiment and the First Law of Thermodynamics.Equivalence between work and heat1 calorie = JoulesWork is done on water. The energy is transferred to the water – i. e. the system- . The energy transferred appears as an increase in temperature.We can replace the insulating walls by conducting walls. We can transfer heat through the walls to the system to produce the same increase in temperature.The increase in temperature of the system is a consequence of an increase in Internal Energy. Internal energy is a state function of the systemSchematic diagram for Joule´s experiment. Insulating walls are necessary to prevent heat transfer from the enclosed water to the surroundings.As the weights fall at constant speed, they turn a paddle wheel, which does work on water.If friction in mechanism is negligible, the work done by the paddle wheel on the water equals the change of potential energy of the weights.The sum of the heat transferred into the system and the work done on the system equals the change in the internal energy of the system
5 The First Law of Thermodynamics Another method of doing work. Electrical work is done on the system by the generator, which is driven by the falling weight.
6 The state of the gas will be described by the Ideal Gas Law. The First Law of Thermodynamics. Application to a particular case: A gas confined in a cylinder with a movable pistonThe state of the gas will be described by the Ideal Gas Law.How does the confined gas exchange energy (heat and work) with the surroundings?.How can we calculate the energy –heat and/or work- transferred, added or subtracted, to the system?What is the value of the internal energy for the gas in the cylinder?First Law“Quasi static processes”: a type of process where the gas moves through a series of equilibrium states. Then, we can apply the Ideal Gas Law. In practice, if we move the piston slowly, it will be possible to approximate quasi-static processes fairly well.
7 Rn – H – λET – G – D - Ph = ΔEint First Law of Thermodynamics. Fluxes of energy and mass on the earth surface. Energy balance.HλETCO2Rn = Rns + RnlDGΔEPhEnergy fluxes: Rn : Net gain of heat energy from radiation λET Latent heat, Energy associated to the flux of water vapor leaving from the system H Sensible Heat G Heat energy by conduction to the soilPh: Net photosynthesis ΔEint: Change of the internal energy of the system D: AdvectionNet fluxes of mass Water vapor Carbon –CO2Energy balance (applying First Law):Rn – H – λET – G – D - Ph = ΔEint
8 What is the value of the internal energy for the gas in the cylinder? The First Law of Thermodynamics. Application to a particular case: A gas confined in a cylinder with a movable pistonInternal Energy for an Ideal Gas It only depends on the temperature of the gas, and not on its volume nor its pressureWhat is the value of the internal energy for the gas in the cylinder?Experiment: Free expansion For a gas at low density – an ideal gas-, a free expansion does not change the temperature of the gas.If heat is added at constant volume, no work is done, so the heat added equals to the increase in thermal energyInternal Energy is a state function, i.e. it is not dependent on the process, it only depends of the initial and final temperature
9 Heat transferred to a system The First Law of Thermodynamics. Application to a particular case: A gas confined in a cylinder with a movable pistonHeat transferred to a systemIf heat is added at constant pressure the heat energy transferred will be used to expand the substance and to increase the internal energy.If the substance expands, it does work on its surroundings.Applying the First Law of ThermodynamicsIf heat is added at constant volume, no work is done, so the heat added equals the increase in thermal energyThe expansion is usually negligible for solids and liquids, so for them CP ~ CV.
10 Heat transferred to a system. A summary The First Law of Thermodynamics. Application to a particular case: A gas confined in a cylinder with a movable pistonHeat transferred to a system. A summaryHeat energy can be added to (or lost from) the system. The value of the heat energy transferred depends on the process.Typical processes are- At constant volume- At constant pressureFor the case of ideal gasRelationship of MayerFrom the Kinetic theory, for monoatomic gasesfor biatomic gasesIdeal GasFor solids and liquids, as the expansion at constant pressure is usually negligible CP ~ CV.Adiabatic: A process in which no heat flows into or out of a system is called an adiabatic process. Such a process can occur when the system is extremely well insulated or when the process happens very quickly.
11 The First Law of Thermodynamics The First Law of Thermodynamics. Application to a particular case: A gas confined in a cylinder with a movable pistonWork done on the system, Won , is the energy transferred as work to the system. When this energy is added to the system its value will be positive.The work done on the gas in an expansion isP- V diagramsConstant pressureIf 5 L of an ideal gas at a pressure of 2 atm is cooled so that it contracts at constant pressure until its volume is 3 L what is the work done on the gas? [405.2 J]
12 P- V diagrams Isothermal The First Law of Thermodynamics. P-V diagramsP- V diagramsIsothermalConecting an initial state and a final state by three pathsConstant pressureConstant VolumeConstant Temperature
13 The First Law of Thermodynamics A biatomic ideal gas undergoes a cycle starting at point A (2 atm, 1L). Process from A to B is an expansion at constant pressure until the volume is 2.5 L, after which, it is cooled at constant volume until its pressure is 1 atm. It is then compressed at constant pressure until the volume is again 1L, after which it is heated at constant volume until it is back to its original state. Find (a) the work, heat and change of internal energy in each process (b) the total work done on the gas and the total heat added to it during the cycle.A system consisting of 0.32 mol of a monoatomic ideal gas occupies a volume of 2.2 L, at a pressure of 2.4 atm. The system is carried through a cycle consisting:The gas is heated at constant pressure until its volume is 4.4L.The gas is cooled at constant volume until the pressure decreases to 1.2 atmThe gas undergoes an isothermal compression back to its initial point.(a) What is the temperature at points A, B and C(b) Find W, Q and ΔEint for each process and for the entire cycle
14 Adiabatic Processes. No heat flows into or out of the system The First Law of Thermodynamics. Processes. P-V DiagramsAdiabatic Processes. No heat flows into or out of the system
15 Adiabatic Processes. No heat flows into or out of the system The First Law of Thermodynamics. Processes. P-V DiagramsAdiabatic Processes. No heat flows into or out of the systemThe equation of curve describing the adiabatic process isWe can use the ideal gas to rewrite the work done on the gas in an adiabatic process in the formA quantity of air is compressed adiabatically and quasi-statically from an initial pressure of 1 atm and a volume of 4 L at temperature of 20ºC to half its original volume. Find (a) the final pressure, (b) the final temperature and (c) the work done on the gas cP = J/(mol•K); cV = J/(mol•K). M=28.84 g
16 The First Law of Thermodynamics. Cyclic Processes. P-V Diagrams Two moles of an ideal monoatomic gas have an initial pressure P1 = 2 atm and an initial volume V1 = 2 L. The gas is taken through the following quasi-static cycle:A.- It is expanded isothermally until it has a volume V2 = 4 L.B.- It is then heated at constant volume until it has a pressure P3= 2 atmC.- It is then cooled at constant pressure until it is back to its initial state.(a) Show this cycle on a PV diagram. (b) Calculate the heat added and the work done by the gas during each part of the cycle. (c) Find the temperatures T1, T2, T3
17 The First Law of Thermodynamics. Cyclic Processes. P-V Diagrams
18 The First Law of Thermodynamics. Cyclic Processes. P-V Diagrams At point D in the figure the pressure and temperature of 2 mol of an ideal monoatomic gas are 2 atm and 360 K. The volume of the gas at point B on the PV diagram is three times that at point D and its pressure is twice that at point C. Paths AB and DC represent isothermal processes. The gas is carried through a complete cycle along the path DABCD. Determine the total work done by the gas and the heat supplied to the gas along each portion of the cycle
19 The First Law of Thermodynamics. Cyclic Processes. P-V Diagrams
20 The First Law of Thermodynamics. Cyclic Processes. P-V Diagrams
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