Thermal physics hl

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Thermal physics hl

  1. 1. Thermal Physics HL
  2. 2. Assumptions of the kinetic theory 1. Molecules behave as if they were hard, smooth, elastic spheres. (i.e. the collisions are perfectly elastic) 2. Molecules are in continuous rapid, random motion. 3. The average kinetic energy of the molecules is proportional to the absolute temperature of the gas. 4. The molecules do not exert any appreciable attraction on each other. 5. The volume of the molecules is infinitesimal when compared with the volume of the gas. 6. The time spent in collisions is small compared with the time between collisions.
  3. 3. Pressure and temperature Gay-Lussac’s Law P1 / T1 = P2 / T2 As the temperature is reduced, the pressure decreases (Why?) If the graph is extrapolated, then the intercept gives the These results do not depend on the type of gas. value of absolute zero
  4. 4. Pressure and volume Boyles’ Law Take measurements of pressure and volume Plot a suitable graph to determine the relationship between these 2 variables.
  5. 5. Boyle’s Law The pressure of a fixed mass of gas at constant temperature is inversely proportional to the volume. p1V1 = p2V2
  6. 6. Boyle’s Law animation
  7. 7. Charles’ Law The pressure of a fixed mass of gas at constant volume is directly proportional to the absolute temperature.
  8. 8. Hyperlink
  9. 9. Charles’ Law animation
  10. 10. Equation of state for an ideal gas Combining the previous gas laws gives PV = (a constant)×T Therefore, for 1 mole of an ideal gas; P = Pressure PV = nRT V = Volume n = number of moles R = Universal gas constant T = Temperature (KELVIN)
  11. 11. Gas equation animation Click on image to activate pV = nRT
  12. 12. Gas properties simulation
  13. 13. Describe the concept of the absolute zero of temperature and the Kelvin scale of temperature.
  14. 14. Questions 1. A diver exhales a bubble of volume 2cm3 at a depth of 30m (Pressure = 4 Atmos). What is the volume of the bubble when it reaches the surface? (Isothermal). (8) 2. A fixed volume of gas is heated from 100kPa at 27 0C to 350kPa. What is it’s new temperature? (777) 3. A sample of Neon gas occupies a volume of 45 litres at 100kPa and 200C. How many moles of gas are there? (1.85) A-level Q’s 1a,2,3,5a(i),6a,6b(i),7(a),8(a)(b)(c),9,10(a),11(a),13
  15. 15. Tsokos Page 181 Q’s 1- 9.
  16. 16. Ideal Gases and Real Gases • Real gases behave as ideal gases at room temp and pressure. • The gas molecules become “interacting” at high temperatures and high pressures. Therefore they lose there ideal properties. • Ideal gases cannot be liquefied. • Ideal gases obey the gas equation.
  17. 17. Work done in compressing a gas Deduce an expression for the work involved in a volume change of a gas at constant pressure. The work done by this force is w = Fs = PAs, since F=PA but As is the change in the volume occupied by the gas, ΔV. therefore; W = P∆V
  18. 18. Conservation of energy If we add energy to a fixed mass of gas, the gas will increase in temperature (internal energy ΔU). ΔU =
  19. 19. 1 Law of Thermodynamics st State the first law of thermodynamics. We can add energy by heating (temperature gradient) = Q Or by working (no temperature difference) = W Students should be familiar with the terms system and surroundings. They should also Q = Heat energy added to the gas appreciate that if a system and its surroundings are at different and the ΔU = Temperature increase of the temperaturesprocess, system undergoes a the gas energy transferred by nonmechanical means to or from the system is referred to as W = Work done by the gas. thermal energy (heat). Q = ΔU + W
  20. 20. 1 Law questions st • 1b • 10(a),(c),(dii) • Tsokos page 193 Q’s 1 Q = ΔU +W Q ΔU W
  21. 21. P-V changes 1. Change of p (and T) at constant volume; an isovolumetric change. 2. Change of V (and T) at constant pressure; an isobaric change. 3. Change in p and V at constant temperature; an isothermal change. 4. Change in p and V in an insulated container (no heating of the gas); an adiabatic change.
  22. 22. Isothermal gas processes For a fixed mass and temperature of gas, state the values of Q, ΔU and W as the gas expands.
  23. 23. Isothermal P-V changes Q= ΔU = W=
  24. 24. Isochoric changes (Volume) Isochoric changes Q= ΔU = W=
  25. 25. Isobaric changes (Pressure) Isobaric changes Q= ΔU = W=
  26. 26. Adiabatic changes Adiabatic changes These are defined as processes where no heat can flow in or out of the system. This occurs when the change happens too rapidly for the heat to be exchanged. Therefore they result in a change in temperature of the gas Q= ΔU = W=
  27. 27. Ideal gas processes Hyperlinkhttp:// www.walterfendt.de/ph14e/ gaslaw.htm Isobaric process: pressure constant V/T constant Isochoric process: volume constant p/T constant Isothermal process: temperature constant pV constant
  28. 28. Conservation of energy for Carnot cycle Curve A Isothermal expansion at TH Work done by the gas Curve B Adiabatic expansion Work done by the gas Curve C Isothermal compression at TC Work done on the gas For each part of the cycle, find Q=? ΔU = ? W=? Curve D Adiabatic compression Work done on the gas
  29. 29. Otto cycle Click to play
  30. 30. Carnot cycle Picture has a hyperlink How does energy enter and leave the gas in a Carnot cycle?
  31. 31. Work done in a thermodynamic process The product of pressure and volume represents a quantity of work. This is represented by the area below a p-V curve. Therefore, the area enclosed by the four curves represents the net work done by the engine during one cycle.
  32. 32. Tsokos Page 193 Q’s 2-5
  33. 33. Second Law: Entropy a measure of the amount of energy which is unavailable to do work a measure of the disorder (of the energy) of a system How “useful” is the energy? Which can do the most work for us? 100j of energy in petrol or 100j of energy as heat? The heat is “disordered” or “higher entropy” Every time we change energy from one form to another, we increase the entropy of the Universe.
  34. 34. Second Law of Thermodynamics The second law of thermodynamics is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines. In so doing, it goes beyond the limitations imposed by the first law of thermodynamics. It's implications may be visualized in terms of the waterfall analogy.
  35. 35. Second Law: Heat Engines Second Law of Thermodynamics: It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W . Some amount of heat QC must be exhausted to a cold reservoir. This precludes a perfect heat engine.
  36. 36. Second Law: Refrigerator Second Law of Thermodynamics: It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator. The statements about refrigerators apply to air conditioners and heat pumps, which embody the same principles.
  37. 37. Tsokos Page 194 Q’s 9-13.

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