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Chapter 12 Thermal Energy

Chapter 12 Thermal Energy. Glencoe 2005 Honors Physics Bloom High School. 12.1 Temperature & Thermal Energy. Thermal Energy- total energy of the molecules in a substance Translational, rotational, vibrational, bending energies of molecules Average energy- related to temperature

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Chapter 12 Thermal Energy

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  1. Chapter 12Thermal Energy Glencoe 2005 Honors Physics Bloom High School

  2. 12.1 Temperature & Thermal Energy • Thermal Energy- total energy of the molecules in a substance • Translational, rotational, vibrational, bending energies of molecules • Average energy- related to temperature • Solids- only exhibit vibrational energies in bonds

  3. Thermal Energy & Temperature • Which would you rather? • 1 drop of 100°C water on your arm • Fall into a pool of 100°C water • Temperature- independent of amount of substance • Thermal energy- dependant on amount of substance

  4. Equilibrium & Thermometry • Conduction- transfer of kinetic energy when particles collide • Thermal equilibrium- rate of energy flow between the two objects is equal

  5. Temperature Scales: Celsius & Kelvin • Celsius scale- 100 degrees difference between freezing and boiling of pure water • 0°C corresponds to the freezing point of pure water • 100°C corresponds to the boiling point of pure water • Kelvin- 100 degrees difference between freezing and boiling of pure water • 273K corresponds to the freezing point of pure water • 373K corresponds to the boiling point of pure water • 0K (absolute zero) represents zero kinetic energy of a substance

  6. Temperature Scales • Kelvin = °C + 273.2 • °C = (°F-32) x 0.555 • °F = (1.8 x °C) +32

  7. Heat and the Flow of Thermal Energy • Heat (Q, Joules)- energy that is transferred between objects • Always flows from high energy area to low energy area • Nothing ever feels cold • Conduction- heat flow due to physical contact • Convection- the rising of a higher temperature fluid • Hot air or water rising; colder fluid sinking • Radiation- transfer of energy through electromagnetic waves • Infrared increases average kinetic energy

  8. Specific Heat • Specific heat capacity (C, J/g°C)- the quantity of heat required to raise the temperature of 1g by 1°C • Ability to store internal energy • A measured value for each substance (Table 12-1) • Q=mCDT • Q=heat (J or kJ) • m=mass(kg or g) • C=specific heat (J/g°C or J/kg•K) • DT=change in temperature

  9. Calorimetry: Measuring Specific Heat • Calorimeter- measures the DT and/or C of unknown substances with a reference substance • Physics Physlet E.19.3

  10. Conservation of E & Calorimetry • EA+EB=constant • Any energy lost by the hotter object goes to colder object • DE=Q=mCDT • In a closed, isolated system, the change in energy is equal to the heat transfer

  11. Thermal Equilibrium • mACADTA+mBCBDTB=0 • Substituting the second equation into the first • Equals zero because all energy is conserved • Can be rearranged to solve for any variable! • For DT, Tf will be equal for both substances

  12. 12.2 Changes of State & the Laws of Thermodynamics • Figure 12-10 • Energy continues to be added throughout • Energy used to increase average kinetic energy or change state • Melting/Freezing point- exists in solid and liquid state • Boiling/Condensation point- exists in liquid and gas state

  13. Phase Diagram of Water

  14. Heat of Fusion • Heat of fusion (Hf, J/kg)- amount of energy needed to melt a kg of substance without DT • Q=mHf • Q is negative when freezing (heat removed) • Heat of Vaporization (Hv, J/kg)- amount of energy needed to vaporize a kg of substance without DT • Q=mHv • Q is negative when condensing (heat removed) • See Table 12-2 for values

  15. Transition of Water 0°Cice-100°Csteam • How much energy does 1kg of ice (0°C) need to change to 1kg of steam (0°C)? • Convert solid to liquid- Q=mHf • Q1=(1kg)(3.34x105J/kg) • Increase temperature of water- Q=mCDT • Q2=(1kg)(4180J/kg•K)(100K) • Convert liquid to gas- Q=mHv • Q3=(1kg)(2.26x105J/kg) • Q1+Q2+Q3=(3.34x105J)+(4.18x105J)+(2.26x105J)

  16. 1st Law of Thermodynamics • First 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 object • DU=Q-W • Heat Engine- converts thermal energy to mechanical energy

  17. Combustion Engine • Physics Physlet E.21.2

  18. Efficiency, Refrigerators & Heat Pumps • Efficiency- ratio of heat in to useful work out • Refrigerator- heat engine in reverse • Heat energy flows from high to low • Refrigerator reverses this energy flow • Heat Pump- a refrigerator that can be run in reverse • Remove heat or add heat to system

  19. 2nd Law of Thermodynamics • 2nd Law- all processes naturally go in the direction of increasing disorder and increases the entropy of the universe • Entropy- a measure of the disorder (chaos) in a system • Dependant on thermal energy of system • DS=Q/T • If heat flows into system, disorder is increased • Hg tube

  20. Entropy • Can be reversed if work is added to the system

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