–A system in energy balance maintains a constanttemperature.–If loss exceeds gain, the system cools.If gain exceeds loss the system warms–If gain exceeds loss, the system warms.Slide 16-27A house in thermal-energy balanceExample:Earth’s Temperature•Earth receives energy from the Sun at the average rate ofEarth receives energy from the Sun at the average rate ofabout 240 watts per square meter.Assuming its emissivity is1, what should be Earth’s average temperature?−In energy balance, the rate ofenergy arriving per square meter(240 W/m2) equals the rate goingout, namelyeT4W/m2. Equatingthe two withe= 1 gives 240 W/m2=T4–Solving gives1/ 42824240 W/m255 K5.6710W/m ·KTT4.–Make sense?This temperature seems in the rightballpark, but a bit low for a global average; it’s –18˚C or0˚FIn fact the natural greenhouse effect keeps EarthSlide 16-280 F.In fact, the natural greenhouse effect keeps Earthsome 33˚C warmer, at about 288 K or 15˚C.
Clicker QuestionSuppose you want to make your morning coffee cool off withinfive minutes to as low a temperature as possible. Do you pourin the cold milk first and then wait five minutes or do you waitin the cold milk first and then wait five minutes, or do you waitfive minutes before adding the cold milk?1.The coffee would be cooler after 5 minutes if the coldmilk were added as soon as possible.2The coffee would be cooler after 5 minutes if the cold2.The coffee would be cooler after 5 minutes if the coldmilk were added at the end of the 5-minute period.3.The final temperature of the coffee would be thesame in either case.4.The one that would be cooler depends on the actualtemperatures and amounts of the coffee and the milkSlide 16-29temperatures and amounts of the coffee and the milk.Summary•Thermodynamic equilibrium is the state reached when•Thermodynamic equilibrium is the state reached whenmacroscopic properties of a system or systems don’tchange.•Systems in thermodynamic equilibrium are at the same•Systems in thermodynamic equilibrium are at the sametemperature.–The kelvin (K) is the SI unit of temperature.Heat is energy in transit because of a temperature difference•Heat is energy in transit because of a temperature differencealone.–The heat required to heat an object by an amount ∆Tdepends on its mass and its specific heatc: ∆Q=mc∆Tdepends on its mass and its specific heat,c:∆Q=mc∆T.–Heat transfer mechanisms include conduction,convection, and radiation.Thlb lit tihi h thtt–Thermal energy balance is a state in which the rate atwhich energy is delivered to a system is equal to the rateat which the system loses energy. A system in energybalance maintains a constant temperatureSlide 16-30balance maintains a constant temperature.
Chapter 17 LectureThe Thermal Behaviorfof MatterSlide 17-1In this lecture you’ll learn•The ideal-gas lawThe ideal gas law–And the relationship betweenmacroscopic andmicroscopic quantitiesmicroscopic quantities,especially temperature andmolecular energy•To describe the three principlephases of matter– To calculate the energiesTo calculate the energiesinvolved in phase transitions–To interpret phase diagrams•To calculate thermal expansionof solids, liquids, and gasesSlide 17-2The Gas Laws•Boyle’s Law:Boyle s Law:PV1(ConstantT)•Charles’ Law:PTV(ConstantP)•Gay-Lussac’s Law:TP(ConstantV)Slide 17-3The Ideal-Gas Law
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