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

2. 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. 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. 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. 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. 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 14.5 o C to 15.5 o 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 63 o F to 64 o F 1 cal = 4.186 J

7. 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  T, then the specific heat is Q = m c  T Units of specific heat are J/kg°C

8. 8 Some Specific Heat Values

9. 9 More Specific Heat Values

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

11. 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. 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. 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: Q cold = - Q hot

14. 14 Calorimetry, final The negative sign in the equation is critical for consistency with the established sign convention Since each Q = m c  T, c unknown can be found Technically, the mass of the water’s container should be included, but if m w >>m container it can be neglected

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17. 17 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

18. 18 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,

19. 19 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

20. 20 Latent Heat, final The latent heat of fusion, L f, is used during melting or freezing The latent heat of vaporization, L v, 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

21. 21 Sample Latent Heat Values

22. 22 Graph of Ice to Steam

23. 23 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 c ice ΔT C ice = 2090 J/kg º C In this case, 62.7 J of energy are added

24. 24 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 L f The energy required is 333 J On the graph, the values move from 62.7 J to 396 J

25. 25 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 c water ΔT 419 J are added The total is now 815 J

26. 26 Boiling Water, Graph Part D At 100º C, a phase change occurs (boiling) Temperature does not change Use Q = m L v This requires2260 J The total is now 3070 J

27. 27 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 c steam ΔT In this case, 40.2 J are needed The temperature is going to 120 o C The total is now 3110 J

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