2FluidsFluids are substances that can flow, such as liquids and gases, and even a few solids.In Physics B, we will limit our discussion of fluids to substances that can easily flow, such as liquids and gases.D = m/VD: density (kg/m3)m: mass (kg)V: volume (m3)You should remember how to do density calculations from chemistry!Density can also be represented by - ρρ= m/Vρ: density (kg/m3)
3Pressure P = F/A P : pressure (Pa) F: force (N) A: area (m2) Pressure unit:Pascal ( 1 Pa = 1 N/m2)The force on a surface caused by pressure is always normal (or perpendicular) to the surface. This means that the pressure of a fluid is exerted in all directions, and is perpendicular to the surface at every location.
4Atmospheric PressureAtmospheric pressure is normally about 100,000 Pascals.Differences in atmospheric pressure cause winds to blow.Low atmospheric pressure inside a hurricane’s eye contributes to the severe winds and the evelopment of the storm surge..
5Calculate the net force on an airplane window if cabin pressure is 90% of the pressure at sea level, and the external pressure is only 50% of that at sea level. Assume the window is 0.43 m tall and 0.30 m wide.Atmospheric pressure = PaPcabin = PaPoutside = PaCabinOutsideP = F/AP = 90000Pa – 50000Pa = 40000PaA = lwA = 0.3m · 0.43m = 0.129m2F = APF = 0.129m2 · 40000Pa = 5160N
6The Pressure of a Liquid P = ρghP: pressure (Pa)ρ : density (kg/m3)g: acceleration constant (9.8 m/s2)h: height of liquid column (m)This type of pressure is often called gauge pressure. Why?The gauges we use to measure pressure do not take into account the atmospheric pressure which is very large!If the liquid is water, this is referred to as hydrostatic pressure. Why?Hydro – water + static - stationary
7Absolute PressureAbsolute pressure is obtained by adding the atmospheric pressure to the hydrostatic pressure.Pabs = Patm + ρghHydrostatic Pressure in Dam DesignThe depth of Lake Mead at the Hoover Dam is 180 m. What is the hydrostatic pressure and what is the absolute pressure at the base of the dam?
8Hydrostatic Pressure in Dam Design The depth of Lake Mead at the Hoover Dam is 180 m. What is the hydrostatic pressure and what is the absolute pressure at the base of the dam?ρfresh water = 1000 kg / m3P = ρghP = 1000 kg / m3 · 9.8m/s2 · 180 m = PaPabs = Patm + ρghPabs = Pa Pa = Pa
9Hydrostatic Pressure in Levee Design Hurricane Katrina August 2005A hurricane’s stormsurge can overtop levees,but a bigger problem canbe increasing thehydrostatic pressure atthe base of the levee.
10New Orleans Elevation Map New Orleans is largely below sea level, and relies upon a system of levees to keep the lake and the river at bayMax ht of the levee 17.5 ftSurge elevation set at 11.5 ftNormal lake level at 1.0ft
11Calculate the increase in hydrostatic pressure experienced by the levee base for an expected (SPH Design) storm surge. How does this compare to the increase that occurred during Hurricane Katrina, where the water rose to the top of the levee?Not to mention there is atmospheric changes during hurricanes which effect the absolute pressure16.5ft = 12in / 1ft · 2.54 cm / 1 in · 1 m / 100 cm = 5.03mhlevee = 16.5ftρsalt water = 1025 kg /m3P = ρghP = 1025 kg /m3 ·9.8m/s2 · 5.03m = Pa
12Pascal’s PrincipleApplies when a pressure is applied to a container holding a fluidApplies to closed systemsPascal’s principle applies in hydraulic systemsF1 / A1 = F2 / A2
13A person stands on a piston. The system is full of hydraulic fluid A person stands on a piston. The system is full of hydraulic fluid. The car with a mass of 1500 kg is sitting on a piston that has a radius of 3.1 m. A person is standing on the other piston that has a radius of 0.40 m. What must the weight of the person be to cause the car to rise?mc = 1500kgrc = 3.1mrp = 0.40mFp = ?Ac =πr2 = π(3.1m)2 = 30.19m2Ac =πr2 = π(0.40m)2 = 0.50m2Fc / Ac = Fp / ApFp = FcAp / AcFp = (1500· 9.8m/s2) · 0.50m2 / 30.19m2 = 243NThis is equal to 54 lbs, so a child could stand on the piston and make the car move!!!
14Calculate the pressure exerted on the bottom of a glass that moves upward in elevator with constant acceleration, increasing from rest to 1.2 m/s in 2.7 s. The height of the water is 15 cm. The radius of the cup is 2 cm. The glass is filled with water which has a density of 1000 kg /m3.Do not include atmospheric contribution in your answer.V = Acircle · h = m2 · 0.15m = m3ΣF = mahFN - mg= mam = Vρ = m3 · 1000 kg / m3 = Kga = (vf – vi) / tFN = ma + mga = (1.2 m/s – 0m/si) / 2.7s = 0.44m/s2AFN = Kg(0.44m/s m/s2) = NA = πr2A = π(0.02m)2 = m2FN = ma + mg
15Calculate the pressure exerted on the bottom of a glass that moves upward in elevator with constant acceleration, increasing from rest to 1.2 m/s in 2.7 s. The height of the water is 15 cm. The radius of the cup is 2 cm. The glass is filled with water which has a density of 1000 kg /m3.Do not include atmospheric contribution in your answer.P1 = FN / AhP2 = ρghP = ρgh + FN / AAP = 1000kg/m3 · 9.8m/s2 · 0.15m N / m2 = 3006Pa
17Floating is a type of equilibrium An upward force counteracts the force of gravity for these objects. This upward force is called the buoyant forceFbuoyancymg
18Archimedes PrincipleArchimedes’ Principle: a body immersed in a fluid is buoyed up by a force that is equal to the weight of the fluid it displaces.The Buoyant ForceFbuoy = ρVgFbuoy: the buoyant force exerted on a submerged or partially submerged object.V: the volume of displaced liquid.ρ : the density of the displaced liquid.When an object floats, the upward buoyant force equals the downward pull of gravity.The buoyant force can float very heavy objects, and acts upon objects in the water whether they are floating, submerged, or even sitting on the bottom.
19Buoyant force on submerged object A sharks body is not neutrally buoyant, so a shark must swim continuously or he will sink deeper.Fbuoy = ρVgmgFbouy ≠ mg
20Buoyant force on submerged object SCUBA divers use a buoyancy control system to maintain neutral buoyancy (equilibrium!).Fbuoy = ρVgmgFbouy = mg
21Buoyant force on submerged object If the diver wants to rise, he inflateshis vest, which increases the amountof water he displaces, and heaccelerates upwardFbuoy = ρVgmgFbouy = mg
22Buoyant force on floating object If the object floats on the surface, weknow for a fact Fbuoy = mg! The volume of displaced water equals the volume of the submerged portion of the ship.Fbuoy = ρVgmgFbouy = mg
23The height above the water is 0.02m Assume a wooden raft has 80.0% of the density of water. The dimensions of the raft are 6.0 meters long by 3.0 meters wide by 0.10 meter tall. How much of the raft rises above the level of the water when it floats?Fbuoy = ρVghywlmgΣF: Fbouy – mg = 0We also know that the volume of displacement is equal to the volume of water displacedρglwy = (0.8ρ)glwhρgVsub – (0.8ρ)gVtotal = 0y = (0.8) hy = 0.8 · 0.1m = 0.08mThe height above the water is 0.02m
24Buoyant Force The buoyant force can be extremely strong. Incredibly massive objects can float, even when they are not intended to…
26When a Fluid Flows……mass is conserved.Provided there are no inlets our outlets in a stream of flowing fluid, the same mass per unit time must flow everywhere in the stream.Fluid Flow ContinuityThe volume per unit time of a liquid flowing in a pipe is constant throughout the pipe.We can say this because liquids are not compressible, so mass conservation is also volume conservation for a liquid.
27Fluid Flow Continuity (cont.) V = Avt V: volume of fluid (m3) A: cross sectional areas at a point in the pipe (m2)v: speed of fluid flow at a point in the pipe (m/s)t: time (s)A1v1 = A2v2A1, A2: cross sectional areas at points 1 and 2v1, v2: speed of fluid flow at points 1 and 2
28A pipe of diameter 6. 0 cm has fluid flowing through it at 1. 6 m/s A pipe of diameter 6.0 cm has fluid flowing through it at 1.6 m/s. How fast is the fluid flowing in an area of the pipe in which the diameter is 3.0 cm? How much water per second flows through the pipe?A1v1 = A2v2πr21 v1 = πr22 v2Apipe = πr2v2 = πr21 v1 / πr22v2 = π(0.06m)2 · (1.6 m/s) / π(0.03m)2 = 6.4 m/s
29The water in a canal flows 0 The water in a canal flows 0.10 m/s where the canal is 12 meters deep and 10 meters across. If the depth of the canal is reduced to 6.5 meters at an area where the canal narrows to 5.0 meters, how fast will the water be moving through this narrower region?What will happen to the water in an open waterway if it cannot flow as fast as it wants to through a narrow region in a channel?It will rise and flow out of the channelV2 = l1w1v1 / l2w2V2 = A1v1 / A2A1v1 = A2v2V2 =(12m · 10m · 0.1 m/s) / (6.5m · 5m) = 0.37 m/s
30Natural WaterwaysFlash flooding can be explained by fluid flow continuity
31Artificial WaterwaysFlooding from the Mississippi River Gulf Outlet was responsible forcatastrophic flooding in eastern New Orleans and St. Bernard during Hurricane Katrina.
32Fluid Flow Continuity in Waterways Mississippi River Gulf Outlet levees are overtopped by Katrina’s storm surge.A hurricane’s storm surge can be “amplified” by waterways that become narrower or shallower as they move inland.
34Bernoulli’s TheoremThe sum of the pressure, the potential energy per unit volume, and the kinetic energy per unit volume at any one location in the fluid is equal to the sum of the pressure, the potential energy per unit volume, and the kinetic energy per unit volume at any other location in the fluid for a non-viscous incompressible fluid in streamline flow.All other considerations being equal, when fluid moves faster, the pressure drops.
35Bernoulli’s Theorem p + ρgh + ½ ρv2 = Constant p : pressure (Pa) ρ : density of fluid (kg/m3)g: gravitational acceleration constant (9.8 m/s2)h: height above lowest point (m)v: speed of fluid flow at a point in the pipe (m/s)
36Knowing what you know about Bernoulli's principle, design an airplane wing that you think will keep an airplane aloft. Draw a cross section of the wing.Longer d, higher v, lower pFliftFthurstFdragFlift = ΔPAmg
37Problem: An above-ground swimming pool has a hole of radius 0 Problem: An above-ground swimming pool has a hole of radius 0.10 cm in the side 2.0 meters below the surface of the water. How fast is the water flowing out of the hole? How much water flows out each second?po + ρgh + ½ ρv2 = Constanth1 = 2mpo + ρgh1 + ½ ρv12 = po + ρgh2 + ½ ρv22ρgh1 + ½ ρv12 = ρgh2 + ½ ρv22h2 = 0 mv1 = 0 m/sρgh1 + = ½ ρv22gh1 = ½v22What about energymgh = ½ mv2v2 = √(2gh1)gh = ½ v2v2 = √(2·9.8 m/s2 · 2.0m) = 6.26 m/sv2 = √(2gh1)Same equation!!!Bernoulli’s principle
38Problem: An above-ground swimming pool has a hole of radius 0 Problem: An above-ground swimming pool has a hole of radius 0.10 cm in the side 2.0 meters below the surface of the water. How fast is the water flowing out of the hole? How much water flows out each second?v2 = 6.26 m/sh1 = 2mV = AvtV/t = AvV/t = πr2vV/t = π(0.001 m)2 · (6.26m/s) = 1.97 x 10-5 m3/s
39Water travels through a 9. 6 cm diameter fire hose with a speed of 1 Water travels through a 9.6 cm diameter fire hose with a speed of 1.3 m/s. At the end of the hose, the water flows out of a nozzle whose diameter is 2.5 cm. What is the speed of the water coming out of the nozzle? If the pressure in the hose is 350 kPa, what is the pressure in the nozzle?A1v1 = A2v2v2 = A1v1 / A2v2 = πr21v1 / πr22p1 = Pav1 = 1.3 m/sp2 = ? Pav2 = ? m/sv2 = π(0.096m)21· 1.3 m/s / π(0.025m)22 = m/s
40Water travels through a 9. 6 cm diameter fire hose with a speed of 1 Water travels through a 9.6 cm diameter fire hose with a speed of 1.3 m/s. At the end of the hose, the water flows out of a nozzle whose diameter is 2.5 cm. What is the speed of the water coming out of the nozzle? If the pressure in the hose is 350 kPa, what is the pressure in the nozzle?p1 + ρgh1 + ½ ρv12 = p2 + ρgh2 + ½ ρv22h1 = 0m h2 = 0mp1 = Pav1 = 1.3 m/sp2 = ? Pav2 = m/sp1 + ½ ρv12 = p2 + ½ ρv22p2 = p1 + ½ ρv12 - ½ ρv22p2 = p1 + ½ ρ(v12 - v22)p2 =350000Pa + ½ · kg / m3((1.3 m/s)2 – (19.17m/s)2) = Pa
41Bernoulli’s Principle and Hurricanes In a hurricane or tornado, the high winds traveling across the roof of a building can actually lift the roof off the building.+Roof&hl=enSee if I can down load video
42Make a device just like the above problem with a whole give them the information and they must predict where the water will hit the floor.
44ThermodynamicsThermodynamics is the study of heat and thermal energy.Thermal properties (heat and temperature) are based on the motion of individual molecules, so thermodynamics is a lot like chemistry.Total energyE = U + K + EintU: potential energyK: kinetic energyEint: internal or thermal energyPotential and kinetic energies are specifically for “big” objects, and represent mechanical energy.Thermal energy is related to the kinetic energy of the molecules of a substance.
45Temperature and HeatTemperature is a measure of the average kinetic energy of the molecules of a substance. Think of it as a measure of how fast the molecules are moving. The unit is ºC or K.Temperature is NOT heat!Heat is the internal energy that is transferred between bodies in contact. The unit is Joules (J), or sometimes calories (cal).A difference in temperature will cause heat energy to be exchanged between bodies in contact. When two bodies are at the same temperature, no heat is transferred. This is called Thermal Equilibrium.
46Thermal ExpansionMost substances expand when their temperature goes up.ΔL = αLo ΔTΔ L is change in lengthα is coefficient of linear expansionLo is original length of substanceΔT is change in temperature
47Sample problem: If the Eiffel tower is 301 m tall on a day when the temperature is 22º C, how much does it shrink when the temperature drops to 0º C? The coefficient of linear expansion is 11 x 10-6 ºC-1 for the iron the tower is made from.L0 = 301mT1 =22º CT2 =0º Cα =11x10-6 /ºCΔL = αL0 ΔTΔL = 11x10-6 /ºC · 301m · (0 ºC - 22 ºC ) = m
48Ideal Gas Law P1 V1 / T1 = P2 V2 / T2 P1, P2: initial and final pressure (any unit)V1, V2: initial and final volume (any unit)T1, T2: initial and final temperature (in Kelvin!)Temperature in K is obtained from temperature in oC by adding 273.
49Suppose an ideal gas occupies 4. 0 liters at 23oC and 2. 3 atm Suppose an ideal gas occupies 4.0 liters at 23oC and 2.3 atm. What will be the volume of the gas if the temperature is lowered to 0oC and the pressure is increased to 3.1 atm.P1 = 2.3 atmV1 = 4.0 LT1 = 23ºC = 296 KP2 = 3.1 atmV2 = ? LT2 = 0ºC = 273 KP1V1/T1 = P2V2/T2V2 = (P1V1 T2) /(T1P2)V2 = (2.3 atm · 4.0 L · 273 K) / (296 K · 3.1 atm) = L
50Ideal Gas EquationP V = n R TP: pressure (in Pa)V: volume (in m3)n: number of molesR: gas law constant8.31 J/(mol K)T: temperature (in K)PV = N/m2 · m3 = Nm = JnRT = mol · J/(mol · K) · K = JBoth sides = J
51Determine the number of moles of an ideal gas that occupy 10 Determine the number of moles of an ideal gas that occupy 10.0 m3 at atmospheric pressure and 25oC.25ºC = 298Kn = PV / RTPV = nRTn = Pa · 10 m3 / (8.31J/ (mol · K) · 298K) = mols
53Ideal Gas Equation PV = nRT (using moles) PV = NkBT (using molecules) P: pressure (Pa)V: volume (m3)N: number of moleculeskB: Boltzman’s constant1.38x10-23 J/Kn: Number of molesR: Universal Gas constant8.31 J / (mol · K)T: Temperature in Kelvin
54Suppose a near vacuum contains molecules of helium in one cubic meter at 0ºC. What is the pressure?PV = NkBTP = NkBT / VP = · 1.38 x J/k 273k / 1m3 = 9.42 x Pa
55What is the relationship between the Universal Gas Law constant (8 What is the relationship between the Universal Gas Law constant (8.31 J / (mol K) and Boltzman’s constant kB (1.38 x J/K)?NA = 6.02 x mol-1R = 8.31 J/(mol K)KB = R/ NAKB = 8.31 J / (mol K) / 6.02 x mol-1 = 1.38 x J/K
56The kinetic theory of gases uses mechanics to describe the motion off each single molecule in a sample off an ideal gas..When a very large number off molecules is considered,, the mechanical properties off the individual molecules are summed in a statistical way to predict the behavior off the gas sample
57Gases consist of a large number of molecules that make elastic collisions with each other and the walls of the container.Molecules are separated, on average, by large distances and exert no forces on each other except when they collide.There is no preferred position for a molecule in the container, and no preferred direction for the velocity.
59Need to change this make connection PV = 3/2 N KB T Kave = 3/2 kBTKave: average kinetic energy (J)kB: Boltzmann’s Constant (1.38 x J/K)T: Temperature (K)The molecules have a range of kinetic energies; kave is just the average of that rangeNeed to change this make connectionPV = 3/2 N KB TPV = to work which is equal to KE we are looking at 1 molecule for N
60What is the average kinetic energy and the average speed of oxygen molecules in a gas sample at 0oC? Use atomic mass number to get the mass per moleatomic mass number = number of Neutrons + number of ProtonsO2 32g/mol · 1 mol / 6.02 x 1023 · 1 kg / 1000g = x kgKave = 3/2 kBT½ mvrms2 = 3/2 kBTvrms =√(3 kBT / m)vrms =√((3 · 1.38 x J/K · 273K) / 5.32 x 10-26kg) = m/svrms =√( J / kg)vrms =√( J/K · K / kg)vrms =√( kg · m2/s2 / kg)vrms =√( kg · m/s2 · m / kg)vrms =√(m2/s2)
61Boundary Environment System (gas) For our purposes, the system will almost always bean ideal gas.System(gas)
62The system boundary controls how the environment affects the system. If the boundary is “closed to mass”, that means that mass can’t get in or out.If the boundary is “closed to energy”, that means energy can’t get in or out.Consider the earth as a system. What type of boundary does it have?What is the boundary?Is the boundary closed to mass?Is the boundary closed to energy?Atmosphere, crust, where gravity is negligibleYes, the increase in mass each day is negligibleNo, the sun adds energy to the system
63Work Q W Heat ΔU ΔU = Q – W System First Law of Thermodynamics U = Δ Total energyΔU = Q – W
64Awkward notation WARNING! We all know that U is potential energy in mechanics. However…U is Eint (thermal energy) in thermodynamics!This means when we are in thermo, U is thermal energy, which is related to temperature. When we are in mechanics, it is potential energy, which is related to configuration or position.
65U is the sum of the kinetic energies of all molecules in a system (or gas). U = N KaveU = N (3/2 kBT)U = n (3/2 R T)Since kB = R /NA
661st Law of Thermodynamics ΔU = Q – WΔ U: change in internal energy of system (J)Q: heat added to the system (J). This heat exchange is driven by temperature difference.W: work done on the system (J). Work will be related to the change in the system’s volume.This law is sometimes paraphrased as “you can’t win”.
67A system absorbs 200 J of heat energy from the environment and does 100 J of work on the environment. What is its change in internal energy?Q = 200 JUW = 100 JΔU = Q – WΔU = 200J – 100J = 100 J
68How much work does the environment do on a system if its internal energy changes from 40,000 J to 45,000 J without the addition of heat?Q = 0 JΔU = 45000J – 40000J = 5000JW = ? JΔU = Q - W5000 J = W therefore W = J
69The thermodynamic state of a gas is defined by pressure, volume, and temperature. A gas process describes how gas gets from one state to another state.Processes depend on the behavior of the boundary and the environment more than they depend on the behavior of the gas.
70Isothermal Process P Constant Temperature PV = nRT V T1 T2 T3 Initial state of the gasIsothermal ProcessFinal state of the gasVΔT = 0 ( constant T)
71Isobaric Process P Constant Pressure PV = nRT V T1 T2 T3 Isobaric ExpansionIsobaric ContractionVΔP = 0 ( constant P)
72Isometric Process P Constant Volume PV = nRT V T1 T2 T3 ΔV = 0 ( constant V)
73Temperature, pressure and volume all change in an adiabatic process InsulatedT1T2PV = nRTIsothermTemperature, pressure and volume all change in an adiabatic processadiabatThe figure compares two processes that begin with the same state and involve expansion to the same final volume. For the isothermal process, the product of P·V remains constant since T remains constant. Since the temperature must decrease for the adiabatic process, it follows that the final pressure must be less for this process. Thus the adiabat lies below the isotherm. Let's look at one more example that incorporates many of the AP points of emphasis.VQ = 0 ( No heat enters or leaves the system)
74WorkCalculation of work done on a system (or by a system) is an important part of thermodynamic calculations.Work depends upon volume change.Work also depends upon the pressure at which the volume change occurs.
75Work Done BY gasWgas = PΔVPositive WorkWenv = -PΔVNegative WorkΔV
76Work Done ON gas Wgas = PΔV Negative since ΔV is negative Wenv = -PΔV Positive since ΔV is negativeΔV
77Calculate the work done by a gas that expands from 0. 020 m3 to 0 Calculate the work done by a gas that expands from m3 to 0.80 m3 at constant atmospheric pressure.How much work is done by the environment when the gas expands this much?V1 = m3V2 = 0.80 m3Wgas = PΔVWgas = Pa·0.078m3 = 78000JWgas = J
78What is the change in volume of a cylinder operating at atmospheric pressure if its internal energy decreases by 230 J when 120 J of heat are removed from it?ΔU = -230 JQ = -120 JΔU = Q+WW = ΔU - QW = -230 J – (-120 J) = 110 JWgas = PΔVΔV = 110 J / PaΔV = m3
79a. WAB = PVAB = P ∙ 0 = 0 J W ΔU Q -53 J A B a. 0 J b. B C -130 J d.C Ae.150 Ja.WAB = PVAB = P ∙ 0 = 0 J
80b. UAB = QAB - WAB = -53 - 0 = -53 J W ΔU Q -53 J A B a. 0 J b. B C-130 Jc.-280 Jd.C Ae.150 Jb.UAB = QAB - WAB= = -53 J
81c. UBC = QCB - WBC = -280 – (-130) = -150 J W ΔU Q -53 J A B a. 0 J d.C Ae.150 Jc.UBC = QCB - WBC= -280 – (-130) = -150 J
82d. UAB + UBC + UCA = 0 UCA = - UAB – UBC = -53 – (-150) = +203 J W ΔU Q-53 JA Ba.0 Jb.-53 JB C-130 Jc.-150 J-280 Jd.+203 JC Ae.150 Jd.UAB + UBC + UCA = 0UCA = - UAB – UBC= -53 – (-150) = +203 J
83e. QCA = UCA + WCA UCA = - 203 + 150 = 353 J W ΔU Q -53 J A B a. 0 J B C-130 Jc.-150 J-280 Jd.+203 JC Ae.+353 J150 Je.QCA = UCA + WCAUCA = = 353 J
84Work (isobaric) WAB > WCD P Where we are considering work done BY the gasABP2WAB = P2ΔVCDP1WCD = P1ΔVVV1V2
85Work is path dependent P Where we are considering work done BY the gas WABD > WACDABP2WABDCP1DWACDV1V2V
86Work done by a cycleWhen a gas undergoes a complete cycle, it starts and ends in the same state. The gas is identical before and after the cycle, so there is no identifiable change in the gas.DU = 0 for a complete cycle.The environment, however, has been changed.
87Work done by a cyclePWork done by the gas is equal to the area circumscribed by the cycleWork done by gas is positive for clockwise cycles, negative for counterclockwise cycles.BAP2WABCDP1CDV1V2VWork done by environment is negative of work done by gas.
88Second Law of Thermodynamics No process is possible whose sole result is the complete conversion of heat from a hot reservoir into mechanical work. (Kelvin-Planck statement.)No process is possible whose sole result is the transfer of heat from a cooler to a hotter body. (Clausius statement.)
89Heat Engines Heat Engines and Carnot Cycle Efficiency
90Heat Engines Heat engines can convert heat into useful work. According to the 2nd Law of Thermodynamics. Heat engines always produce some waste heat.Efficiency can be used to tell how much heat is needed to produce a given amount of work.NOTE: A heat engine is not something that produces heat. A heat engine transfers heat from hot to cold, and does mechanical work in the process.
93Work and Heat Engine QH = W + QC Heat Source (High Temperature)QH = W + QCQH: Heat that is put into the system and comes from the hot reservoir in the environment.W: Work that is done by the system on the environment.QC: Waste heat that is dumped into the cold reservoir in the environment.QHWQCHeat Sink (Low Temperature)
94Efficiency of Heat Engine Efficiency = W/QH = (QH - QC)/QH W: Work done by engine on environmentQH: Heat absorbed from hot reservoirQC: Waste heat dumped to cold reservoirEfficiency is often given as percent efficiency.
95A piston absorbs 3600 J of heat and dumps 1500 J of heat during a complete cycle. How much work does it do during the cycle?QH = 3600JQC = 1500JQH = W + QCW = QH - QCW = 3600 J – 1500 J = 2100J
96A certain coal-fired steam plant is operating with 33% thermodynamic efficiency. If this is a 120 MW plant, at what rate is heat energy used?Eff = W/QH = (QH - QC)/QH
97Carnot Cycle P Heat In QH = QC + W Eff = W / QH Work V Heat Out Isothermal expansionWorkAdiabatic compressionAdiabatic expansionIsothermal compressionVHeat Out
98Efficiency of Carnot Cycle For a Carnot engine, the efficiency can be calculated from the temperatures of the hot and cold reservoirs.Carnot Efficiency = (TH - TC)/THTH: Temperature of hot reservoir (K)TC: Temperature of cold reservoir (K)
99Calculate the Carnot efficiency of a heat engine operating between the temperatures of 60 and 1500 oC.TC = 60 ºCTH = 1500 ºCEfficiency = (TH - TC)/THEfficiency = ((1500 ºC + 273) – (60 ºC + 273) )/ (1500 ºC + 273) = 0.81 or 81 %
101Entropy… Entropy is disorder, or randomness. The entropy of the universe is increasing. Ultimately, this will lead to what is affectionately known as “Heat Death of the Universe”.Does the entropy in your room tend to increase or decrease?