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Chapter 17

Chapter 17. Energy in Thermal Processes: First Law of Thermodynamics. 17.1 Thermodynamics – Historical Background. Until around 1850, thermodynamics and mechanics were considered to be two distinct branches of science.

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Chapter 17

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  1. Chapter 17 Energy in Thermal Processes: First Law of Thermodynamics

  2. 17.1 Thermodynamics – Historical Background • Until around 1850, thermodynamics and mechanics were considered to be two distinct branches of science. • Experiments by James Joule and others showed a connection between the two branches of science. • A connection was found to be between the transfer of energy by heat in thermal processes and the transfer of energy by work in mechanical processes

  3. Internal Energy • Internal energy is all the energy of a system that is associated with its microscopic components • These components are its atoms and molecules • The system is viewed from a reference frame at rest with respect to the system

  4. Internal Energy and Other Energies • Internal energy includes kinetic energies due to • Random translational motion • Rotational motion • Vibrational motion • Internal energy also includes intermolecular potential energy

  5. Heat • Heat is a mechanism by which energy is transferred between a system and its environment due to a temperature difference between them • Heat flows from the one at higher temperature to the one at lower temperature. • Heat is also represented by the amount of energy, Q, transferred by this mechanism

  6. Units of Heat • Calorie is the unit used for heat • One calorie is the heat necessary to raise the temperature of 1 g of water from 14.5o C to 15.5o C • The Calorie used for food is actually 1 kcal • In the US Customary system, the unit is a BTU (British Thermal Unit) • One BTU is the heat required to raise the temperature of 1 lb of water from 63o F to 64o F • 1 cal = 4.186 J

  7. 17.2 Specific Heat • Specific heat, c, is the heat amount of energy per unit mass required to change the temperature of a substance by 1°C • If energy Q transfers to a sample of a substance of mass m and the temperature changes by DT, then the specific heat is Q = m c DT Units of specific heat are J/kg°C

  8. Some Specific Heat Values

  9. More Specific Heat Values

  10. Sign Conventions • If the temperature increases: • Q and DT are positive • Energy transfers into the system • If the temperature decreases: • Q and DT are negative • Energy transfers out of the system

  11. Specific Heat of Water • Water has a high specific heat relative to most common materials • Hydrogen and helium have higher specific heats • This is responsible for many weather phenomena • Moderate temperatures near large bodies of water • Global wind systems • Land and sea breezes

  12. Calorimetry • One technique for measuring specific heat involves heating a material, adding it to a sample of water, and recording the final temperature • This technique is known as calorimetry • A calorimeter is a device for the measurement

  13. Calorimetry, cont • The system of the sample and the water is isolated • The calorimeter allows no energy to enter or leave the system • Conservation of energy requires that the amount of energy that leaves the sample equals the amount of energy that enters the water • Conservation of Energy gives a mathematical expression of this: Qcold= - Qhot

  14. Calorimetry, final • The negative sign in the equation is critical for consistency with the established sign convention • Since each Q = m c DT, cunknown can be found • Technically, the mass of the water’s container should be included, but if mw>>mcontainerit can be neglected

  15. Phase Changes • A phase change is when a substance changes from one form to another • Two common phase changes are • Solid to liquid (melting) • Liquid to gas (boiling) • During a phase change, there is no change in temperature of the substance

  16. 17.3 Latent Heat • During phase changes, the energy added to a substance modifies or breaks the bonds between the molecules or makes different molecular arrangements. • The amount of added energy also depends on the mass of the sample • If an amount of energy Q is required to change the phase of a sample of mass m,

  17. Latent Heat, cont • The quantity L, called the latent heat of the substance, is the energy required for transforming the substance per unit mass from one phase to another phase • Latent means hidden • The value of L depends on the substance as well as the actual phase change

  18. Latent Heat, final • The latent heat of fusion, Lf, is used during melting or freezing • The latent heat of vaporization, Lv, is used when the phase change boiling or condensing • The positive sign is used when the energy is transferred into the system • This will result in melting or boiling • The negative sign is used when energy is transferred out of the system • This will result in freezing or condensation

  19. Sample Latent Heat Values

  20. Graph of Ice to Steam

  21. Warming Ice, Graph Part A • Start with one gram of ice at –30.0º C • During A, the temperature of the ice changes from –30.0º C to 0º C • Use Q = m cice ΔT • Cice= 2090 J/kg º C • In this case, 62.7 J of energy are added

  22. Melting Ice, Graph Part B • Once at 0º C, the phase change (melting) starts • The temperature stays the same although energy is still being added • Use Q = m Lf • The energy required is 333 J • On the graph, the values move from 62.7 J to 396 J

  23. Warming Water, Graph Part C • Between 0º C and 100º C, the material is liquid and no phase changes take place • Energy added increases the temperature • Use Q = m cwater ΔT • 419 J are added • The total is now 815 J

  24. Boiling Water, Graph Part D • At 100º C, a phase change occurs (boiling) • Temperature does not change • Use Q = m Lv • This requires2260 J • The total is now 3070 J

  25. Heating Steam • After all the water is converted to steam, the steam will heat up • No phase change occurs • The added energy goes to increasing the temperature • Use Q = m csteam ΔT • In this case, 40.2 J are needed • The temperature is going to 120o C • The total is now 3110 J

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