Theories of Heat

1. Theories of Heat

2. Kinetic Theory • all substances contain tiny, constantly moving particles

3. Thermal Energy • the sum of the kinetic energy of the random motion of the particles • average kinetic energy of particles is proportional to the temperature

4. Diffusion • matter can be subdivided • diffusion: the spreading of a substance through particle motion alone • much faster in gases than liquids

5. Diffusion • gas molecules move faster than liquid molecules • easily demonstrated with substances like ammonia and bromine vapor

6. Brownian Motion • affects microscopic particles • caused by random, asymmetrical collisions of liquid or gas molecules against the particles

7. Caloric Theory • claimed heat is a material fluid (caloric) that flows from hot bodies to cold bodies • evidence for the kinetic theory eventually destroyed this idea

8. Joule’s Research • studied conversion of mechanical energy to thermal energy • mechanical equivalent of heat

9. Joule’s Research • various experiments gave slightly different results • currently accepted value of the mechanical equivalent of thermal energy: 4.186 N·m = 1 cal (at 15°C)

10. Joule’s Research In his honor, the N·m was renamed the “joule,” the SI derived unit of energy, work, and heat.

11. Thermal Energy and Matter

12. Heat Capacity • It is not always the hottest object that has the greatest amount of thermal energy! • Heat Capacity (C): amount of thermal energy required to raise the temperature of entire object 1°C.

13. Qobject C = Δt Heat Capacity • Heat (Q): amount of thermal energy added to or taken from a system • SI unit: J/°C

14. Qobject C = Δt Heat Capacity • Δt = change in temperature • technically incorrect to say that a system has a certain amount of heat

15. Specific Heat • analogous to the specific density of a material • specific heat capacity = heat capacity per gram

16. Specific Heat • specific heat (csp) of a substance is the amount of thermal energy required to raise the temperature of 1 g of the substance by 1°C • SI units: J/g·°C

17. Specific Heat • specific heat of water: • 1 cal/g·°C (at 15°C) • 4.18 J/g·°C (near room temperature)

18. Q csp = mΔt Specific Heat • to calculate specific heat: • and by definition: C = m(csp)

19. Conservation • when an object gains heat, its surroundings lose that same amount of heat • heat-balance equations: Qsystem = -Qsurroundings Qsystem + Qsurroundings = 0 J

20. Conservation • adiabatic vessel: one that allows no heat to enter or leave its contents • calorimeter: container designed to minimize the exchange of thermal energy with its surroundings

21. Conservation • In computational work, it must be remembered to include the calorimeter’s gain or loss of heat.

22. Heat and Phase Transitions • an amount of heat is required to melt a solid or to vaporize a liquid • adding this heat will not change the temperature while melting or vaporizing occurs

23. Qmelt Lf = m Heat and Phase Transitions • latent heat of fusion (Lf): amount of thermal energy required to melt 1 kg of the substance at its melting point

24. Qboil Lv = m Heat and Phase Transitions • latent heat of vaporization (Lv): amount of thermal energy required to vaporize 1 kg of the substance at its boiling point

25. Heat and Phase Transitions • Example 15-6: Why are there five parts to this?

26. Mechanisms for Heat Transfer

27. Conduction • the flow of thermal energy from one object to another through contact • conductors: materials that conduct thermal energy easily

28. Conduction • conductors are more likely to have free electrons • insulators: materials that do not conduct thermal energy easily

29. Convection • the transfer of thermal energy from one place to another by the physical translation of particles between locations

30. Convection • most liquids and gases rise when heated • water has unusual properties

31. Radiation • travels without the use of an intervening medium • converted to thermal energy when absorbed by matter • all objects radiate thermal energy

32. Radiation • Stefan’s law gives the correspondence between absolute temperature (T) and the power of its radiant energy (S): S = σT4

33. Radiation • Stefan-Boltzmann constant: σ = 5.67 × 10-8 W/(m2·K4) • S is proportional to temperature to the fourth power S = σT4

34. Radiation • black objects absorb and radiate radiant energy better than other objects • blackbody: a perfect (ideal) radiator and absorber