Thermal Physics

1. Thermal Physics AHL Topic 10

2. Thermodynamics • Thermodynamics is the study of heat and • its transformation into mechanical energy, • as heat and work. • The word is derived from the Greek meaning • ‘movement of heat’. • It was developed in the mid 1800’s • before atomic and molecular theory was developed.

3. Thermodynamics • Work is defined as: • The quantity of energy transferred from one system to another by ordinary mechanical processes. • Heat is defined as: • A transfer of energy from one body to another body at a lower temperature.

4. Thermodynamics • From this we can see that thermodynamics describes the relationship between heat and work. • To distinguish the two: • Heat is the transfer of energy due to a temperature difference. • Work is the transfer of energy that is not due to a temperature difference.

5. Thermodynamics • The foundation of this area of study is • The law of conservation of energy • and the fact that heat flows from hot to cold. • In discussing thermodynamics, we will refer to different systems. • A system is just a group of objects we wish to consider. • Everything else in the universe will be called the environment.

6. Thermodynamics • Consider a hot gas separated from a cold gas by a glass wall. • In macroscopic terms, • we know that the hot gas gets cooler and the cool gas gets hotter. • The molecules in the hot gas hit the glass • and set those molecules in faster motion.

7. Thermodynamics • This then sets in train a set of collisions • which sees the energy being transferred to the cold gas. • If we were to observe a single collision, • we could analyse the energy transfer • using the laws of mechanics.

8. Thermodynamics • We could say that one molecule has transferred energy • by doing work on another. • Heat is therefore the work done on a molecular level.

9. Thermodynamics • This isnot the complete story. • Although the cool gas contains, on average, • slower molecules than in the hot gas, • it does contain some fast moving molecules. • Likewise, the hot gas contains slow moving molecules.

10. Thermodynamics • From above, it should be possible to for the cold gas to transfer • energy to the hot gas • So the cool gas would get cooler • and the hot gas hotter. • This does not disobey any classical theory of mechanics. • We do know however that this cannot occur.

11. Thermodynamics • To explain this, we cannot look at this the effects of single molecules • or even a few molecules. • We must, when discussing heat, • look at the overall effects of a large number of molecules • and the average energies • and distribution of energies and velocities.

12. Thermodynamics • This is what is meant by a system of particles in thermodynamics. • A system could be any group of atoms, • molecule or particles we wish to deal with. • It may be the steam in a steam engine, • the earth’s atmosphere • or the body of a living creature.

13. Thermodynamics • The operation of changing the system from its initial state to a final state is called the, • Thermodynamic process.

14. Thermodynamics • During this process, • heat may be transferred into or out of the system, • and work may be done on or by the system. • We assume all processes are carried out very slowly, • so that the system remains in thermal equilibrium at all stages.

15. Isothermal & Adiabatic Processes • Previously, we discussed the relationship between • pressure and volume and found that: •  P  1/V

16. Isothermal & Adiabatic Processes • We also stated that this was true, • the temperature was constant. • A graph of P vs V is shown below.

17. Isothermal & Adiabatic Processes • The volume has increased from Vi to Vf while the pressure has decreased. • The solid line is an isotherm, • a curve giving the relationship • between V and P • at a constant temperature. • This is known as isothermic expansion.

18. Isothermal & Adiabatic Processes • The process of compression or expansion of a gas; • so that no heat enters or leaves the system, • is said to be adiabatic. • This comes from the Greek which means ‘impassable’.

19. Isothermal & Adiabatic Processes • Adiabatic changes of volume can be achieved by; • performing the process so rapidly that, • heat has little time to enter or leave the system, • like a bicycle pump. • Or by thermally insulating a system; • from its surroundings, • with Styrofoam.

20. Isothermal & Adiabatic Processes • A common example of a near adiabatic system; • is the compression and expansion of gases, • in the cylinders of a car engine. • Compression and expansion occur too rapidly; • for heat to leave the system.

21. Isothermal & Adiabatic Processes • When work is done on a gas by; • adiabatically compressing it, • the gas gains internal energy, • and becomes warmer. • When the gas adiabatically expands; • it does work on the surroundings, • and gives up its internal energy, • and becomes cooler.

22. Isothermal & Adiabatic Processes • Adiabatic processes occur in the atmosphere in large masses of air. • Due to their large size, • mixing of different pressures • and temperatures • only occur at the edges of these large masses • and do little to change • the composition of these air masses.

23. Isothermal & Adiabatic Processes • As it flows up the side of a mountain; • its pressure reduces, • allowing it to expand and cool. • The reduced pressure results • in a reduced temperature.

24. Isothermal & Adiabatic Processes • It has been shown that dry air will drop • by 10oC • for every kilometre it rises. • Air can flow over high mountains • or rise in thunderstorms • or cyclones • many kilometres.

25. Isothermal & Adiabatic Processes • If a mass was 25oC at sea level and was lifted 6 kilometres, • its temperature would become -35oC. • An air mass that was -20OC at 6 km, • would be 40oC at sea level.

26. Isothermal & Adiabatic Processes • An example of this is when cold air is blown over the Mt Lofty Ranges. • Warm moist air is cooled as it rises over the ranges • starts to rain. • On the other side, the air begins to warm as it flows down the other side • causing a warm wind.

27. Isothermal & Adiabatic Processes • As the Mt Lofty ranges are not very high; • the change in temperature is not as great, • compared to the Rocky Mountains in the USA..

28. P – V Diagrams • Thermodynamic processes can be represented by pressure - volume graphs.

29. P – V Diagrams • In the, an ideal gas is expanding isothermally, • absorbing heat Q, • and doing work W. • The system has not been restored; • to its original state, • at the end of the process.

30. P – V Diagrams

31. P – V Diagrams • The previous diagram is from a reversible heat engine. • Process 1-2 takes place at a constant volume; • Isochoric • process 2-3 is adiabatic, • process 3-1 is at a constant pressure; • Isobaric.

32. P – V Diagrams • In the next case; • the volume of an ideal gas is decreased, • by adding weight to the piston. • The process is adiabatic (Q = 0).

33. P – V Diagrams

34. P – V Diagrams • The process is as shown below on a graph.

35. P – V Diagrams • In the next case, • the temperature of an ideal gas is raised from T; • to T + T, • by a constant pressure process. • Heat is added; • and work is done, • in lifting the loaded piston.

36. P – V Diagrams

37. P – V Diagrams • The process is shown below on a P-V diagram

38. P – V Diagrams • The work; • PV, • is the shaded area under the line, • connecting the initial and final states.

39. Work Done By a Gas • To calculate the work done in a process, • some Year 10 knowledge is important. • Imagine the pressure is kept constant during a process.

40. Work Done By a Gas

41. Work Done By a Gas • If the gas expands slowly against the piston; • the work done to raise the piston is the force F multiplied by the distance d. • But the force is just the pressure P of the gas; • multiplied the area A of the piston, • F = PA.

42. Work Done By a Gas • W = Fd = PAd • W = PV = p(V2 – V1) • The sign of the work done depends on whether the gas expands or is compressed.

43. Work Done By a Gas • If the gas expands, • V is +ive and •  work is +ive. • The equation also is valid if the gas is compressed.

44. 1st Law of Thermodynamics • A long, long time ago; • heat was thought to be an invisible fluid, • called a caloric, • which flowed like water, • from hot objects, • to cold objects.

45. 1st Law of Thermodynamics • Caloric was conserved in its interactions which led to the discovery of the conservation of energy. • Within any system, • the less heat energy it has, • the more ordered is the motion of its molecules.

46. 1st Law of Thermodynamics • This can be seen in solids; • where the molecules all vibrate, • about a mean position. • As heat is added; • the more disorderly the motion until in a gas, • we say that all molecules, • move in random motion.

47. 1st Law of Thermodynamics • In a sense then, heat is the disordered energy of molecules. • There can be no heat in a single molecule. • Heat is a statistical concept; • applies only to a large number of molecules.

48. 1st Law of Thermodynamics • It is only when there is a great number of molecules • does the concept of random • or disorderly movement have meaning.

49. 1st Law of Thermodynamics • The discussion of heat, • internal energy and temperature. • Has given rise to the law of conservation of energy, • and when applied to thermal systems, • is often referred to as the, • First law of thermodynamics.

50. 1st Law of Thermodynamics • In a general form it is: • Whenever heat is added to a system, • it transforms to an equal amount of some other form of energy.