chapter 7 - thermal processes

• • all matter contains energy in its internal (thermal) store.
  • amount of energy in the internal store of an object is the sum of all potential energy due to intermolecular forces and all kinetic energy due to the motion of the particles
• • energy transferred from a region of higher temp. to a region of lower temp. until both have the same temp. = net flow --> occurs when there is a temp, difference
  • when there is no net flow of energy between two objects by heating = thermal equilibrium
• thermal equilibrium

  > when two bodies are at thermal equilibrium, both bodies are at the same temp. and there is no net flow of thermal energy between them
• 3 ways of thermal energy transfer

  • conduction
  • convection
  • thermal radiation
• conduction

  • transfer of energy through a medium without any flow
  • process of transferring energy through the passing of vibrational motion from one particles to another
  • 2 ways conduction happens in solid : vibration, diffusion
• conduction

  • vibration of particles

  > particles at the heated end of the medium gain energy in their thermal stores and vibrate more vigorously abt their fixed positions

  > collide with their neighboring particles-->transferring energy towards the cooler end of medium
• conduction

  • free electrons diffusion (metal only) --> energy of the kinetic stores of the free electrons at the heated end increases
  • they move freely throughout the metal and collide with the particles at the lower temperature end of the medium, transferring energy to their thermal stores
• conduction

  • flow of thermal energy is from high temp. end to low temp. end
  • movement of more energetic electrons is from high temp. end to low temp. end
• which of the state is the good conductor if thermal energy?

  • solid, atoms and molecules are closer together in solid
• conduction

  • liquids and gases are poor conductors cuz their particles are further apart which slows the transfer of energy in the thermal store of the gas
  • metals are better conductors of heat compared to non-metals cuz metals have free electrons
• convection

  • transfer of thermal energy by movement of particles due to change in the density of medium
  • air is fluid cuz air can flow
• convection

  • when fluid is heated, the particles in it receive thermal energy and move further apart --> hence, , the heated fluid expands with its volume increases and density decreases --> cause it to rise
  • cooler, denser fluid flows down to fill up the space and become heated
  • the process repeats until all parts of the fluid have the same temp.
• infra-red (thermal) radiation

  • transfer of energy of the thermal store by electromagnetic waves
  • does not require a medium and can pass through a vacuum
  • energy from sun reaches the earth with thermal radiation
• infra-red(thermal) radiation

  • object emits thermal radiation when its temp. is higher than that of the surrounding, and absorbs thermal radiation when its temp. is lower than that of the surrounding
  • when an object is at the same temp. as the surrounding, it does not emit or absorb thermal radiation
• infra-red (thermal) radiation

  • amount of thermal radiation transferred by an object depends on its

  > temp. --> higher temp. = higher rate of emission of infrared radiation

  > surface area --> larger surface area = emit/ absorb infrared radiation at a higher rate

  > colour and texture (smooth or rough) of the surface
• infra-red (thermal ) radiation

  • good emitters and absorbers

  > black

  > dull surface

  > rough surface

  > poor reflector

  • poor emitters and absorbers

  > white/bright colours

  > shiny surface

  > smooth surface

  > good reflector