Module 3- Thermal Physics

Module 3- Thermal Physics

1. 3.1- Thermal Concepts

   • Molecular Theory
     • In a solid, the molecules can only vibrate, they cannot translate
     • In a liquid the molecules can vibrate and move about freely in a fixed volume
     • In going from a solid to a liquid, some of the intermolecular bonds are broken giving the molecules more freedom of motion
       • Melting (Opposite is freezing)
     • In going from a liquid to a gas, most of the intermolecular bonds are broken
       • Boiling (opposite is condensing)
     • Solid to gas is sublimation. Gas to solid is deposition
   • Internal Energy
     • All substances are composed of individual molecules that are vibration
     • As we heat up a substance, its vibrations become more energetic. This is an increase in kinetic energy of the molecules
     • As heat energy is being added the molecules are also moving further apart. This is an increase in the potential energy of the substance
     • Internal Energy = Ek+Ep
     • When thermal energy is added to a substance it is stored as internal energy
   • Temperature
     • Absorption of thermal energy causes materials to expand
     • Temperature only reveals the internal kinetic energy
     • Expansion reveals the internal potential energy
     • Absolute zero is -273 degrees where pressure for all gases is 0
       • Kelvin Scale starts from absolute 0
     • T(k)= T(degrees)+273
   • Specific Heat Capacity
     • One kilocalorie is the amount of heat needed to raise the temperature of 1kg of water by 1 degree
       • 1kcal=4.186kJ
     • Different materials absorb heat in different ways
     • Specific heat capacity is the amount of mass needed per unit temperature increase per unit mass
     • Q=mc(change in T)
   • Phase Change Graphs
     • The thermometer only measures kinetic energy and temperature does not change during phase change
       • The heat must only be stored as potential energy during phase change
   • Specific Latent Heat
     • Latent heat means hidden heat by which we mean there is no temperature indication that heat is being lost or gained by a substance
     • Q=mL
     • Latent heat of fusion- solid to liquid. Latent heat of vaporisation- liquid to gas
   • Conduction, Convection and Thermal Radiation
     • Thermal energy can be transferred from a warmer mass to a cooler mass by Conduction, Convection and Radiation
       • Heating or Cooling
     • Thermal radiation transfers heat without any physical medium such as solid, liquid or gas
     • Conduction- two objects touch transfer thermal energy from hotter to cooler
     • Vibration is transferred from atom to atom
     • Thermal equilibrium at which point there is no net transfer of heat
     • Convection requires a fluid as a medium of heat transfer
       • Hot air rising and cool air sinking is a convection current

2. 3.2 Modelling a Gas

   • Pressure
     • Walls of a container feel pressure as due to Newton's Third Law, molecules exert a force on the container
       • p=F/A
   • Ideal Gas
     • Imaginary gas that is used to model real gases
     • Molecules that make up the ideal gas....
       • Are identical perfect spheres
       • Are perfectly elastic, no energy lost in collisions
       • have no intermolecular forces, their potential energy does not change
       • are so small that their volume is much smaller than the volume of the container
     • If temperature increases, so does the average speed of the molecules
       • Number of collisions increases so pressure will increase if temperature increases
     • Smaller volume means that molecules have less distance to hit the container so pressure is bigger if volume is small
     • Equation of state for an ideal gas pV=nRT (R=8.31)
     • Average Kinetic Energy = 3/2KbT=3RT/2Na (Kb is 1.38x^-23)
   • Mole and Molar Mass
     • 1 mole is the number of atoms of an elements that will have a mass in grams equal to its gram atomic weight
       • Avogadro Constant = 6.02x10^23
     • n=N/Na (n is number of moles, N is number of atoms)
   • Constant Processes
     • Pressure
       • Isobaric process is where pressure does not change
       • Work done by an expanding gas =p change in v
     • Volume
       • Isovolumetric is constant volume
     • Temperature
       • Isothermic process is where temperature does not change
     • Thermodynamic process involves moving from one state to another state
       • A thermodynamic cycle is a set of processes which return a gas to an original state