Lecture 23 Goals: • Chapter 16 • Use the ideal-gas law. Use pV diagrams for ideal-gas processes. Chapter 17 Employ energy conservation in terms of 1st law of TD Understand the concept of heat. Relate heat to temperature change Apply heat and energy transfer processes in real situations Recognize adiabatic processes. • Assignment HW9, Due Wednesday, Apr. 15th HW10, Due Wednesday, Apr. 22nd (9 AM) Physics 207: Lecture 23, Pg 1 Thermodynamics: A macroscopic description of matter Recall “3” Phases of matter: Solid, liquid & gas All 3 phases exist at different p,T conditions Triple point of water: p = 0.06 atm T = 0.01°C Triple point of CO2: p = 5 atm T = -56°C Physics 207: Lecture 23, Pg 2 Modern Definition of Kelvin Scale Water’s triple point on the Kelvin scale is 273.16 K One degrees Kelvin is defined to be 1/273.16 of the temperature at the triple point of water Accurate water phase diagram Triple point Physics 207: Lecture 23, Pg 3 Transferring energy to a solid (ice) 1. Temperature increase or 2. State Change If a gas, then V, p and T are interrelated….equation of state Physics 207: Lecture 23, Pg 4 Temperature scales Three main scales Farenheit Celcius Kelvin 212 100 373.15 32 0 273.15 -459.67 -273.15 0 Water boils Water freezes Absolute Zero Physics 207: Lecture 23, Pg 7 Some interesting facts In 1724, Gabriel Fahrenheit made thermometers using mercury. The zero point of his scale is attained by mixing equal parts of water, ice, and salt. A second point was obtained when pure water froze (originally set at 30oF), and a third (set at 96°F) “when placing the thermometer in the mouth of a healthy man”. On that scale, water boiled at 212. Later, Fahrenheit moved the freezing point of water to 32 (so that the scale had 180 increments). In 1745, Carolus Linnaeus of Upsula, Sweden, described a scale in which the freezing point of water was zero, and the boiling point 100, making it a centigrade (one hundred steps) scale. Anders Celsius (1701-1744) used the reverse scale in which 100 represented the freezing point and zero the boiling point of water, still, of course, with 100 degrees between the two defining points. T (K) 108 Hydrogen bomb 107 Sun’s interior 106 Solar corona 105 104 103 100 10 1 Sun’s surface Copper melts Water freezes Liquid nitrogen Liquid hydrogen Liquid helium 0.1 Lowest T~ 10-9K Physics 207: Lecture 23, Pg 8 Ideal gas: Macroscopic description Consider a gas in a container of volume V, at pressure P, and at temperature T Equation of state Links these quantities Generally very complicated: but not for ideal gas Equation of state for an ideal gas Collection of atoms/molecules moving randomly No long-range forces Their size (volume) is negligible Density is low Temperature is well above the condensation point PV = nRT R is called the universal gas constant In SI units, R =8.315 J / mol·K n = m/M : number of moles Physics 207: Lecture 23, Pg 9 Boltzmann’s constant Number of moles: n = m/M m=mass M=mass of one mole One mole contains NA=6.022 X 1023 particles : Avogadro’s number = number of carbon atoms in 12 g of carbon In terms of the total number of particles N PV = nRT = (N/NA ) RT PV = N kB T kB = R/NA = 1.38 X 10-23 J/K kB is called the Boltzmann’s constant P, V, and T are the thermodynamics variables Physics 207: Lecture 23, Pg 10 The Ideal Gas Law pV nRT What is the volume of 1 mol of gas at STP ? T = 0 °C = 273 K 5 p = 1 atm = 1.01 x 10 Pa nRT V P 8.31 J / mol K 273 K 1.0110 5 Pa 0.0224 m 3 22.4 Physics 207: Lecture 23, Pg 11 Example A spray can containing a propellant gas at twice atmospheric pressure (202 kPa) and having a volume of 125.00 cm3 is at 27oC. It is then tossed into an open fire. When the temperature of the gas in the can reaches 327oC, what is the pressure inside the can? Assume any change in the volume of the can is negligible. Steps 1. Convert to Kelvin (From 300 K to 600 K) 2. Use P/T = nR/V = constant P1/T1 = P2/T2 3. Solve for final pressure P2 = P1 T2/T1 http://www.thehumorarchives.com/joke/WD40_Stupidity Physics 207: Lecture 23, Pg 12 Example problem: Air bubble rising A diver produces an air bubble underwater, where the absolute pressure is p1 = 3.5 atm. The bubble rises to the surface, where the pressure is p2 = 1.0 atm. The water temperatures at the bottom and the surface are, respectively, T1 = 4°C, T2 = 23°C What is the ratio of the volume of the bubble as it reaches the surface,V2, to its volume at the bottom, V1? (Ans.V2/V1 = 3.74) Is it safe for the diver to ascend while holding his breath? No! Air in the lungs would expand, and the lung could rupture. Physics 207: Lecture 23, Pg 13 Example problem: Air bubble rising A diver produces an air bubble underwater, where the absolute pressure is p1 = 3.5 atm. The bubble rises to the surface, where the pressure is p2 = 1 atm. The water temperatures at the bottom and the surface are, respectively, T1 = 4°C, T2 = 23°C What is the ratio of the volume of the bubble as it reaches the surface,V2, to its volume at the bottom, V1? (Ans.V2/V1 = 3.74) pV=nRT pV/T = const so p1V1/T1 = p2V2/T2 V2/V1 = p1T2/ (T1 p2) = 3.5 296 / (277 1) If thermal transfer is efficient. [More than likely the expansion will be “adiabatic” and, for a diatomic gas, PVg = const. where g = 7/5, see Ch. 17 & 18] Physics 207: Lecture 23, Pg 15 Buoyancy and the Ideal Gas Law A typical 5 passenger hot air balloon has approximately 700 kg of total mass and the balloon itself can be thought as spherical with a radius of 10.0 m. If the balloon is launched on a day with conditions of 1.0 atm and 273 K, how hot would you have to heat the air inside (assuming the density of the surrounding air is 1.2 kg/m3 and the air behaves and as an ideal gas) in order to keep the balloon at a constant altitude? Hint: Remember the weight of the air inside the balloon. Balloon weight = Buoyant force – Weight of hot air Ideal gas law: pV = nRT nT= pV/R = const. or r T = const. = 1.2 x 273 kg K/m3 Physics 207: Lecture 23, Pg 16 Buoyancy and the Ideal Gas Law mballoon g = rair at 273 K V g – rair at T V g mballoon = rair at 273 K V – rair at T V mballoon = (1.2 – 330 / T) V 700 / 4200 = 1.2 – 330 / T 330 / T = (1.2 - 0.2) T = 330 K 57 C Physics 207: Lecture 23, Pg 17 PV diagrams: Important processes Isochoric process: V = const (aka isovolumetric) Isobaric process: p = const pV Isothermal process: T = const constant T 1 p1V1 p2V2 1 Volume Pressure p1 p2 T1 T2 Isobaric Isothermal Pressure Pressure Isochoric 2 V1 V2 T1 T2 1 2 2 Volume Volume Physics 207: Lecture 23, Pg 18 Work and Energy Transfer (Ch. 16) K reflects the kinetic energy of the system ΔK =Wconservative + Wdissipative + Wexternal Wconservative = - ΔU (e.g., gravity) Wdissipative = - ΔEThermal Wexternal Typically, work done by contact forces ΔK + ΔU + ΔETh = Wexternal= ΔEsys Physics 207: Lecture 23, Pg 19 Work and Energy Transfer (Ch. 17) ΔK + ΔU + ΔETh = Wexternal= ΔEsys But we can transfer energy without doing work Q ≡ thermal energy transfer ΔK + ΔU + ΔETh = W + Q = ΔEsys If ΔK + ΔU = ΔEMech = 0 ΔETh = W + Q Physics 207: Lecture 23, Pg 20 1st Law of Thermodynamics ΔEth =W + Q W & Q with respect to the system Thermal energy Eth : Microscopic energy of moving molecules and stretching molecular bonds. ΔEth depends on the initial and final states but is independent of the process. Work W : Energy transferred to the system by forces in a mechanical interaction. Heat Q : Energy transferred to the system via atomic-level collisions when there is a temperature difference. Physics 207: Lecture 23, Pg 21 1st Law of Thermodynamics ΔEth =W + Q W & Q with respect to the system Work W and heat Q depend on the step process by which the system is changed (path dependent). The change of energy in the system, ΔEth depends only on the total energy exchanged W+Q, not on the process. Physics 207: Lecture 23, Pg 22 1st Law: Work & Heat Work done on system (an ideal gas) Won final p dV (area under curve ) initial Won system < 0 Moving left to right [where (Vf > Vi)] If ideal gas, PV = nRT, and given Pi & Vi fixes Ti Wby system > 0 Moving left to right Physics 207: Lecture 23, Pg 23 Work: 1st Law: Work & Heat Depends on the path taken in the PV-diagram (It is not just the destination but the path…) Won system > 0 Moving right to left Physics 207: Lecture 23, Pg 24 1st Law: Work (“Area” under the curve) Work depends on the path taken in the PV-diagram : 3 3 2 1 2 1 (a) Wa = W1 to 2 + W2 to 3 (here either P or V constant) Wa (on) = - Pi (Vf - Vi) + 0 > 0 (b) Wb = W1 to 2 + W2 to 3 (here either P or V constant) Wb (on) = 0 - Pf (Vf - Vi) > Wa > 0 (c) Need explicit form of P versus V but Wc (on) > 0 Physics 207: Lecture 23, Pg 25 Recap • Assignment HW9, Due Wednesday, Apr. 15th HW10, Due Wednesday, Apr. 22nd (9 AM) Physics 207: Lecture 23, Pg 26