(455) Thermal energy basics [IB Physics SL/HL]

Molecular Theory

• Molecular theory posits that all states of matter (gases, particles, solids, liquids) are composed of molecules.

• Solids: Fixed shape and volume; molecules vibrate relative to each other.

• Liquids: No fixed shape but fixed volume; molecules vibrate and move freely.

• Gases: Neither fixed shape nor volume; molecules are free to move around.

Temperature Scales

• Common temperature scales include Fahrenheit, Celsius, and Kelvin.

• Celsius (°C):

  • 0°C: Freezing point of water.

  • 100°C: Boiling point of water.

• Kelvin (K):

  • Cannot experience negative values; 0 K is absolute zero.

  • 0°C is equivalent to 273 K.

  • Temperature changes (∆T) are equivalent in Celsius and Kelvin (e.g., an increase of 3°C is also an increase of 3 K).

Important Definitions

• Density (ρ): Mass per volume represented as

  • ρ = M/V

  • Units: kg/m³

• Average Kinetic Energy (E_k): Related to temperature through the equation:

  • E_k = (3/2)kT

  • Boltzmann constant (k) = 1.38 × 10⁻²³ J/K

  • Hotter objects have faster particles, colder objects have slower particles.

Heat and Energy

• Heat (Q): Transfer of energy, measured in joules.

  • Heat naturally transfers from hot to cold.

• Internal Energy (U): Total energy of a system.

  • U = Potential Energy (E_p) + Kinetic Energy (E_k)

  • Accounts for forces between molecules and their movement.

Summary

• Key topics covered include molecular theory, temperature scales (Celsius & Kelvin), density, average kinetic energy, heat, and internal energy.

• Understanding the relationship between temperature and kinetic energy is crucial in thermal energy studies.