heat and temperature matter is made of atoms electron microscope photo of germanium atoms
TRANSCRIPT
Heat and Temperature
Matter is made of Atoms
Electron Microscope Photo of Germanium Atoms
Atoms are in Constant Motion
Temperature
• Measure of how hot or cold an object is
• Measured by thermometers
• Work by expansion of a liquid
• Other types use bimetallic strip
Temperature Scales
• Fahrenheit T(0F) = 9/5T(0C) + 32
• Celsius (centigrade) T(0C ) = 5/9[T(0F) –32]
(degree is 9/5 that of Fahrenheit)
• Kelvin (Celsius + 273)
Examples
• Zero degrees Celsius is what Kelvin?
Answer: 273o
• What is the boiling point of water in degrees Kelvin?
Answer: 373o
• 200 degrees Celsius is what in Kelvin?
Answer: 473o
Zeroth Law of Thermodynamics
• If two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other
• Thermal equilibrium occurs when objects in contact are at same temperature and no energy flows between them
AB C
Thermal Expansion
• Nearly all substances expand when heated and contract when cooled
• Exception- water below 4 0C• For solids change in length is proportional
to length and change in temperature• L = L0T is coefficient of linear expansion,
different for different substances
Metals Expand the Most
• In solid object all sections expand with increased temperature
• Aluminum expands more than iron or brass• Engine pistons are made of aluminum,
cylinders of iron• Overheating engine ruins (scuffs) piston• Bridge and sidewalk sections must be spaced
Expansion Example
• Aluminum has a coefficient of linear expansion of 25 x 10-6 An aluminum beam 3.0 m in length is heated from 200C to 800C. What is the increase in length?
L = L0T =
25 x 10-6 x 3.0 x 60 = 4.5 x 10-3 m = 4.5 mm
Bi-metallic Strip demo
Ideal Gases
• When pressure is less than 1 atm and gas is not near liquifaction temperature
V goes as/p (inverse proportion)
V goes asT (direct proportion)
Gay-Lussac’s Law
• At constant volume, the pressure of a gas is proportional to the absolute (Kelvin) temperature
• Example: What would happen if you throw a closed aerosol can into a fire?
Ideal Gas Law
• Combines laws of Boyle, Charles and Gay-Lussac• PV = nRT (equation of state for ideal gas)• n = number of moles of gas• R is gas constant = 8.315 J/(mol K)
or 0.0821 (L atm)/(mol K)• Pressure in Pascals (Pa) or atmospheres• 1 atm = 1.013 x 105 Pa
PV = nRT• Memorize
• Make sure you understand what P, V, n, R, and T are
Example
• One mole of hydrogen fills a pressure bottle one liter in volume at room temperature (200 C). What is the absolute pressure(in Pa and atm)?
P = nRT/V =
8.315 J/(mol K) (273 + 20) /10-3 m3 =
2.4 x 106 N/m2 = 24 atm
Mole
• Amount of substance that contains as many atoms or molecules as there are in 12 grams of Carbon 12
• That number of grams of a substance numerically equal to the molecular mass of the substance
What is the Molecular Mass?
• How many grams per mole?
• H2
• O2
• H2O
• CO2
• He
Volume of One Mole at STP
• Standard temp = 00C = 273 0K• Standard P = 1.00 atm = 1.013 x 105 N/m2
• V = nRT/P = (1.00 mol)(8.315 J/molK)(273 K)
(1.013 x 105 N/m2)
= 22.4 x 10-3 m3 = 22.4 liters
Remember: 103 liters = 1 cubic meter
Use degrees Kelvin (Celsius + 273)
Another Example
• A 200 liter tank contains hydrogen gas at room temperature (200 C) and absolute pressure of 5 atmospheres. How many moles and grams of hydrogen are in the tank?
• Hint n = PV/RT
5atm x 1.013 x 105 Pa x 200 x 10-3 / 8.315 / 293 = 41.6 moles = 83 grams
Alternate form of Ideal Gas Law
• P1V1/T1 = P2V2/T2
• Use to solve problems where one of these six variables is unknown
• Isolate that one and
plug in given information
Ideal Gas Law in Terms of Molecules
• PV = NkT = nRT• N = number of molecules in sample• NA = Avogadro’s number = number of
molecules in a mole = 6.02 x 1023 mol-1
• k = R/NA = Boltzmann’s constant = 1.38 x 10-23 J/K = gas constant per
molecule
N/NA = n
Nk = nR• Memorize and know how to derive
Assumptions of Kinetic Theory
• In ideal gas large numbers of molecules move with varying speeds in random directions
• Average molecules are much further apart than their size
• Molecules interact only when they collide
• Collisions are assumed perfectly elastic
Molecules in a Box
Consequences of Kinetic Theory
• The average translational kinetic energy of molecules in a gas is proportional to the absolute temperature
• KEav = ½ m (v2 ) = (3/2) kT– k is Boltzmann’s constant = 1.38 x 10-23 J/K
• vrms = ( 3kT/m)1/2 not equal to vav
Question: If you double the rms speed of molecules in a gas, what happens to the temperature?
Example
• What is the rms speed of an oxygen molecule at room temperature?
vrms2 = 3 x 1.38x 10-23 x 293 / (32 x 1.67 x 10-27)
vrms = 480 m/s
1 amu = 1.66 x 10-27 kg proton mass = 1.67 x 10-27 kg
Distribution of Molecular Speeds
Courtesy Hyperphysics, Georgia State University
Kinetic Theory Simulations
• http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html