Thermal Physics

• States of matter


  • Solids
  • Liquids
  • Gases
• Solids


  • Molecules are closely packed, regularly arranged and vibrate in fixed positions
  • Incompressible (No intermolecular spaces + Strong repulsive forces)
  • Fixed volume and shape
• Liquids

  • Molecules are loosely packed, irregularly arranged and slide over each other (move in clusters)
  • Incompressible (as molecules are touching and due to strong intermolecular repulsive forces)
  • take the shape of their container
  • Fixed volume
• Gases


  • Molecules are far apart, irregularly arranged and move Fast, Random and Free
  • Compressible
  • Shape and volume not fixed
• Evaporation


  1. Most energetic molecules on the surface of the liquid break down their bonds (Work Done against intermolecular forces) and escape as gas

  2. Leaves behind the least energetic molecules so the average KE of molecules decreases, so the temp decreases

  3. The evaporated liquid absorbs heat energy from the remaining liquid
• Evaporation is affected by the surface area / temperature / air current (draughts)
• Boiling point/Melting Point


  The temperature at which the liquid changes to gas (Boiling) or Solid to Liquid (Melting)
• Heat energy is used in


  • Breaking down bonds (WD against intermolecular forces)
  • Separating the molecules (molecules gain potential energy)
  • Changing the state (Liquid to gas in Boiling or Solid to liquid in Melting)
• Brownian Motion:

  Observations: Bright specks (Smoke particles/Pollen grains) move randomly, changing directions, zigzag motion, Haphazard, coming in focus and out of focus of the microscope
• Conclusion and explanation: Dust particles/pollen grains are being hit by air molecules/water molecules that are Fast, small, move randomly AND collisions happen in all directions
• Temperature


  The measure of the average kinetic energy of molecules
• Absolute zero = 0 K
• Relationship between °C and K

  T (°C) + 273 = T(K)
• Absolute zero


  The lowest possible temperature at which the gas stops moving (Pressure & KE = 0)
• How gases exert pressure

  1. Particles move randomly and freely, they collide with the walls of container

  2. Rebound Changing direction

  3. Change in momentum (AP)

  4. Exerting a force

  5. Producing pressure
• As Temperature increases


  Pressure increases (at constant Volume)
• As Temperature increases


  Volume increases (at constant Pressure)
• As Volume decreases


  Pressure increases (at constant Temperature)
• Pressure and volume


  Inversely proportional (at constant Temperature)
• Thermometric property


  A property of the thermometer that changes with the change in temperature
• Thermometric properties


  • Volume of gases (Thermal expansion)
  • Colour of some solids
  • Volume of liquids (Thermal expansion)
  • Resistance of electrical conductors
• Conduction


  • The transfer of energy due to contact (molecules are touching)
  • Metals depend on: Free moving electrons that gain thermal energy to travel through the metal and hit the distant molecules (transfers heat from high temp to low temp)
  • Lattice vibration: every molecule gains energy and vibrates to hit the neighboring molecules
  • Non metals depend only on lattice vibrations
• Convection


  As particles gain kinetic energy, they start to move more faster and further apart, so the volume of the liquid increases → the density decreases causing the water to rise and to be replaced with the cold water moving down (happens in liquids and gases)
• Radiation


  • The fastest type of heat energy transfer, as it depends on infrared radiation
  • Moves with the speed of light (3.0x108 m/s)
  • Occurs in all directions
  • Affected by: Colour of object (Matt black is the best absorber and emitter), Shiny/polished surfaces are used to insulate heat, Temperature of object, Surface area
• Specific heat capacity


  • The amount if heat energy needed to change the temperature of 1kg (1g) by 1°C (1K)
  • Units: J/Kg.ºC
• Heat capacity


  • The amount of heat energy required to change the temperature of a body by 1°C
  • Units: J/°C