Thermal energy

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Thermal energy

  1. 1. Chapter 10 Heat
  2. 2. Thermal Energy A. Temperature & Heat 1. Temperature is the measure of the average kinetic energy of the particles in a substance.
  3. 3. 2. SI unit for temp. is the Kelvin (but you will see Celsius used) a. K = C + 273 (10C = 283K) b. C = K – 273 (10K = -263C) 3. Thermal Energy – the total of all the kinetic and potential energy of all the particles in a substance.
  4. 4. Quantity of Heat • It is measured in joules • Common usage is a heat unit called the calorie (the energy needed to raise the temperature of 1 gram of water by 1o C) • Chemists use the kilocalorie (1000 calories=4200 Joules) • Nutritionists call it a food Calorie
  5. 5. 4. Thermal energy relationships a. As temperature increases, so does thermal energy (because the kinetic energy of the particles increased). b. Even if the temperature doesn’t change, the thermal energy in a more massive substance is higher (because it is a total measure of energy).
  6. 6. 5. Heat a. The flow of thermal energy from one object to another. b. Heat always flows from warmer to cooler objects. Ice gets warmer while hand gets cooler Cup gets cooler while hand gets warmer
  7. 7. Heat Transfer • Heat flows from hot to cold. – If you hold something cold, heat flows from hand to object. – If you hold something hot, heat flows from object to hand • Conduction- transfer of thermal energy through matter by the direct contact of particles – Occurs because particles are in constant motion – KE transferred as particles collide
  8. 8. • http://www.physchem.co.za/Heat/Transfer.htm#C onduction
  9. 9. Conduction• Heating of metal pan- – Particles in handle of pan move slowly – Fast moving particles from the bottom bump into slower particles and speed them up – Occurs until all particles move the same speed • Conduction works best in solids- especially metals- because particles are close together
  10. 10. Conduction and Convection • Metals- good conductors-because electrons move easily & transfer KE to nearby particles • Fluid- any materials that flows • Convection- transfer of energy in a fluid by the movement of heated particles • Convection currents transfer heat from warmer to cooler parts of a fluid. • Convection vs. Conduction- – Conduction involves collisions and transfers of energy. – Convection involves movement of the energetic particles from one location to another
  11. 11.                        
  12. 12. Convection • Convection- results in changes in density – As particles move faster, they get farther apart – Fluid expands as temperature increases – Larger volume = smaller density – Decreasing density results in the rise of the warmer fluid • Lava Lamp- – Cool oil = dense = sits on the bottom – Warmer oil = less dense than alcohol & rises – As it rises, it loses energy through conduction • Causes decrease in density = sinking
  13. 13. • When oil is cool  Oil isOil is warm, so itwarm, so it risesrises Oil starts to lose heat by conduction and falls
  14. 14. Convection Currents • Currents in which warm portions of the fluid move through the substance- convection • The warm portions transfer energy to the cool section through conduction
  15. 15. Heat Transfer on Earth • At equator- earth experiences the most heat from the sun. – Result: evaporation of water and large accumulations of clouds. – As the water vapor rises, it cools and condenses, forming rain • After the rain = dry air – Dry air causes moisture to evaporate, drying out the ground – causes desert • Convection currents create deserts and rain forests over different regions of Earth
  16. 16. Radiation • Transfer of heat to the earth – occurs through radiation • Radiation- the transfer of energy by electromagnetic waves. The waves travel through space even without matter
  17. 17. Controlling the Flow of Heat • To control the flow of heat: Use clothing, blankets, layers of fat, fur, etc. • Insulator- material that does not allow heat to flow through easily • Gases – like air- are good insulators because: – Gas particles are very far apart & can’t transmit E through conduction. – If the gas is also held in place, particles can’t move around and warm up the rest of the gas
  18. 18. Insulation • Insulation is made of fluffy materials containing pockets of trapped air – prevents heat loss • Thermos- vacuum layer between 2 layers of glass – Vacuum contains few particles so conduction & convection don’t occur. • Thermos- coated in aluminum – Reflects electromagnetic waves that would either heat the substance or allow the substance to cool
  19. 19. 6. Specific Heat a. Some things heat up or cool down faster than others. Land heats up and cools down faster than water
  20. 20. b. Specific heat is the amount of thermal energy required to raise the temperature of 1 kg of a substance one degree (C or K). 1) C water = 4184 J / kg C 2) C sand = 664 J / kg C This is why land heats up quickly during the day and cools quickly at night and why water takes longer.
  21. 21. Specific Heat • The higher the specific heat, the more energy is required to cause a change in temperature. • Substances with higher specific heats must lose more thermal energy to lower their temperature than do substances with a low specific heat. • Water is slower to heat but is also slower to lose heat
  22. 22. Why does water have such a high specific heat? Water molecules form strong bonds with each other; therefore it takes more heat energy to break them. Metals have weak bonds and do not need as much energy to break them. water metal
  23. 23. c. A calorimeter is used to help measure the specific heat of a substance. First, mass and temperature of water are measured Then heated sample is put inside and heat flows into water T is measured for water to help get its heat gain This gives the heat lost by the substance
  24. 24. Expansion of Water • Remarkably interesting case
  25. 25. Expansion of Water
  26. 26. Expansion of Water • This is why lakes and ponds and rivers freeze with the ice on top • If they didn’t, no aquatic life would be possible
  27. 27. Thermostat •A thermostat is a device that controls the temperature •The switch of a thermostat is a bimetallic strip
  28. 28. Bimetallic Strip • Two different metals that are bound together • They expand at different rates when heated • Used as a switch in a thermostat
  29. 29. Copyright 1999, PRENTICE HALL Chapter 11 30 Phase ChangesPhase Changes • Sublimation: solid → gas. • Vaporization: liquid → gas. • Melting or fusion: solid → liquid. • Deposition: gas → solid. • Condensation: gas → liquid. • Freezing: liquid → solid. Energy Changes AccompanyingEnergy Changes Accompanying Phase ChangesPhase Changes • Energy change of the system for the above processes are:
  30. 30. Copyright 1999, PRENTICE HALL Chapter 11 31 Phase ChangesPhase Changes Energy Changes AccompanyingEnergy Changes Accompanying Phase ChangesPhase Changes – Sublimation: (endothermic). – Vaporization: (endothermic). – Melting or Fusion: (endothermic). – Deposition: (exothermic). – Condensation: (exothermic). – Freezing: (exothermic). • Generally heat of fusion (enthalpy of fusion) is less than heat of vaporization: – it takes more energy to completely separate molecules, than partially separate them.
  31. 31. Copyright 1999, PRENTICE HALL Chapter 11 32 Phase ChangesPhase Changes Energy Changes AccompanyingEnergy Changes Accompanying Phase ChangesPhase Changes • All phase changes are possible under the right conditions (e.g. water sublimes when snow disappears without forming puddles). • The sequence heat solid → melt → heat liquid → boil → heat gas is endothermic. • The sequence cool gas → condense → cool liquid → freeze → cool solid is exothermic.
  32. 32. Copyright 1999, PRENTICE HALL Chapter 11 33 Phase ChangesPhase Changes Energy Changes AccompanyingEnergy Changes Accompanying Phase ChangesPhase Changes
  33. 33. Copyright 1999, PRENTICE HALL Chapter 11 34 Phase ChangesPhase Changes Heating CurvesHeating Curves • Plot of temperature change versus heat added is a heating curve. • During a phase change, adding heat causes no temperature change. –These points are used to calculate ∆Hfus and ∆Hvap. • Supercooling: When a liquid is cooled below its melting point and it still remains a liquid. • Achieved by keeping the temperature low and increasing kinetic energy to
  34. 34. Copyright 1999, PRENTICE HALL Chapter 11 35 Phase ChangesPhase Changes Heating CurvesHeating Curves
  35. 35. Copyright 1999, PRENTICE HALL Chapter 11 36 Heating Curve Illustrated
  36. 36. Copyright 1999, PRENTICE HALL Chapter 11 37 Phase ChangesPhase Changes Critical Temperature and PressureCritical Temperature and Pressure • Gases liquefied by increasing pressure at some temperature. • Critical temperature: the minimum temperature for liquefaction of a gas using pressure. • Critical pressure: pressure required for liquefaction.
  37. 37. Copyright 1999, PRENTICE HALL Chapter 11 38 Critical Temperature, Tc
  38. 38. Phase Diagrams Copyright 1999, PRENTICE HALL Chapter 11 39 A phase diagrams show what phases exist at equilibrium and what phase transformations we can expect when we change one of the parameters of the system (T, P, composition).
  39. 39. Copyright 1999, PRENTICE HALL Chapter10 40 Phase DiagramsPhase Diagrams • Phase diagram: plot of pressure vs. Temperature summarizing all equilibrium between phases. • Given a temperature and pressure, phase diagrams tell us which phase will exist. • Features of a phase diagram: – Triple point: temperature and pressure at which all three phases are in equilibrium. – Vapor-pressure curve: generally as pressure increases, temperature increases. – Critical point: critical temperature and pressure for the gas. – Melting point curve: as pressure increases, the solid phase is favored if the solid is more dense than the liquid. – Normal melting point: melting point at 1 atm.
  40. 40. Copyright 1999, PRENTICE HALL 41 Phase DiagramsPhase Diagrams • Any temperature and pressure combination not on a curve represents a single phase.
  41. 41. Copyright 1999, PRENTICE HALL Chapter 11 42 Phase DiagramsPhase Diagrams The Phase Diagrams of HThe Phase Diagrams of H22 O and COO and CO22 • Water: – The melting point curve slopes to the left because ice is less dense than water. – Triple point occurs at 0.0098°C and 4.58 mmHg. – Normal melting (freezing) point is 0°C. – Normal boiling point is 100°C. – Critical point is 374°C and 218 atm.
  42. 42. Copyright 1999, PRENTICE HALL Chapter 11 43 Phase DiagramsPhase Diagrams The Phase Diagrams of HThe Phase Diagrams of H22 O and COO and CO22 • Carbon Dioxide: – Triple point occurs at -56.4°C and 5.11 atm. – Normal sublimation point is -78.5°C. (At 1 atm CO2 sublimes it does not melt.) – Critical point occurs at 31.1°C and 73 atm.

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