* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The Laws of Thermodinamics
Survey
Document related concepts
Equipartition theorem wikipedia , lookup
Temperature wikipedia , lookup
Heat transfer wikipedia , lookup
Conservation of energy wikipedia , lookup
Chemical thermodynamics wikipedia , lookup
First law of thermodynamics wikipedia , lookup
Thermal conduction wikipedia , lookup
Internal energy wikipedia , lookup
Heat transfer physics wikipedia , lookup
Thermodynamic system wikipedia , lookup
Second law of thermodynamics wikipedia , lookup
Adiabatic process wikipedia , lookup
Transcript
The Laws of Thermodinamics • Work in Thermodynamic process • Energy can be transferred by heat and by work done on the system • W=-FΔy =-PA Δy • The work W done on a gas at constant pressure is given by: W= -P ΔV P-pressure through the gas ΔV- the change in volume W= -P ΔV –can be used to calculate the work done on the system only when the pressure of the gas remain ct. during the expansion or compression ISOBARIC PROCESS- a process in which the pressure remain constant • The area under the graph in a PV diagram is equal in magnitude to do work done on a gas: A=P ΔV • The first law of thermodynamics (energy conservation law that relates changes in internal energy): If a system undergoes a change from an initial state to a final state, where Q is the energy transferred to the system by heat and W is the work done on the system, the change in the internal energy of the system, ΔU is given by: ΔU= Uf-Ui = Q+W • Q>0- when energy is transferred into the system by heat • Q<0- when energy is transferred out of the system by heat • W>0 –when work is done on the system • W<0 – when the system does work on its environment • The internal system of any isolated system must remain constant, ΔU=0 • If the system isn’t isolated, ΔU=0if the system goes through a cyclic process, which P, V, T, n (nr of moles) return to original values • U=3/2 n R T • ΔU=3/2 n R ΔT • Molar specific heat at V=ct. of a monatomic ideal gas, Cv is: Cv =3/2 R • ΔU=n Cv ΔT • Q= ΔU-W= ΔU+ P ΔV • P ΔV=n R ΔT • Q= 3/2n R ΔT+ n R ΔT= 5/2 n R ΔT • Molar heat capacity at P=ct., Cp is: Cp=5/2R • Q=n Cp ΔT • Cp = Cv +R • Adiabatic processno energy enters or leaves the system by heat (can still do work) • Q=0 ΔU=W • For an ideal gas undergoing an adiabatic process: PVγ =ct γ= Cp / Cv γ - adiabatic index of the gas • Isothermal process- the temperature of the system doesn’t change • ΔT=0; ΔU=0; W=-Q • For an ideal gas at T=ct P=nRT/V • The work done on the environment during an isothermal process is given by: Wenv=nRTln(Vf/Vi) • The PV diagram of a typical isothermal process , contrasted with an adiabatic process • A heat engine takes energy by heat and converts into electrical or mechanical energy • A heat engine carries some working substance through cyclic process during which: • 1. energy is transferred by heat from a source at a high temperature • 2. work is done by the engine • 3. energy is expelled by the engine by heat to a source at lower temperature • The absorbs energy Qh from a hot reservoir, does work Weng , that gives up energy Qc to the cold reservoir • W= -Weng (work is done on the engine) • ΔU=0=Q+W Q net=-W =Weng • The work done by the engine= the net energy absorbed by the engine • Q net=|Qh|-|Qc| • The thermal efficiency e of a heat engine is the work done by the engine divided by the energy absorbed during one cycle: • e= Weng / |Qh|=|Qh|-|Qc|/ |Qh|=1-|Qc|/|Qh| • e=1- engine has a 100% efficiency The heat pumps and refrigerator The coefficient of performance for a refrigerator or an air conditioner (the cooling mode): COP=|Qc|/W • The coefficient of performance of a heat pump operating in a heat mode : COP=|Qh|/W THE SECON LAW OF THERMODINAMICS: No heat engine operating in a cycle can absorb energy from a reservoir and use it entirely for the performance of na equal amount of work (e<1) Ist law: we can’t get a greater amount of energy out of a cyclic process that we put in 2nd law: we can’t break it even • Reversible process- every state along the path in an equilibrium state, so the system can return to its initial conditions by going along the same path in the reverse direction. • Irreversible process- a process that doesn’t satisfy this requirement • Most natural processes are known to be ireversible • The Carnot Engine: a heat engine operating an ideal, reversible cycle (the Carnot cycle) between two reservoirs is the most efficient engine possible • Carnot’s theorem: no real engine operating between 2 energy reservoirs can be more efficient than a Carnot engine operating between the same 2 reservoirs Carnot Cycles: • An ideal gas is contained in a cylinder with a movable piston at one end • 1. The process A→B is an isothermal expansion at temperature T (the gas is placed in thermal contact with a hot reservoir, and the gas absorbs energy Qh from the reservoir and does work WAB in raising the piston) • 2. The process B→C , the gas expends adiabatically (energy enter or leave by heat). The temperature falls from Th to Tc, and the gas does work WBC to the raising piston • 3. The process C→D, the gas is placed in thermal contact with a cold reservoir at temperature Tc and is compress isothermally at temperatures Tc (the gas expels energy Qc to the reservoir and the work done on the gas is WCD) • 4. The process D→A , the gas is compressed adiabatically (the temperature increases to Th, and the work done on the gas is WDA) • |Qc|/|Qh|=Tc/Th • e=1-Tc/Th • All carnot engines operating reversibly between the same 2 temperatures have the same efficiency The third Law of Thermodynamics: its impossible to lower the temperature of a system to absolute zero (such reservoirs are not available) All real engines operate irreversibly, due to friction and the brevity of their cycles, are less efficient than the Carnot engine • Entropy • Let Qr be the energy absorbed or expelled during a reversible, constant temperature process between two equilibrium states. Then the change in entropy during any constant temperature process connecting the two equilibrium states is defined as: ΔS =Qr/T SI unit: J/K (Joules/Kelvin)