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Thermal Physics

Thermal Physics. Chapter 10. Thermal Physics. Thermal physics looks at temperature, heat, and internal energy Heat and temperature are not the same thing although we use them interchangeably in our everyday language. Thermometer.

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Thermal Physics

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  1. Thermal Physics Chapter 10

  2. Thermal Physics • Thermal physics looks at temperature, heat, and internal energy • Heat and temperature are not the same thing although we use them interchangeably in our everyday language

  3. Thermometer • A device calibrated to measure the temperature (not heat) of an object

  4. Zeroth Law of Thermodynamics • AKA the Law of Equilibrium • If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other.

  5. Temperature • Defn: the property that determines whether or not an object is in thermal equilibrium with other objects

  6. Temperature • Temperature is a measure of the intensity of heat or how hot a system is regardless of size. • Kelvin is the official metric unit • To convert degrees Celsius to Kelvin by adding 273. • To convert Kelvin to degrees Celsius subtract 273.

  7. Thermal Expansion • Defn: as temperature increases, volume increases • Ex. Useful in building designs, concrete highways, and bridges

  8. Expressing Thermal Expansion • If the thermal expansion of an object is sufficiently small compared with the object’s initial dimensions, then the change in any dimension is proportional to the first power of the temperature change.

  9. Defining the Variables • L0 initial length along some direction at some temperature • DL increase in length • DT change in temperature • a = coefficient of linear expansion for a given material and has untis of 0C-1 • DL = aL0DT

  10. Area Expansion • g= coefficient of area expansion • A = area • DA = A-A0 = gA0DT

  11. Coefficient of Volume Expansion • b= coefficient of volume expansion • DV = bV0DT

  12. Application • Why would a glass break if it hot liquid is poured into it too quickly? • You have a metallic lid stuck on a glass jar. Describe how you would loosen it without any tools.

  13. Kinetic Molecular Theory of Gases • A gas consists of small particles (atoms/molecules) that move randomly with rapid velocities • The attractive forces between particles of a gas can be neglected • The actual volume occupied by a gas molecule is extremely small compared to the volume that gas occupies. • The average kinetic energy of a gas molecule is proportional to Kelvin temperature • Gas particles are in constant motion, moving rapidly in straight paths.

  14. Properties of a gas • Pressure: kPa, atm, mm of Hg, torr • Conversion 760 mm Hg = 760 torr = 1 atm= 101.3 kPa • Volume: L, mL or cm3 • Conversions 1000 mL = 1L • 1 mL = 1 cm • Temperature: 0C or K • Conversions 0C + 273 = K or 0C = K -273

  15. Boyle’s Law: • Pressure and volume are inversely proportional • As pressure increases volume decreases • As pressure decreases volume increases • Pressure and Volume units must be the same on both sides • P1V1 = P2V2

  16. Charles’ Law: • Temperature and Volume are directly proportional • As temperature increases volume increases • As temperature decreases volume decreases • Temperature must be in Kelvin (add 273) • Volume units must be consistent on both sides • V1 / T1 = V2/T2

  17. Gay-Lussac’s Law: • Pressure and Temperature are directly proportional • As pressure increases temperature increases • As pressure decreases temperature decreases • Pressure units must be consistent on both sides • Temperature units must be in Kelvin (add 273) • P1/T1 = P2/T2

  18. Combined Gas Law : P1V1= P2V2 T1 T2 • Pressure and volume and temperature vary according to this equation when all three change • Temperature must be in Kelvin

  19. The Mole and Avagadro’s Law • Avagadro’s Law: V1 / n1 = V2/n2 • n = number of moles, moles are large quantities of very small objects like molecules of a gas • STP = Standard Temperature and Pressure 00C or 273 K and 1 atm • Molar volume: 22.4 L

  20. Ideal Gas Law: (Eqn. of State) • PV = nRT • R is the universal gas constant and varies depending on which unit is used for measuring pressure • R = 0.0821 L x atm. /mol • or if using kPa R = 8.31 J/mol x K

  21. Ideal Gas Law Using Boltzmann’s Constant • PV= NkBT • N = total number of molecules • kB = 1.38 x 10-23 J/K

  22. Molecular Model of an Ideal Gas • The pressure is proportional to the number of molecules per unit volume and the average translational kinetic energy of a molecule • Temperature of a a gas is a direct measure of average molecular kinetic energy

  23. Internal Energy, U, for a monatomic gas • U = 3/2(nRT) • Again, temperature must be in Kelvin

  24. Root-mean-square (rms) speed • Vrms = square root of (3kBT/m) or • = square root of (3RT/M) • M is molar mass in kg/mol These speeds can be found on Table 10.2 on p. 324

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