1 Thermal Physics Chapter 10

2 Thermodynamics Concerned with the concepts of energy transfers between a system and its environment and the resulting temperature variations Concerned with the concepts of energy transfers between a system and its environment and the resulting temperature variations Historically, the development of thermodynamics paralleled the development of atomic theory Historically, the development of thermodynamics paralleled the development of atomic theory Concerns itself with the physical and chemical transformations of matter in all of its forms: solid, liquid, and gas Concerns itself with the physical and chemical transformations of matter in all of its forms: solid, liquid, and gas

3 Heat The process by which energy is exchanged between objects because of temperature differences is called heat The process by which energy is exchanged between objects because of temperature differences is called heat Objects are in thermal contact if energy can be exchanged between them Objects are in thermal contact if energy can be exchanged between them Thermal equilibrium exists when two objects in thermal contact with each other cease to exchange energy Thermal equilibrium exists when two objects in thermal contact with each other cease to exchange energy

4 Zeroth Law of Thermodynamics If objects A and B are in thermal equilibrium with a third object, C, then A and B are in thermal contact with each other. Allows a definition of temperature

5 Temperature from the Zeroth Law of Thermodynamics Two objects in thermal equilibrium with each other are at the same temperature Two objects in thermal equilibrium with each other are at the same temperature Temperature is the property that determines whether or not an object is in thermal equilibrium with other objects Temperature is the property that determines whether or not an object is in thermal equilibrium with other objects

6 Thermometers Thermometers are devices used to measure the temperature of an object or a system Thermometers are devices used to measure the temperature of an object or a system Mercury thermometer is an example of a common thermometer Mercury thermometer is an example of a common thermometer

7 Thermometers Make use of physical properties that change with temperature Make use of physical properties that change with temperature Many physical properties can be used Many physical properties can be used volume of a liquid volume of a liquid length of a solid length of a solid pressure of a gas held at constant volume pressure of a gas held at constant volume volume of a gas held at constant pressure volume of a gas held at constant pressure electric resistance of a conductor electric resistance of a conductor color of a very hot object color of a very hot object

8 Temperature Scales Thermometers can be calibrated by placing them in thermal contact with an environment that remains at constant temperature Thermometers can be calibrated by placing them in thermal contact with an environment that remains at constant temperature Environment could be mixture of ice and water in thermal equilibrium Environment could be mixture of ice and water in thermal equilibrium Also commonly used is water and steam in thermal equilibrium Also commonly used is water and steam in thermal equilibrium

9 Celsius Scale Temperature of an ice-water mixture is defined as 0º C Temperature of an ice-water mixture is defined as 0º C This is the freezing point of water This is the freezing point of water Temperature of a water-steam mixture is defined as 100º C Temperature of a water-steam mixture is defined as 100º C This is the boiling point of water This is the boiling point of water Distance between these points is divided into 100 segments Distance between these points is divided into 100 segments

10 Kelvin Scale When the pressure of a gas goes to zero, its temperature is –273.15º C When the pressure of a gas goes to zero, its temperature is –273.15º C This temperature is called absolute zero This temperature is called absolute zero This is the zero point of the Kelvin scale This is the zero point of the Kelvin scale –273.15º C = 0 K –273.15º C = 0 K To convert: T C = T K – To convert: T C = T K –

11 Gas Thermometer Temperature readings are nearly independent of the gas Temperature readings are nearly independent of the gas Pressure varies with temperature when maintaining a constant volume Pressure varies with temperature when maintaining a constant volume

12 Pressure-Temperature Graph All gases extrapolate to the same temperature at zero pressure All gases extrapolate to the same temperature at zero pressure This temperature is absolute zero This temperature is absolute zero

13 Modern Definition of Kelvin Scale Defined in terms of two points Defined in terms of two points Agreed upon by International Committee on Weights and Measures in 1954 Agreed upon by International Committee on Weights and Measures in 1954 First point is absolute zero First point is absolute zero Second point is the triple point of water Second point is the triple point of water Triple point is the single point where water can exist as solid, liquid, and gas Triple point is the single point where water can exist as solid, liquid, and gas Single temperature and pressure Single temperature and pressure Occurs at 0.01º C and P = 4.58 mm Hg Occurs at 0.01º C and P = 4.58 mm Hg

14 Some Kelvin Temperatures Some representative Kelvin temperatures Some representative Kelvin temperatures Note, this scale is logarithmic Note, this scale is logarithmic Absolute zero has never been reached Absolute zero has never been reached

15 Fahrenheit Scales Most common scale used in the US Most common scale used in the US Temperature of the freezing point is 32º Temperature of the freezing point is 32º Temperature of the boiling point is 212º Temperature of the boiling point is 212º 180 divisions between the points 180 divisions between the points

16 Comparing Temperature Scales

17 Converting Among Temperature Scales

18 Thermal Expansion The thermal expansion of an object is a consequence of the change in the average separation between its constituent atoms or molecules The thermal expansion of an object is a consequence of the change in the average separation between its constituent atoms or molecules At ordinary temperatures, molecules vibrate with a small amplitude At ordinary temperatures, molecules vibrate with a small amplitude As temperature increases, the amplitude increases As temperature increases, the amplitude increases This causes the overall object as a whole to expand This causes the overall object as a whole to expand

19 Linear Expansion For small changes in temperature For small changes in temperature The coefficient of linear expansion,, depends on the material The coefficient of linear expansion,, depends on the material See table 10.1 See table 10.1 These are average coefficients, they can vary somewhat with temperature These are average coefficients, they can vary somewhat with temperature

20 Area Expansion Two dimensions expand Two dimensions expand

21 Volume Expansion Three dimensions expand Three dimensions expand For liquids, the coefficient of volume expansion is given in the table For liquids, the coefficient of volume expansion is given in the table

22 Applications of Thermal Expansion – Bimetallic Strip Thermostats Use a bimetallic strip Two metals expand differently

23 Unusual Behavior of Water At the temperature of water increases from 0ºC to 4 ºC, it contracts and its density increases Above 4 ºC, water exhibits the expected expansion with increasing temperature Maximum density of water is 1000 kg/m 3 at 4 ºC

24 Ideal Gas A gas does not have a fixed volume or pressure A gas does not have a fixed volume or pressure In a container, the gas expands to fill the container In a container, the gas expands to fill the container Most gases at room temperature and pressure behave approximately as an ideal gas Most gases at room temperature and pressure behave approximately as an ideal gas

25 Characteristics of an Ideal Gas Collection of atoms or molecules that move randomly Collection of atoms or molecules that move randomly Exert no long-range force on one another Exert no long-range force on one another Occupy a negligible fraction of the volume of their container Occupy a negligible fraction of the volume of their container

26 Moles It’s convenient to express the amount of gas in a given volume in terms of the number of moles, n It’s convenient to express the amount of gas in a given volume in terms of the number of moles, n One mole is the amount of the substance that contains as many particles as there are atoms in 12 g of carbon-12 One mole is the amount of the substance that contains as many particles as there are atoms in 12 g of carbon-12

27 Avogadro’s Number The number of particles in a mole is called Avogadro’s Number The number of particles in a mole is called Avogadro’s Number N A =6.02 x particles / mole N A =6.02 x particles / mole The mass of an individual atom can be calculated: The mass of an individual atom can be calculated:

28 Equation of State for an Ideal Gas Boyle’s Law Boyle’s Law At a constant temperature, pressure is inversely proportional to the volume At a constant temperature, pressure is inversely proportional to the volume Charles’ Law Charles’ Law At a constant pressure, the temperature is directly proportional to the volume At a constant pressure, the temperature is directly proportional to the volume Gay-Lussac’s Law Gay-Lussac’s Law At a constant volume, the pressure is directly proportional to the temperature At a constant volume, the pressure is directly proportional to the temperature

29 Ideal Gas Law Summarizes Boyle’s Law, Charles’ Law, and Guy-Lussac’s Law Summarizes Boyle’s Law, Charles’ Law, and Guy-Lussac’s Law PV = n R T PV = n R T R is the Universal Gas Constant R is the Universal Gas Constant R = 8.31 J / mole K R = 8.31 J / mole K R = L atm / mole K R = L atm / mole K

30 Avogadro’s Hypothesis Equal volumes of gas at the same temperature and pressure contain the same numbers of molecules Equal volumes of gas at the same temperature and pressure contain the same numbers of molecules Corollary: At standard temperature and pressure, one mole quantities of all gases contain the same number of molecules Corollary: At standard temperature and pressure, one mole quantities of all gases contain the same number of molecules This number is N A This number is N A Can also look at the total number of particles: N = n N A Can also look at the total number of particles: N = n N A

31 Ideal Gas Law revisited P V = N k B T P V = N k B T k B is Boltzmann’s Constant k B is Boltzmann’s Constant k B = R / N A = 1.38 x J/ K k B = R / N A = 1.38 x J/ K

32 Kinetic Theory of Gases -- Assumptions The number of molecules in the gas is large and the average separation between them is large compared to their dimensions The number of molecules in the gas is large and the average separation between them is large compared to their dimensions The molecules obey Newton’s laws of motion, but as a whole they move randomly The molecules obey Newton’s laws of motion, but as a whole they move randomly

33 Kinetic Theory of Gases – Assumptions, cont. The molecules interact only by short- range forces during elastic collisions The molecules interact only by short- range forces during elastic collisions The molecules make elastic collisions with the walls The molecules make elastic collisions with the walls The gas under consideration is a pure substance, all the molecules are identical The gas under consideration is a pure substance, all the molecules are identical

34 Pressure of an Ideal Gas The pressure is proportional to the number of molecules per unit volume and to the average translational kinetic energy of a molecule The pressure is proportional to the number of molecules per unit volume and to the average translational kinetic energy of a molecule

35 Molecular Interpretation of Temperature Temperature is proportional to the average kinetic energy of the molecules Temperature is proportional to the average kinetic energy of the molecules The total kinetic energy is proportional to the absolute temperature The total kinetic energy is proportional to the absolute temperature

36 Internal Energy In a monatomic gas, the KE is the only type of energy the molecules can have In a monatomic gas, the KE is the only type of energy the molecules can have U is the internal energy of the gas U is the internal energy of the gas In a polyatomic gas, additional possibilities for contributions to the internal energy are rotational and vibrational energy in the molecules In a polyatomic gas, additional possibilities for contributions to the internal energy are rotational and vibrational energy in the molecules

37 Speed of the Molecules Expressed as the root-mean-square (rms) speed Expressed as the root-mean-square (rms) speed At a given temperature, lighter molecules move faster, on average, than heavier ones At a given temperature, lighter molecules move faster, on average, than heavier ones Lighter molecules can more easily reach escape speed from the earth Lighter molecules can more easily reach escape speed from the earth