2 Could This Happen?Could all the air in a box rush to one side of the box leaving a vacuum in the other side?Will heat flow spontaneously from a cold object to a hot object?Nature’s processes have preferred directionsDirection is important in Thermodynamic processes
3 Reversible vs. Irreversible Processes A reversible process is a transition from one state of a thermodynamic system to another, during which the system is always very close to a state of mechanical and thermal equilibrium (including uniform temperature and pressure) In other words…a small change in the system that can be reversed. Ex. a gas that expands slowly and adiabatically can be compressed slowly and adiabatically by a small increase in pressureA process that despite any small change in conditions cannot be reversed. Ex. Conversion of work to heat by friction
4 Heat EnginesA device that converts heat partly into work or mechanical energy. In a heat engine a quantity of matter, called the working substance, inside the engine undergoes addition and subtraction of heat, expansion and compression, and sometimes, a phase change.Ex. of Heat EnginesWorking Substance
5 Heat EnginesThe simplest engines to analyze are those that have a working substance which undergoes a cyclic process.Cyclic processa sequence of processes that eventually leaves the substance in the same state as it startedwhen a cycle is complete the system starts and ends with the same internal energyCycleAbsorb Heat at High Temp. Do Work Discard Heat at Low Temp.
6 Heat EnginesInternal Energy doesn’t change after a cycle.
7 Fuel ConsumptionA gasoline engine in a large truck takes in 2500 J of heat and delivers 500 J of mechanical work per cycle. The heat is obtained by burning gasoline with heat of combustion Lc = 5.0 x 104 J/g.What is the thermal efficiency of this engine?How much heat is discarded in each cycle?How much gasoline is burned during each cycle?If the engine goes through 100 cycles per second, what is its power output in watts? In horsepower? [1hp = 746 W]How much gasoline is burned per second? per hour?
8 The Otto Cycleidealized model of the thermodynamic processes in a gasoline engine
9 The Otto Cycleidealized model of the thermodynamic processes in a gasoline engine
10 The Diesel Cycleidealized model of the thermodynamic processes in a diesel enginethere are no spark plugs for ignition in a diesel enginethe fuel is injected just before the power strokethe fuel ignites spontaneously from the high temperature developed during the adiabatic compressionProsCons
11 Refrigerator heat engine operating in reverse take heat from a cold place (inside the fridge) and expel it to a warmer place (outside the fridge)I thought heat didn’t flow from cold to hot?!?!...well it doesn’t…to move heat from cold to hot requires some work!
12 Refrigeration Process Don’t Write ThisThe compressor takes in the refrigerant fluid (working substance) and compresses it adiabatically (this requires work)The refrigerant is then sent to the condenser coil at high temperatureHeat transfers from the hot refrigerant to the surroundings (outside the fridge) which partially condenses the fluidRefrigerant fluid expands adiabatically into the evaporator (the expansion valve controls this). The expansion cools the fluid enough so that it is colder than the inside of the fridge.The fluid is then transferred into the fridge where it absorbs the heat from the inside of the fridge and partially vaporizes45321
13 Air Conditioners & Heat Pumps Air conditioners operate the same as a refrigeratorDifference – inside of room/building is like the inside of fridge;outside of fridge is now outside of room/buildingHeat Pumps are similar to anair conditioner exceptthey’re backwards.
14 2nd Law of Thermodynamics (2LT) 2LT for EnginesIt is impossible for any system to undergo a process in which it absorbs heat from a reservoir at a single temperature and converts the heat completely into mechanical work with the system ending in the same state in which it began.A 100% efficient engine does not exist.
15 2nd Law of Thermodynamics (2LT) 2LT for RefrigeratorsIt is impossible for any process to have as its sole result the transfer of heat from a cooler object to a hotter object.It is impossible to have a workless refrigerator.
16 How Efficient Can An Engine Be? 2LT – No engine is 100% efficient In 1824 Sadi Carnot (a French engineer) determined the hypothetical maximum possible efficiency for an engine based on the 2LT. Carnot Cycle To maximize efficiency we must avoid irreversible processes. Heat flow as a result of a temperature difference is an irreversible process. To avoid this irreversible process the only time that heat can flow must be during an isothermal process.
17 Carnot Cycle Isothermal Expansion (a b) (Heat Absorbed) W is the work done by the systemIsothermal Expansion (a b) (Heat Absorbed)Adiabatic Expansion(b c)Isothermal Compression(c d)(Heat Released)4. Adiabatic Compression(d a)
18 Thermal Efficiency of Carnot Cycle The thermal efficiency of a Carnot Engine is dependent only on the absolute temperature (temperature in Kelvin) of the heat reservoirs.
19 A Carnot EngineA Carnot engine takes 200o J of heat from a reservoir at 500 K, does some work, and discards some heat to a reservoir at 350 K.What is the engine’s efficiency?How much work does it do?How much heat is discarded by the engine?
20 Entropy A quantitative measure of disorder. Nature (irreversible processes) tends towards randomness (disorder).No process is possible in which the total entropy decreases when all systems taking part in the process are included.Reversible ProcessesChange in Entropy is the heat transferred divided by the absolute temperature (Kelvin).Thermo ProcessHeat (Q)EntropyIsothermalQ = -WIsobaricQ = ΔU - WIsochoricQ = ΔUAdiabaticQ = 0