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Chapter 10 Thermal Physics

Chapter 10 Thermal Physics. William Thomson (Lord Kelvin) (1824 - 1907). Temperature Thermodynamics – branch of physics studying thermal energy of systems Temperature ( T ), a scalar – measure of the thermal (internal) energy of a system SI unit: K (Kelvin)

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Chapter 10 Thermal Physics

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  1. Chapter 10 Thermal Physics

  2. William Thomson (Lord Kelvin)(1824 - 1907) • Temperature • Thermodynamics – branch of physics studying thermal energy of systems • Temperature (T), a scalar – measure of the thermal (internal) energy of a system • SI unit: K (Kelvin) • Kelvin scale has a lower limit (absolute • zero) and has no upper limit

  3. Kelvin scale • Kelvin scale is defined by the temperature of the triple point of pure water • Triple point – set of pressure and temperature values at which solid, liquid, and gas phases can coexist • International convention: • T of the triple point of water is

  4. The zeroth law of thermodynamics • If two (or more) bodies in contact don’t change their internal energy with time, they are in thermal equilibrium • 0th law of thermodynamics: if bodies are in thermal equilibrium, their temperatures are equal

  5. Measuring temperature • Temperature measurement principle: if bodies A and B are each in thermal equilibrium with a third body C, then A and B are in thermal equilibrium with each other (and their temperatures are equal) • The standard temperature for the Kelvin scale is measured by the constant-volume gas thermometer

  6. Constant-volume gas thermometer

  7. Anders Cornelius Celsius (1701 - 1744) Gabriel Daniel Fahrenheit (1686 - 1736) • Celsius and Fahrenheit scales • Celsius scale: • Fahrenheit scale:

  8. Chapter 10 Problem 3 Convert the following temperatures to their values on the Fahrenheit and Kelvin scales: (a) the boiling point of liquid hydrogen, –252.87°C; (b) the temperature of a room at 20°C.

  9. Thermal expansion • Thermal expansion: increase in size with an increase of a temperature • Linear expansion: • Volume expansion:

  10. Thermal expansion

  11. Chapter 10 Problem 14 A cube of solid aluminum has a volume of 1.00 m3 at 20°C. What temperature change is required to produce a 100-cm3 increase in the volume of the cube?

  12. Temperature and heat • Heat (Q): energy transferred between a system and its environment because of a temperature difference that exists between them • SI Unit: Joule • Alternative unit: calorie (cal):

  13. Amedeo Avogadro (1776 -1856) • Avogadro’s number • Mole – amount of substance containing a number of atoms (molecules) equal to the number of atoms in a 12 g sample of 12C • This number is known as Avogadro’s number (NA): • NA = 6.02 x 1023 mol -1 • The number of moles in a sample • N – total number of atoms (molecules) • m – total mass of a sample, m0 – mass of a single atom (molecule); M – molar mass

  14. Ludwig Eduard Boltzmann (1844-1906) • Ideal gases • Ideal gas – a gas obeying the ideal gas law: • R – gas constant • R = 8.31 J/mol ∙ K • kB – Boltzmann constant • kB = 1.38 x 1023 J/K

  15. Ideal gases • The gas under consideration is a pure substance • All molecules are identical • Macroscopic properties of a gas: P, V, T • The number of molecules in the gas is large, and the average separation between the molecules is large compared with their dimensions – the molecules occupy a negligible volume within the container • The molecules obey Newton’s laws of motion, but as a whole they move randomly (any molecule can move in any direction with any speed)

  16. Ideal gases • The molecules interact only by short-range forces during elastic collisions • The molecules make elastic collisions with the walls and these collisions lead to the macroscopic pressure on the walls of the container • At low pressures the behavior of molecular gases approximate that of ideal gases quite well

  17. Ideal gases

  18. Ideal gases • Root-mean-square (RMS) speed:

  19. Translational kinetic energy • Average translational kinetic energy: • At a given temperature, ideal gas molecules have the same average translational kinetic energy • Temperature is proportional to the average translational kinetic energy of a gas

  20. Internal energy • For the sample of n moles, the internal energy: • Internal energy of an ideal gas is a function of gas temperature only

  21. Chapter 10 Problem 30 A tank having a volume of 0.100 m3 contains helium gas at 150 atm. How many balloons can the tank blow up if each filled balloon is a sphere 0.300 m in diameter at an absolute pressure of 1.20 atm?

  22. James Clerk Maxwell (1831-1879) • Distribution of molecular speeds • Not all the molecules have the same speed • Maxwell’s speed distribution law: • NvΔv – fraction of molecules with speeds in the range from v to v + Δv

  23. Distribution of molecular speeds • Average speed: • RMS speed: • Most probable speed:

  24. Questions?

  25. Answers to the even-numbered problems • Chapter 10 • Problem 28 • 3.0 mol • (b) 1.80 × 1024 molecules

  26. Answers to the even-numbered problems Chapter 10 Problem 42 3.34 × 105 Pa

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