Thermal physics slides 2011 student part1

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Thermal physics slides 2011 student part1

  1. 1. flipperworks.com Applications• Engineering• Chemistry• Earth Science• Astrophysics• Everyday life
  2. 2. flipperworks.comApplications To design pressure cookers
  3. 3. flipperworks.com
  4. 4. flipperworks.com
  5. 5. flipperworks.comApplications Automobile engine
  6. 6. flipperworks.comApplications
  7. 7. flipperworks.comApplications Household refrigerator
  8. 8. Power Plant flipperworks.comApplications
  9. 9. flipperworks.comApplications Solar Heater
  10. 10. flipperworks.com1 Introduction • In the study of thermal physics there is a need to approach it in two levels. – macroscopic level – microscopic level
  11. 11. flipperworks.com1 Introduction Macroscopic Approach • Look at macroscopic variables that can be measured by simple experiments: – temperature – pressure – volume • Derive general relationships and equations that can describe the experiments. – Advantage: • easily related to experiment – Disadvantage: • lacks fundamental explanations
  12. 12. flipperworks.com1 Introduction Microscopic Approach • Explain macroscopic properties by looking closely into the atoms and molecules that make up matter to understand what is happening . – Advantage: • Gives a real explanation of what is actually happening. – Disadvantage: • Too many particles will require the use of complex computation.
  13. 13. flipperworks.com1 Introduction • Lets begin by asking ourselves, – What is Temperature? – What is Heat?
  14. 14. flipperworks.com2. Temperature • In a macroscopic view: – Temperature is the physical property which determines the direction flow of heat. heat • e.g Heat flows from higher temperature to lower temperature Hot Cold Heat flows
  15. 15. flipperworks.com2. Temperature • In a macroscopic view: – Temperature is a physical property of a system that provide a measure of hotness or coldness.
  16. 16. flipperworks.com2. Temperature • In a microscopic view: – Temperature is a measure of the average kinetic energy of molecules in a body.
  17. 17. flipperworks.com2.1 Heat and Thermal Equilibrium • When two bodies are in thermal contact, energy can be transferred between them. Hot Cold
  18. 18. flipperworks.com2.1 Heat and Thermal Equilibrium • Microscopically the energy transfer occurs in two ways. Higher kinetic Lower kinetic energy molecules energy molecules Hot Cold
  19. 19. flipperworks.com2.1 Heat and Thermal Equilibrium • But the rate of transfer of energy from a hotter body is always greater than that from a cooler body. Higher kinetic Lower kinetic energy molecules energy molecules Hot Cold Energy Energy
  20. 20. flipperworks.com2.1 Heat and Thermal Equilibrium • This net energy transfer from a body of a higher temperature to a lower temperature is known as heat. Hot Cold Heat flows
  21. 21. flipperworks.com2.1 Heat and Thermal Equilibrium • Heat will flow between two bodies as long as there is temperature difference between them. Hot Cold Heat flows
  22. 22. flipperworks.com2.1 Heat and Thermal Equilibrium • When two bodies are in thermal contact and there is no flow of heat from one body to another, they are said to be in thermal equilibrium. • At thermal equilibrium (microscopically)  The rate of transfer of energy is the same from both bodies.  Thus these two bodies are said to be at the same temperature.
  23. 23. flipperworks.com2.1 Heat and Thermal EquilibriumConsider the 2 systems, X and Y: insulator No thermal contact No flow of heat between X and Y X Y Fig 1
  24. 24. flipperworks.com2.1 Heat and Thermal Equilibrium X and Y are in thermal contact Temperature of X > Temperature of Y Rate of transfer of energy from X to Y > Rate of transfer of energy from Y to X X Y Heat flows from X to Y Fig 2 heat Temperature of X decreases and Temperature of Y increases
  25. 25. flipperworks.com2.1 Heat and Thermal Equilibrium X and Y in thermal equilibrium Rate of transfer of energy from X to Y = Rate of transfer of energy from Y to X X Y No flow of heat between X and Y Fig 3 Temperature of X = Temperature of Y
  26. 26. flipperworks.com2.2 Zeroth Law (law of thermal equilibrium) Zeroth law of thermodynamics states that if bodies A and B are in thermal equilibrium with a third body C, then A and B are in thermal equilibrium with each other. C A ⇒ TA = TB C B TA = T C TB = TC
  27. 27. flipperworks.com2.2 Zeroth Law (law of thermal equilibrium) • The significance of the Zeroth Law: – It allows us to claim that two objects in thermal equilibrium with each other must be at the same temperature. – It allows us to know whether objects are at the same temperature, even when we can’t place them in thermal contact. – It allows temperature to become reproducible, and quantifiable. (Temperature can be a physical quantity)
  28. 28. flipperworks.com2.2 Zeroth Law (law of thermal equilibrium) In other words We could create a thermometer to measure temperature.
  29. 29. flipperworks.comExample 1 A solid X is in thermal equilibrium with a solid Y, which is at the same temperature as a third solid Z. The three bodies are of different materials and masses. Which one of the following statements is certainly true? A X and Y have the same heat capacity. B There is no net transfer of energy if X is placed in thermal contact with Z. C It is not necessary that Y should be in thermal equilibrium with Z. D It is not necessary that X should be at the same temperature as Z.Tx = Ty Therefore Tx = Tz Ans: BTy = Tz
  30. 30. flipperworks.com2.3 Thermodynamic Temperature Scale • To measure temperature quantitatively, we need to have a scale. Empirical Scale 2 Scales Thermodynamic Temperature Scale • An empirical temperature scale is a temperature scale based on experimental results. – e.g. Centigrade Scale
  31. 31. flipperworks.com2.3 Thermodynamic Temperature Scale • An empirical scale requires a thermometric property and two fixed points (i.e ice point and steam point) • Thermometric Property: A physical property that changes in a known way with temperature.
  32. 32. flipperworks.com2.3 Thermodynamic Temperature Scale Examples of thermometric properties used in various thermometers: Type of Thermometer Thermometric Property liquid-in-glass length of mercury in a thermometer capillary tube Resistance resistance of platinum wire thermometer Thermocouple EMF of a copper- thermometer constantan thermocouple constant volume gas pressure of a fixed mass thermometer of gas at constant volume
  33. 33. flipperworks.comThermomete Constant volume gas thermometerrs Thermocouple Resistance Thermometers mV iron iron constantan Junction 2 Junction 1
  34. 34. flipperworks.com2.3 Thermodynamic Temperature Scale • Problem with empirical scales based on particular thermometers: – scales agree only at calibration points. Ideal • A need to have a scale which is independent of any thermometric property (absolute scale). – reliable – reproducible
  35. 35. flipperworks.com2.3 Thermodynamic Temperature Scale • The THERMODYNAMIC temperature scale is theoretical and is independent of the properties of any particular thermometric substance. • The scale is also known as the Absolute Temperature Scale.
  36. 36. flipperworks.com2.3 Thermodynamic Temperature Scale The two fixed points in the Thermodynamic Temperature Scale are: (a) absolute zero which is the temperature at which the pressure of an ideal gas becomes zero. It is arbitrarily given the value 0 K. (b) the triple point of water which is the temperature at which ice, water and water vapour coexist in dynamic equilibrium.
  37. 37. flipperworks.com2.3 Thermodynamic Temperature Scale Phase diagram for water
  38. 38. flipperworks.com2.3 Thermodynamic Temperature Scale Diagram of a triple point cell Thermometer well Water Dewar vapour vessel Ice Ice water sheath mixture Thermal contact liquid Water
  39. 39. flipperworks.com2.3 Thermodynamic Temperature Scale • The triple point of water is chosen because it – is unique, invariant and occurs only at one definite temperature and pressure. (T = 273.16 K and Pressure = 611.73 Pa) – can be easily and accurately reproduced using a triple point cell.
  40. 40. flipperworks.com2.3 Thermodynamic Temperature Scale The unit of temperature in the thermodynamic scale is the kelvin, symbol K. Kelvin is also the S.I. unit of temperatureOne kelvin is defined to be 1 of the 273.16thermodynamic temperature of the triple point of water 1 If, × Ttr = 1 K 273.16 Ttr = 273.16 K
  41. 41. flipperworks.com2.4 The Celsius ScaleThe Celsius scale is related to the Thermodynamicscale by the exact equation: t/oC = T/K – 273.15 The unit for this scale is degree Celsius, symbol oC (same symbol as for degree Centigrade).
  42. 42. flipperworks.comExample 2 What is the change in temperature in kelvin when the temperature falls from 540.85 °C to 502.02 °C? A 38.83 K B 311.98 K C 273.15 K D 228.85 K Ans: A
  43. 43. flipperworks.comWhat we have covered• Definition of Temperature – Macroscopic – Microscopic• Heat and Thermal Equilibrium• Zeroth Law and its significance – Thermodynamic Temperature Scale – Celsius Scale
  44. 44. flipperworks.com What happen when you heat a substance?ExpandsTemperature increase
  45. 45. flipperworks.com3 Heat Capacity Heat Capacity HEAT CAPACITY, C, of a body is defined as the quantity of heat absorbed / liberated, Q, by the body per unit temperature change. Q = C ∆θ S.I. unit for heat capacity is J K-1
  46. 46. flipperworks.com3 Heat Capacity Specific Heat Capacity SPECIFIC HEAT CAPACITY, c, of a material, is defined as the quantity of heat absorbed / liberated, Q, per unit mass of the material per unit temperature change. Q = m c ∆θ The S.I. unit of specific heat capacity is J kg-1 K-1 C = mc
  47. 47. flipperworks.comExample 3The specific heat capacity of copper is 400 J kg -1 K-1.(a) What is the heat capacity of 5 kg of copper?(b) If the copper temperature rises by 10 oC, what would be the heat gained? (a) heat capacity C = m c = 5 x 400 = 2,000 J K-1
  48. 48. flipperworks.comExample 3The specific heat capacity of copper is 400 J kg -1 K-1.(a) What is the heat capacity of 5 kg of copper?(b) If the copper temperature rises by 10 oC, what would be the heat gained? (b)Heat gained, Q = m c ∆θ = 5 x 400 x 10 = 20,000 J
  49. 49. flipperworks.com3.1.1 Determination of specific heat capacity, c (Electrical Method) For Solid Conductors: (Insulation)
  50. 50. flipperworks.comBy the principle of conservation of energy,Assuming negligible heat loss to the surroundings.Electrical energy supplied = Heat absorbed by block VIt = m c (θ2 - θ1) V It c= m(θ 2 − θ1 ) What happens to the value of c if heat loss is not negligible?
  51. 51. flipperworks.comBy the principle of conservation of energy,Assuming negligible heat loss to the surroundings.Electrical energy supplied = Heat absorbed by block V I t2 = m cL (θ2 - θ1) + h t2 t2 would be longer than t (ideal time taken) cL = V It 2 m(θ 2 − θ1 ) The calculated cL will be higher than the actual value c.

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