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