Thermodynamics

1. Thermodynamics

2. A few reminders TEMPERATURE determines the direction of flow of thermal energy between two bodies in thermal equilibrium HOT COLD

3. A few reminders TEMPERATURE is also a measure of the average kinetic energy of particles in a substance

4. A few reminders INTERNAL ENERGY is the sum of the kinetic energy and potential energies of particles in a substance K.E. + P.E.

5. Internal energy The sum of the KE and PE of the particles in a system NOTE, THIS IS NOT THE SAME AS THE TOTAL ENERGY.

6. A few reminders In an ideal gas, the INTERNAL ENERGY is all kinetic energy.

7. What is THERMODYNAMICS? A study of the connection between thermal energy entering or leaving a system and the work done on or by the system.

8. A few words to consider

9. Thermodynamic system The system/machine that we are considering the flow of heat energy in/out of and work done on/by the system.

10. The surroundings Everything else!

11. Heat The quantity of heat/thermal energy (transferred by a temperature difference).

12. Work The energy transferred (changed) E.g. Work = Force x distance or Work = VIt

13. Example Finding the work done on or by a gas when it expands at constant pressure (i.e. a small change in volume!) (most of the systems we consider will involve the compression or expansion of gases under different conditions)

14. Work done by a gas (constant pressure) Work = force x distance Work = force x Δx (Pressure = F/A so F = PA) Work = PAΔx (AΔx = ΔV) Work = pΔV A P P Δx

15. The 1st law of thermodynamics Q = ΔU + W

16. The 1st law of thermodynamics Q = ΔU + W Q = The thermal energy given to a system (if this is negative, thermal energy is leaving the system)

17. The 1st law of thermodynamics Q = ΔU + W ΔU = The increase in internal energy (if this is negative the internal energy is decreasing)

18. The 1st law of thermodynamics Q = ΔU + W W = The work done on the surroundings (if this is negative the surroundings are doing work on the system)

19. The 1st law of thermodynamics Q = ΔU + W This is really just another form of the principle of energy conservation

20. Ideal gas processes • In most cases we will be considering changes to an ideal gas (this will be the “system)

21. p V pV diagrams and work done Changes that happen during a thermodynamic process can usefully be shown on a pV diagram

22. pV diagrams and work done The area under the graph represents the work done A p This area represents the work done by the gas (on the surroundings) when it expands from state A to state B B V What happens if the gas is going from state B to A?

23. p V ISOCHORIC (isovolumetric) processes These take place at constant volume V = constant, so p/T = constant Q = negative ΔU = negative W = zero A B Isochoric decrease in pressure

24. p V ISOBARIC processes These take place at constant pressure p = constant, so V/T = constant Q = positive ΔU = positive W = positive A B Isobaric expansion

25. p V ISOTHERMAL processes These take place at constant temperature T = constant, so pV = constant Q = positive ΔU = zero W = positive A B Isothermal expansion

26. p V ADIABATIC processes No thermal energy transfer with the surroundings (approximately a rapid expansion or contraction) Q = zero ΔU = negative W = positive A B Adiabatic expansion

27. Heat engines and heat pumps A heat engine is any device that uses a source of heat energy to do work. Examples include the internal combustion engine of a car.

28. Heat engine “Reservoir” implies a constant heat source Below is a generalised diagram showing the essential parts of any heat engine. Work done ΔW Hot reservoir Thot Cold reservoir Tcold Engine Thermal energy Qhot Thermal energy Qcold

29. p V A simple example of using an ideal gas in a heat engine Heat in ΔU = (3/2)nRΔT Isobaric expansion Heat in A B Area = work done by gas Isovolumetric decrease in pressure Isovolumetric increase in pressure D C Isobaric compression Heat out Heat out

30. Let’s read! • Page 191 to 192 “An example of a heat engine”

31. Heat pump Simply a heat engine run in reverse! (Put work in, transfer heat from cold reservoir to hot reservoir) Input work ΔW Hot reservoir Thot Cold reservoir Tcold Engine Thermal energy Qhot Thermal energy Qcold

32. p V Heat pump Heat out Heat out Isobaric compression Area = work done on gas Isovolumetric increase in pressure Isovolumetric decrease in pressure Isobaric expansion Heat in Heat in

33. Questions • Page 193 • Questions 1 to 5 • Page 194 • Questions 10

34. 2nd Law of Thermodynamics and entropy There are many ways of stating the 2nd law, below is the Kelvin-Planck formulation “No heat engine, operating over a cycle, can take in heat from its surroundings and totally convert it totally into work” (some heat has to be transferred to the cold reservoir) This is possible in a single process however

35. 2nd Law of Thermodynamics and entropy Other statements of the 2nd law include • No heat pump can transfer thermal energy from a low temperature to a higher temperature reservoir without work being done on it (Clausius) • The entropy of the universe can never decrease

36. Entropy • This is a measure of the disorder of a system • Most systems, when left, tend towards more disorder (think of your bedroom! • This is why heat spreads from hot to cold. • Entropy can decrease in a small part of a system

37. Entropy Decrease in entropy = Q/Thot Increase in entropy = Q/Tcold Thot Tcold ΔQ

38. 1st and 2nd laws • These laws MUST apply in all situations • A refrigerator does transfer heat from cold to hot, but work must be done (electricity supplied and some converted into heat) to do this • A boat could use the temperature difference between the sea and atmosphere to run, but eventually the two reservoirs would reach the same temperature

39. Degradation The more spread energy becomes, the less useful it is. The heat produced in the brakes of a car is still energy, but not really in a useful form. We call this energy degradation

40. That’s it!

41. Now let’s try some questions Page 193 Questions 1 to 5 Page 194 Questions 10 to 13. Let’s also have a test on 4th November