Thermal PhysicsThermal 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
• A device calibrated to measure the temperature (not heat) of an object
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.
Temperature
• Defn: the property that determines whether or not an object is in thermal equilibrium with other objects
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.
Thermal Expansion
• Defn: as temperature increases, volume increases
• Ex. Useful in building designs, concrete highways, and bridges
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.
Defining the Variables• L0 initial length along some direction at
some temperature L increase in length change in temperature coefficient of linear expansion for a
given material and has untis of 0C-1
L = L0T
Area Expansion
= coefficient of area expansion• A = area A = A-A0 = A0T
Coefficient of Volume Expansion
= coefficient of volume expansion V = V0T
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.
Kinetic Molecular Theory of Gases1. A gas consists of small particles (atoms/molecules) that
move randomly with rapid velocities2. The attractive forces between particles of a gas can be
neglected3. The actual volume occupied by a gas molecule is extremely
small compared to the volume that gas occupies.4. The average kinetic energy of a gas molecule is
proportional to Kelvin temperature5. Gas particles are in constant motion, moving rapidly in
straight paths.
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
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
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
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
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
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
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
Ideal Gas Law Using Boltzmann’s Constant
• PV= NkBT• N = total number of molecules• kB = 1.38 x 10-23 J/K
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
Internal Energy, U, for a monatomic gas
• U = 3/2(nRT)• Again, temperature must be in Kelvin
Root-mean-square (rms) speed
• Vrms = square root of (3kBT/m) or • = square root of (3RT/M)• M is molar mass in kg/molThese speeds can be found on Table 10.2 on p. 324