Chapter 12 Thermal Energy

Chapter 12Thermal EnergyGlencoe 2005Honors PhysicsBloom High School12.1 Temperature & Thermal EnergyThermal Energy- total energy of the molecules in a substanceTranslational, rotational, vibrational, bending energies of molecules

Average energy- related to temperature

Solids- only exhibit vibrational energies in bondsThermal Energy & TemperatureWhich would you rather?1 drop of 100C water on your armFall into a pool of 100C water

Temperature- independent of amount of substance

Thermal energy- dependant on amount of substance

Equilibrium & ThermometryConduction- transfer of kinetic energy when particles collideThermal equilibrium- rate of energy flow between the two objects is equal

Temperature Scales: Celsius & KelvinCelsius scale- 100 degrees difference between freezing and boiling of pure water0C corresponds to the freezing point of pure water100C corresponds to the boiling point of pure water

Kelvin- 100 degrees difference between freezing and boiling of pure water273K corresponds to the freezing point of pure water373K corresponds to the boiling point of pure water0K (absolute zero) represents zero kinetic energy of a substanceTemperature ScalesKelvin = C + 273.2

C = (F-32) x 0.555F = (1.8 x C) +32

Heat and the Flow of Thermal EnergyHeat (Q, Joules)- energy that is transferred between objectsAlways flows from high energy area to low energy areaNothing ever feels coldConduction- heat flow due to physical contactConvection- the rising of a higher temperature fluidHot air or water rising; colder fluid sinkingRadiation- transfer of energy through electromagnetic wavesInfrared increases average kinetic energySpecific HeatSpecific heat capacity (C, J/gC)- the quantity of heat required to raise the temperature of 1g by 1CAbility to store internal energyA measured value for each substance (Table 12-1)Q=mCDTQ=heat (J or kJ)m=mass(kg or g)C=specific heat (J/gC or J/kgK)DT=change in temperatureCalorimetry: Measuring Specific HeatCalorimeter- measures the DT and/or C of unknown substances with a reference substancePhysics Physlet E.19.3

Conservation of E & CalorimetryEA+EB=constantAny energy lost by the hotter object goes to colder objectDE=Q=mCDTIn a closed, isolated system, the change in energy is equal to the heat transferThermal EquilibriummACADTA+mBCBDTB=0Substituting the second equation into the firstEquals zero because all energy is conservedCan be rearranged to solve for any variable!For DT, Tf will be equal for both substances

12.2 Changes of State & the Laws of ThermodynamicsFigure 12-10Energy continues to be added throughoutEnergy used to increase average kinetic energy or change stateMelting/Freezing point- exists in solid and liquid stateBoiling/Condensation point- exists in liquid and gas state

Phase Diagram of Water

Heat of FusionHeat of fusion (Hf, J/kg)- amount of energy needed to melt a kg of substance without DTQ=mHfQ is negative when solidifying (heat removed)Heat of Vaporization (Hv, J/kg)- amount of energy needed to vaporize a kg of substance without DTQ=mHvQ is negative when condensing (heat removed)

See Table 12-2 for valuesTransition of Water 0Cice-100CsteamHow much energy does 1kg of ice (0C) need to change to 1kg of steam (0C)?Convert solid to liquid- Q=mHfQ1=(1kg)(3.34x105J/kg)Increase temperature of water- Q=mCDTQ2=(1kg)(4180J/kgK)(100K)Convert liquid to gas- Q=mHvQ3=(1kg)(2.26x105J/kg)Q1+Q2+Q3=(3.34x105J)+(4.18x105J)+(2.26x105J)1st Law of ThermodynamicsFirst Law- changes in internal thermal energy (DU) of an object are equal to the heat (Q) that is added to the system minus the work (W) done by the objectDU=Q-W

Heat Engine- converts thermal energy to mechanical energy

Combustion EnginePhysics Physlet E.21.2

Efficiency, Refrigerators & Heat PumpsEfficiency- ratio of heat in to useful work out

Refrigerator- heat engine in reverseHeat energy flows from high to lowRefrigerator reverses this energy flow

Heat Pump- a refrigerator that can be run in reverseRemove heat or add heat to system2nd Law of Thermodynamics2nd Law- all processes naturally go in the direction of increasing disorder and increases the entropy of the universeEntropy- a measure of the disorder in a systemDependant on thermal energy of systemDS=Q/TIf heat flows into system, disorder is increasedHg tubeEntropyCan be reversed if work is added to the system