Thermal Physics

• Thermal expansion of solids

  • Particles within the solid gain more kinetic energy as the temperature goes up, and therefore vibrate more vigorously and gain more separation from neighboring particles
  • Expansion is usually too small to see with the naked eye
• Thermal expansion of railway tracks

  Can cause misalignment problems
• Railway tracks

  Made with small gaps between each rail to account for the slight expansion on a hot day
• Thermal expansion of liquids

  • Liquids expand more than solids when heated, and is easily visible
• Liquid-in-glass thermometer

  Uses thermal expansion of liquid (alcohol or mercury) to measure temperature
• Thermal expansion of gases

  • Gases expand more than liquids when heated
  • Particles gain more kinetic energy and move faster, taking up more space
• Physical properties that vary with temperature

  • Thermal expansion
  • Electrical resistance
  • Potential difference
• Fixed points

  Temperatures at which certain particular physical properties manifest themselves (e.g. melting, boiling)
• Celsius scale

  Defined by the freezing point of water (0) and boiling point of water (100)
• Liquid-in-glass thermometers

  • Expansion of liquid is not perfectly linear, so thermometer must be calibrated
  • Sensitivity describes how accurately it was calibrated
  • Limited range (0-100)
• Thermocouple

  • Uses voltage differences to measure temperature
  • Can measure a much larger range of temperatures and more accurately than liquid-in-glass
  • Can provide instant temperature readings
• Internal energy

  Increases when an object is heated as particles gain more kinetic energy
• Thermal capacity (heat capacity)

  Amount of energy required to change the temperature of an object by 1°C
• Specific heat capacity

  Amount of energy needed to raise the temperature of an object per unit mass of that object
• Experiment to determine specific heat capacity of water

  1. Measure mass of water (m)

  2. Measure temperature change (ΔT)

  3. Measure energy supplied (E)

  4. Apply formula: c = E / (m * ΔT)
• Melting


  Change in state of a solid to a liquid, at the melting point temperature
• Solidification

  Reverse of melting, change from liquid to solid at freezing point
• Latent heat of fusion

  Energy added to melt a solid at melting point, or given out when a liquid solidifies into a solid at freezing point
• Boiling


  Change in state from a liquid to a gas, at the boiling point temperature
• Condensation


  Reverse of boiling, change from gas to liquid
• Latent heat of vaporization

  Energy added to vaporize a liquid at boiling point, or given out when a gas condenses
• Differences between boiling and evaporation

  • Boiling occurs at a fixed temperature, evaporation can occur at all temperatures
  • Evaporation decreases the temperature of the remaining liquid, boiling temperature remains constant
• Specific latent heat of fusion

  Energy required to melt 1 kg of solid at its melting point, with no change in temperature
• Experiment to calculate specific latent heat of fusion of ice


  1. Fill a funnel with ice and place a beaker beneath it

  2. Place a 50W heater in the ice

  3. Turn on the heater & start the timer

  4. After 10 minutes turn off the heater

  5. Measure the mass of the accumulated water in the beaker

  6. Calculate specific latent heat using formula: L = E / Δm
• Specific latent heat of vaporization

  Energy required to vaporize 1 kg of liquid at boiling point, with no change in temperature
• Experiment to calculate specific latent heat of vaporization of steam


  1. Part fill a beaker with boiling water and place on a balance

  2. Place a 50W heater in the water

  3. Switch the heater and wait for water to boil

  4. Once water is boiling start the timer and take the balance reading

  5. When the mass reading has decreased by 0.1 kg, stop the timer

  6. Calculate specific latent heat using formula: L = E / Δm
• Conduction

  The process by which heat or electricity is directly transmitted through the material of a substance
• Good conductors

  • Metallic structure composed of a lattice of metal ions and a sea of 'free electrons' that move throughout the structure
  • When one end is heated, the metallic ions gain energy and vibrate faster, passing on kinetic energy in the form of vibrations to neighboring ions
  • Free electrons gain kinetic energy at the hot end and can pass on their kinetic energy via collisions with other electrons and metal ions as they randomly move throughout the structure, greatly enhancing the conduction process
• The presence of free electrons is the main reason why metals are great conductors
• Experiment to show copper is a good conductor

  • Copper bar heated at one end, drawing pins fall off one by one as the metal conducts heat from the hot end to the cold end, melting the blobs of wax
• Poor conductors (insulators)

  • Materials that are very poor conductors, such as water, liquids and non-metallic solids
  • The absence of free electrons means molecules cannot easily pass on kinetic energy to their neighbors, so heat can only be transmitted through particle vibrations and collisions
• Experiment to show water is a poor conductor

  • Water in boiling tube heated at top, ice at bottom does not melt, demonstrating heat is not reaching the bottom
• Convection

  An important method of heat transfer in liquids and gases
• Radiation

  Infra-red radiation is part of the electromagnetic spectrum, emitted by any hot object, and can travel across vacuum without a medium
• Infra-red radiation can be emitted, absorbed, or reflected
• Experiment to demonstrate which surface is best emitter of infra-red radiation

  • Metal cube with 4 different painted surfaces (matt black, shiny black, white, silver), filled with boiling water, heat detector measures emission levels in order: matt black (highest), shiny black, white, silver (lowest)
• Experiment to demonstrate which surface is best absorber of infra-red radiation

  • Radiant heater between two plates (one matt black, one silver), temperatures increase quicker on matt black plate, demonstrating it is a better absorber