Chapters 13 & 14

1. Chapters 13 & 14 Temperature and Heat Alta Physics

2. Heat • Heat is a form of Energy Transfer • Heat flows from areas of high energy to areas of lower energy • Heat is transferred three way • Conduction – requires contact • Convection – mass movement of molecules • Radiation – transfer over a distance Alta Physics

3. Temperature Scales • Temperature is defined as a measure of the average kinetic energy of the molecules. • Temperature scales were developed using the freezing and boiling points of water at sea level as the standard reference points. Alta Physics

4. Temperature Scales • There are three Temperature Scales used in Science • Fahrenheit – Used primarily in the United States • Celsius – the standard for the Metric System • Kelvin – Also know as the “Absolute Zero” Scale. Alta Physics

5. Standard Reference Points Alta Physics

6. Conversion Equations • Fahrenheit to Centigrade C = 5/9 (F – 32) Ex: C = 5/9 (212-32) = 100 • Centigrade to Fahrenheit F = (9/5 C) + 32 Ex: F = (9/5)(100) + 32 = 212 Alta Physics

7. Conversion Equations • Centigrade to Kelvin K = C +273 Ex: K = 100 + 273 = 373 • Kelvin to Centigrade C = K - 273 Ex: K = 373 - 273 = 100 Alta Physics

8. Thermal Expansion • Most objects tend to expand when their temperature rises and to contract when the temperature drops. • Do you know what the one notable exception is? Alta Physics

9. Thermal Expansion • Objects can expand linearly whereas liquids expand volumetrically! • Each substance has a constant for which describes its ability to expand. • Linear constants are denoted by the Greek letter alpha, α and the volumetric constant is denoted by the Greek letter beta, β. Alta Physics

10. Thermal Expansion • Linear Expansion Δl = (α)(l)(Δt) l’ = l + Δl • Volumetric Expansion Δl = (β)(V)(Δt) V’ = V + ΔV Alta Physics

11. Sample Problem • A Brass strip is 3 cm long at 0° C. How long will it be at 100° C if the coefficient of linear expansion for Brass is 19 x 10 -6? Δl = (α)(l)(Δt) = (19 x 10 -6)(0.03 m)(100 °C)= 0.000057 m l’ = Δl + l = 0.000057 m + 0.03 m = 3.0057 cm Alta Physics

12. Conduction • Each material has an innate ability to absorb or give off heat – specific heat • The amount of heat an object can transfer depends upon three things: • The mass of the object, m, (in Kg) • The specific heat of the object, Cp,( in J/g°C) • The temperature change of the object, t, (in ºC) Alta Physics

13. Calorimetry • The symbol for heat is a capitol Q • Law of Conservation of Energy says that Qlost= Qgained • Mathematically Q = mCpΔt Alta Physics

14. Sample Problem • How much energy is required to raise the temperature of 5 Kg of water from 0°C to 100°C ? (Cp = 4186 J/kg°C) • Q= m CpΔt = (5 kg)(Cp = 4186 J/kg°C)(100°C) = 493000 Joules Alta Physics

15. Sample Problem • A 5 Kg copper ball is heated to 180 ºC and dropped into a container of water at 100 °C. When the temperature of the ball/water system equalizes the final temperature is found to be 110 ºC. How much water is in the container? (Cp= 390 J/Kg°C) Alta Physics

16. Solution Qlost=Qgained Q copper = Q water mC copperΔt= mC water Δt (5 kg)(390 J/Kg°C)(70°C)=m(4186 J/Kg°C)(10°C) m = 3.02 kg Alta Physics

17. Phases Changes • Phase changes require that a substance absorb energy or release energy to occur. • There is NO Change in Temperature associated with a phase change! • Different words are used to denote direction when dealing with a phase change. Alta Physics

18. Vocabulary of a Phase Change • Freezing – change from liquid to solid. • Energy is released! • Melting – change from solid to liquid • Energy is absorbed! Both of these changes happen at the same point; 0° for water! Alta Physics

19. Vocabulary of a Phase Change • Condensing – change from gas or vapor to a liquid. • Energy is released! • Boiling – change from liquid to gas or vapor • Energy is absorbed! Vapor is the gas phase of any substance which is normally a liquid at room temperature! Alta Physics

20. Energy Required for a Phase Change • Just like specific heat is a set amount of energy for each substance – the amount of energy required for a phase change is also substance specific. • Heat of Fusion – ΔHf , is the amount of energy absorbed or released when a substance melts or freezes! Alta Physics

21. Energy Required for a Phase Change • Heat of Vaporization – ΔHf , is the amount of energy absorbed or released when a substance boils or condenses! • Mathematically: Q = mΔHf for Fusion or Q = mΔHv for Vaporization Alta Physics

22. Sample Problem How much heat is required to melt 5 kg of ice at 0° C? Solution Q = mΔHf = (5 kg)(3.33 x 105 J/kg °C) = 1.665 x 106 J Alta Physics

23. Phase Diagram Water-Steam 120 Steam 100 Water Ice - Water 0 -5 Ice Alta Physics

24. Sample Problem How much energy is required to convert 5 kg of ice at -5 °C to Steam at 120 °C? Steps Raise temperature of ice to melting point Melt Ice Raise temperature of Water to boiling point Vaporize Water Raise Temperature of Steam to 120 °C Alta Physics

25. Solution Step 1: Raise temperature of Ice Q = mCΔt = (5 Kg)(2100 J/kg°C)(5 °C)= 52500 J Step 2: Melt Ice Q = mΔHf = (5 Kg)(3.33 x 105 J/kg) = 1665000 J Step 3: Raise temperature of water to boiling Q = mCΔt = (5 Kg)(4186 J/kg°C)(100 °C)= 2093000 J Step 4: Vaporize Water Q = mΔHv = (5 Kg)(22.6 x 105 J/kg) = 11300000 J Step 5: Raise temperature of Steam to 120°C Q = mCΔt = (5 Kg)(2010 J/kg°C)(20 °C)= 201000 J Step 6: Get Total Qtotal= 52500 + 1665000 + 2093000 + 11300000 + 201000 = 15311500J Alta Physics

26. Convection & Radiation • Convection – transfer of energy by mass movement of molecules • Most common form is Wind • Radiation – transfer of energy through waves – most common form is light or electromagnetic waves • Will discuss both in more detail in later chapters Alta Physics

27. Problem Types • Temperature Scales and Conversions • Thermal Expansion • Conduction • Calorimetry • Latent Heat • Fusion • Vaporization Alta Physics