CC

Thermal Energy Concepts

Molecular Theory and States of Matter

  • Solids:
    • Fixed shape and fixed volume.
    • Molecules vibrate with respect to each other.
  • Liquids:
    • No fixed shape but fixed volume.
    • Molecules vibrate.
  • Gases:
    • No fixed shape and no fixed volume.
    • Molecules bounce around.

Visual Representation using PHET Simulations

  • Neon (Ne):
    • Demonstrates solid, liquid, and gas states.
    • Molecules vibrate in all states.
  • Water (H2O):
    • Solid (ice), liquid (water), and gas (steam) states.

Temperature Scales: Celsius and Kelvin

  • Celsius (°C):
    • 0°C: Freezing point of water.
    • 100°C: Boiling point of water.
  • Kelvin (K):
    • 0 K: Absolute zero (theoretically the coldest temperature).
    • 0°C = 273 K

Kelvin-Celsius Conversion

  • Equation (from data booklet):
    • T(K) = T(°C) + 273
  • Exam Tip:
    • Change in temperature (\Delta T) is the same in Celsius and Kelvin.
    • Use Kelvin for actual temperature (T) calculations.

Density

  • Definition:
    • \rho = \frac{m}{V}, where \rho is density, m is mass, and V is volume.
  • Units:
    • Mass (m): kilograms (kg).
    • Volume (V): meters cubed (m^3).
    • Density ($\rho$): kilograms per meter cubed (kg/m^3).

Temperature Definition

  • Temperature: Average kinetic energy of the molecules.
  • Kinetic Energy:
    • Ek = \frac{1}{2}mv^2, where Ek is kinetic energy, m is mass, and v is velocity.
  • Relationship:
    • Hotter: Faster moving molecules.
    • Cooler: Slower moving molecules.
    • Same Temperature: Same average kinetic energy and speed.

Average Kinetic Energy Equation

  • Equation:
    • Ek = \frac{3}{2}kB T, where Ek is average kinetic energy, kB is Boltzmann constant, and T is temperature in Kelvin.
  • Boltzmann's Constant (k_B):
    • k_B = 1.38 \times 10^{-23} J/K (from data booklet).
    • Units: Joules per Kelvin (J/K).

Animation Reinforcement

  • Cooling a gas:
    • Temperature decreases.
    • Particles move slower.
    • At near zero Kelvin, particles move very slowly.

Heat

  • Definition: Transfer of energy between a system and its surroundings.
  • Variable: q
  • Units: Joules (J)
  • Direction: Flows naturally from hot to cold.
  • Analogy: If air conditioning is on a hot air will go to the cold air.

Internal Energy

  • Definition: Total sum of intermolecular potential energy and kinetic energy due to random motion.
  • Variable: U
  • Units: Joules (J).
  • Components:
    • Kinetic Energy (due to motion).
    • Potential Energy (due to intermolecular forces).
  • Equation (conceptual):
    • U = KE + PE