Download Thermal Physics - Eastside Physics

Thermal Physics
• Thermal Physics is the study of temperature
and heat and how they effect matter.
• Heat leads to change in internal energy
which shows as a change of temperature
and is evident with the expansion or
contraction of matter
Temperature
• Temperature is the hotness or coldness of
matter
• Heat energy travels from a hot object to a
cold object
• If two objects are in contact thermal contact
energy can be exchanged between them
• The exchange of energy is called heat
Thermal Equilibrium
• Two objects are in thermal equilibrium if
they are in contact and no exchange of
energy takes place
• Zeroth Law of Thermodynamics states that
if object A and B are in thermal equilibrium
with object C then A and B are in thermal
equilibrium with each other.
• Two objects in thermal equilibrium have the
same temperature.
Thermometers
• A thermometer is a calibrated device to
measure temperature. They are much
smaller than the system so they can reach
equilibrium without great loss of energy
from the system.
Types of Thermometers
• Change in volume of liquid (Mercury)
• Length change of a solid
• Change of pressure of gas with constant
volume. (change of v with constant p)
• Electric resistance of a conductor
• Change of color of a hot object
Temperature Scales
• Kelvin calibrated using a gas thermometer
Absolute zero = 0 Kelvin = - 273.15c
Triple point of water is where ice, water and
water vapor coexist. At 0.01oc and 4.58 mm
Hg is used to establish Kelvin scale.
• Celcius scale TC = TK- 273.15
• Fahrenheit scale TF = 9/5TC + 32
Thermal expansion of solids and
liquids
• As the temperature of a substance increases
the volume increases. Thermal expansion
occurs due to a change in the average
separation of the constituent atoms or
molecules.
• Atoms in a solid a separated by an average
of 10-10m and vibrate. As temperature
increases so does the separation.
Linear Expansion
• Let Lo be the original length
 be the coefficient of linear expansion
ΔT be the change in temperature
Then ΔL =  Lo ΔT
• Coefficient are published values particular
to the type of material
Area Expansion
• Let the lengths of the sides be = L
then A = L2 let Ao = original area
ΔA = Ao ΔT
•  is the coefficient of area expansion
Volume Expansion
• Similar to both length and area expansion
volume expansion can be shown as
Δv = vo ΔT
 is the coefficient of volume expansion
• Note that  = 2  and  = 3 
• Liquids generally have volume coefficients
ten times greater than solids
Ideal Gas
• An ideal gas is one that has atoms or
molecules that move randomly and have no
long range forces on each other. Each
particle is like a point.
• 1 Mole of gas has 6.02*1023 particles
• 1 mole of gas occupies 22.4 liters
Ideal Gas Equation
• Pv = nRT
• R is the ideal gas constant
R= 8.31 when using Pa and m3
R= 0.0821 when using atmospheres and
liters
Kinetic Theory of Gases
• The number of atoms/ molecules in a gas
are large and the average separation is great
compared to their size
• particles obey Newton’s laws of motion and
move randomly
• Particles interact only through short range
forces having elastic collision, including
walls
• All molecules in a gas are identical
Boltzmann’s Constant
• From Pv = nRT
you get Pv = kBRT where kB = n/NA
NA = Avogadro’s number = 6.02 * 1023
Force on Container Walls
• F = N/3(mv2/d)
• where N = number of particles
m = mass of one particle
v = the average speed of the particles
d = the length of the edge of the container
• Total pressure on the walls of the container
• P = 2/3(N/vc)(1/2mv2) vc= container volume
Molecular Interpretation of
Temperature
• Temperature of a gas is a direct measure of
the average molecular kinetic energy of the
gas particles. 1/2mv2 = 3/2kBT
• Total translational kinetic energy of N
particles KEtotal = N(1/2mv2) = 3/2NkBT
• For monatomic gases translational KE is the
only type of energy the particles have.
• Where U = 3/2nRT
Root-Mean-Square
• Diatomic and polyatomic gases have
additional energies due to vibration and
rotation. Their average velocity is calculated
from
• vrms = (3RT/M)
• m = molar mass in kg per mole