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IB PHYSICS Topic 3: Thermal Physics
3.1 Thermal Concepts
Molecular Theory of Solids, Liquids, and Gasses
Molecules are held together by intermolecular forces.
Plasma state is a highly ionized, electrically conductive gas with charged ions and free electrons; it exhibits conductivity in high-temperature environments
Temperature and Absolute Temperature
Temperatures describe an object's hotness or coldness, determining heat flow direction.
Heat transfer: higher to lower temperature; termed "heat."
Thermal equilibrium: objects share the same temperature.
Kelvin temperature is calculated by adding 273.15 to the Celsius temperature.
Absolute temperature (Kelvin) is proportional to the average kinetic energy per molecule.
Absolute zero: 0K or -273°C, particles at zero average kinetic energy.
Internal Energy
Internal energy: sum of total kinetic and potential energy.
Kinetic energy is associated with molecular motions.
Potential energy is linked to intermolecular forces.
Specific Heat Capacity
Substance-specific heat capacity: heat needed to raise 1kg by 1K.
Different substances have distinct heat capacities due to varying properties.
Thermal capacity: Q = cθ (or Q = cΔT) - or the heat to raise the object's temperature by 1K.
Heat (Q): The unit of heat is typically measured in joules (J) in the International System of Units (SI).
Thermal capacity (c): The unit of thermal capacity is also joules per Kelvin (J/K) in the SI system.
Temperature change (θ or ΔT): The unit of temperature change is Kelvin (K) in the SI system.
Phase Change
Solid to Liquid (Melting)
Kinetic energy is unchanged; potential energy increases.
Liquid to Solid (Freezing)
Kinetic energy is unchanged; potential energy decreases.
Liquid to Gas (Boiling)
Kinetic energy is unchanged; potential energy increases.
Gas to Liquid (Condensation)
Kinetic energy is unchanged; potential energy decreases.
During phase change, temperature and kinetic energy remain constant, while potential energy changes.
Specific Latent Heat
Specific latent heat: heat needed for phase change without temperature change.
Latent Heat of Fusion
Heat to change 1kg from solid to liquid.
Latent Heat of Vaporization
Heat to change 1kg from liquid to gas.
3.2 Modeling a Gas
Pressure
Pressure: normal force per unit area.
Equation: P = F/A.
Pressure (P): The unit of pressure is measured in pascals (Pa) in the International System of Units (SI). One pascal is equivalent to one newton per square meter (N/m²).
Force (F): The unit of force is measured in newtons (N) in the SI system.
Area (A): The unit of area is measured in square meters (m²) in the SI system.
Equation of State for an Ideal Gas
Ideal gas equation: PV = nRT, where R = 8.31 (J/mol/K).
Pressure (P): The unit of pressure is typically measured in pascals (Pa) in the International System of Units (SI).
Volume (V): The unit of volume is typically measured in cubic meters (m³) in the SI system.
Amount of substance (n): The unit of amount of substance is measured in moles (mol).
Gas constant (R): The unit of the gas constant depends on the units used for pressure, volume, and temperature in the equation. In this case, R = 8.31 J/mol/K, where the unit of R is joules per mole per Kelvin (J/mol/K).
Temperature (T): The unit of temperature is measured in Kelvin (K) in the SI system.
Kinetic Model of an Ideal Gas
Assumptions:
Perfectly elastic molecule collisions.
Identical spherical molecules.
Negligible molecular volume compared to gas volume.
No interaction except during collisions.
Implications:
Absolute temperature is directly proportional to average kinetic energy and speed.
Mole, Molar Mass, and Avogadro Constant
Mole:
Unit of quantity, like a “dozen.”
1 mole = 6.022*1023 atoms or molecules (Avogadro’s constant).
Molar Mass:
Mass of 1 mole of any element or compound.
Avogadro’s Constant:
6.022*1023.
Differences Between Real and Ideal Gasses
Real gasses deviate from ideal assumptions.
Intermolecular forces exist in real gasses.
Molecular volume is not negligible.
Real gasses may resemble ideal gasses under high temperatures and low pressure.