2. Thermal Physics

• Matter exists in one of three different states.
• Label the states of matter:

  A) Solid

  B) Liquid

  C) Gas

  
• Solids:

  • Strong forces of attraction between particles.

  • Have a fixed pattern (lattice).

  • Atoms vibrate but cannot change position.

  • Fixed shape and volume.
• Liquid:

  • Weaker attractive forces that solids.

  • No fixed pattern.

  • Particles slide past each other.

  • Fixed volume but changes shape depending on container.
• Gases:

  • Almost no intermolecular forces

  • Particles are far apart, and move quickly, gases spread out to fill up the container and exert equal pressure on all surfaces.

  • They collide with each other and bounce in all directions.

  • No fixed shape volume, gases fill up their containers.
• Matter can change from solid to liquid by melting and liquid to solid by freezing. These changes occur at the melting point.
• Liquid can also change to gas by boiling and gas to liquid by condensing. These changes occur at the boiling point.
• Molecules in a gas move around randomly and very quickly.
• The temperature of a gas is related to the average kinetic energy of the molecules.
• The higher the kinetic energy of the molecules, the higher its temperature.
• The lowest possible temperature that can be achieved in this universe is – 273 °C, and it is known as absolute zero.
• Absolute zero is the temperature at which particles have no kinetic energy.
• The motion of the molecules often cause them to collide with the surface of nearby walls. This collision causes a change in momentum when the molecule bounces off the wall (recall from chapter 1 that change in momentum over time gives you force).
• Each collision of a particle against a wall applies a force across a surface area of the walls. Recall from previous chapter force per unit area is pressure.
• Brownian motion is the erratic motion of small particles when observed through a microscope which is caused by collision between said particles and the molecules of the fluid.
• The SI unit of temperature is in Kelvin. However, Celsius is more frequently used.
• You can convert Celsius to Kelvin by using the formula:

  K = T +273

  where K is the temperature in Kelvins and T is the temperature in degrees Celsius.
• When the temperature of a gas in a fixed container is increased, the kinetic energy (speed) of molecules increase. This causes the molecules to collide more frequently against the container thus increasing pressure.
• If you decrease the volume while keeping the temperature of the gas constant (as in the case of the piston below) the pressure will increase. This is due to more collisions of the molecules with the container.
• Boyle's Law:

  P₁V₁ = P₂V₂

  where P₁ is the initial pressure, V₁ is the initial volume, P₂ is the final pressure and V₂ is the final volume.
• When most substances are heated, they expand due to higher average kinetic energy.
• When heated, molecules start knocking into each other and push each other apart.
• Due to an increase in temperature, solids expand a little due to the stronger bonds holding each molecule. Liquids expand more than solid but less than gas since the molecular bond strength holding them is between solid and gas. Gas expands the most due to it have the weakest molecular bonds.
• A thermometer is an example of the expansion of liquids when heated. As the temperature of thermometer increases, the liquid inside of the thermometer expands.
• Temperature is related to the average speed (or kinetic energy) of individual molecules. The SI unit for temperature is in Kelvin.
• Heat is a form of energy (not a force). As such its SI unit is Joules.
• Specific heat capacity is the amount of heat required to increase the temperature by 1°C for a mass of 1kg of the substance.
• A lower heat capacity means the object heats up easier, while a higher heat capacity means an object heats slower.
• The specific heat capacity (c) can be calculated by using the formula:

  c = Q/(mθ)

  where Q is the thermal energy (heat), θ is the change in temperature and m is the mass of the substance.
• The SI unit of specific heat capacity is J/(kg°C).
• When a substance is heated its temperate would normally increase due to the average kinetic energy increasing. However, when the substance is changing state either solid to liquid or liquid to gas, the temperature stays the same. This happens because the energy is being used to break the bonds between the molecules instead of increasing the kinetic energy.
• Liquid can change to gas through either boiling or evaporation.
• Evaporation:

  • Evaporation constantly occurs on the surface of liquids.
  • It is the escape of the more energetic particles.
  • If the more energetic particles escape, the liquid contains fewer high energy particles and lower energy particles so the average temperature decreases.
• Evaporation can be accelerated by:

  • increasing temperature: more particles have enough energy to escape.
  • increasing surface area: more molecules are close to the surface.
  • reducing the humidity level in the air: if the air is less humid, fewer particles are condensing.
  • blowing air across the surface: removes molecules before they can return to the liquid.
• Evaporation can cool objects down if the surface of the object is in contact with the liquid.
• Thermal energy is transferred via 3 mechanisms:

  • Conduction
  • Convection
  • Radiation
• Conduction

  • In non-metals - when heat is supplied to something, its atoms vibrate faster and pass on their vibrations to the adjacent atoms.
  • In metals - conduction happens in the previous way and in a quicker way - some electrons are free to move, they travel randomly in the metal and collide with atoms and pass on the vibrations.
• Convection

  • As a fluid (liquid or gas) warms up, the particles which are warmer become less dense and rise.
  • They then cool and fall back to the heat source, creating a cycle called convection current.
  • As particles circulate they transfer energy to other particles.
• Radiation

  • Thermal radiation is mainly infra-red waves but very hot objects also give out light waves.
  • Infra-red radiation is part of the electromagnetic spectrum.
• Infra-red radiation is part of the electromagnetic spectrum. Unlike the other two mechanism, thermal radiation can travel through a vacuum and does not need a medium.