13 Temperature and Ideal Gases

13 Temperature and Ideal Gases • Homework: • Problems: 1, 7, 41. • Thermal Equilibrium • Temperature Scales • Ideal Gases • Thermal Expansion

Temperature T ~ avg. KE/molecule Thermal Expansion Scales: Kelvin, K C° = K – 273 F° = (9/5)C° + 32 2

Thermal Equilibrium • Heat flows from hotter object to cooler object. • When the heat flow ceases the objects are in thermal equilibrium. • Objects in thermal equilibrium are at the same temperature. • /

Ideal Gas

Gas Thermometer • PV ~ NT • P ~ T (V, N constant) • Gas cools, • avg. KE  0, • (absolute zero), • P  0, • ≈ -273 °C

Constant Pressure • What % increase in V occurs for an ideal gas heated from 20C to 40C? (V ~ T) • (It does not double, b/c C is not a thermodynamic temperature scale) • V2/V1 = T2/T1 = (273+40)/(273+20) = 1.068 • 6.8% increase in volume.

Linear Thermal Expansion DL = a LoDT Example: 100C increase in Aluminum causes a fractional increase in length of 0.0024 = 0.24% change.

Bi-Metallic Strips

Summary • Thermal Equilibrium • Temperature Scales • Ideal Gases • Thermal Expansion

Water Expansion Expansion from 4°Cto 100°C (normal) Contraction from 0°C to 4°C. (anomalous, transient ice melting) 10 10

Superheating Mythbusters 12 12

Ideal Gases • N molecules (few intermolecular collisions) • v = average speed • P due to wall-collisions (P ~ Nv/t) • t = time between same-wall collision

Ideal gas pressure

Ideal Gas Law

13 Temperature and Ideal Gases