Thermal Physics

Thermal Physics

1. Thermal Energy Transfer

   • Internal energy
     • Energy can be classified into 2 forms, Kinetic and potential energy. Kinetic energy is due to the speed of the molecules and potential energy is due to the seperation between the molecules
     • Internal energy (U) is the sum of the randomly distributed kinetic and potential energies of the particles in a body
       • The Internal energy of a system is determined by Temperature, the random motions of molecules, the phase of matter (solid, liquid or gas) with gases have highest internal energy, Intermolecular forces between the particles (higher potential energy for greater intermolecular forces).
     • particles are randomly distributed so they all have different speeds and seperations
     • Internal energy can be increased by doing work on it, or increased by heating
       • Internal energy can be decreased by losing heat to surrounds or changing state
   • The first law of thermodynamics
     • It applies to all situations, not just gases.
     • Expansion/compression of a gas
       • When a gas expands (volume increases), work is done by the gas on the surrounds , which decreases the internal energy of the gas
       • When a gas is compressed (volume decreases), work is done on the gas by the surroundings, which increases the internal energy
       • Example: a gas in a cylindrical container with moveable piston
         • When the piston moves down, it compresses the gas molecules (work is done on the gas) the molecules are pushed closer together therefore higher kinetic energy as they move faster, hence increases internal energy of the gas
         • When the piston moves up, it expands the gas (work is done by the gas) so molecules are further apart, therefore lower kinetic energy as they move slower which decreases the internal energy
         • Heating the gas will increase the temperature so molecules move around faster, as they have higher kinetic energy internal energy increases
   • Specific heat capacity (c) is the amount of the thermal energy required to raise the the temperature of 1kg of a substance by 1°C (or 1 K) without changing state
     • ΔQ=mcΔθ
       • ΔQ= change in thermal energy (j) m= mass of substance (kg) c=specific heat capacity of substance (joules per kg per kelvin/Celcius Δθ= change in temperature (K or °C)
     • A low specific heat capacity, it heats and cools down quickly
       • A high specific heat capacity, it heats and cools down slowly
   • Specific Latent heat
     • The thermal energy required to change the state of 1kg of mass of a substance without any change of temperature
       • 2 types: Specific latent heat of fusion (melting) , Specific latent heat of vaporisation (boiling)
         • Latent heat of fusion: The thermal energy required to convert 1 kg of solid to liquid with no change in temperature. Applies to melting a solid or freezing a liquid
         • Latent heat of vaporisation: The thermal energy required to convert 1 kg of liquid to gas with no change in temperature . Applies to evaporating a liquid or condensing a gas
     • During a change of state, there is no change in temperature and the potential energies of the molecules change but not kinetic
       • The potential energy of molecules is due to seperation and intermolecular bonds
         • when heat is absorbed in melting/boiling causes molecules to move apart by overcoming intermolecular force of attraction
         • When heat is released allows molecules to move closer together and the intermolecular forces of attraction become stronger. This is because the kinetic energy is proportional to temperature and np change in temperature means no change in kinetic energy

2. Ideal gases

   • Absolute Zero: is the Lowest temperature possible. Equal to 0 K or -273.5°C
     • The temperature at which molecules in a substance have zero kinetic energy

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