PHY020 FE

• Convection


  1. Transfer of heat happens when a fluid such as air or water is in motion

  2. Driven by temperature differences across that fluid

  3. When a fluid is heated, the region in closest contact with the heat source becomes less dense due to increased kinetic energy in the particles

  4. The less dense portion of fluid rises, while the denser portion of fluid sinks

  5. The process repeats itself as the less dense fluids cool down and sink, while the denser fluids heat up and rise, creating convection currents
• Force Convection


  Can achieve the same effects as natural convection, the process is simply aided through devices like fans
• The heat current due to convection


  Is directly proportional to the surface area
• The viscosity of fluids

  Slows natural convection near a stationary surface
• The heat current due to convection

  Is found to be approximately proportional to 5/4
• Mechanisms of heat transfer

  • Conduction - occurs body or between two bodies contact
  • Convection - motion of mass
  • Radiation - heat transfer by electromagnetic radiation
• Radiation


  A body that is a good observer must also be a good emitter
• Black body


  An ideal body that absorbs all radiation that falls on its surface
• Reflector absorbs no radiation
• Vacuum flask air is pumped out of the spaces between the walls
• The average surface temperature of the earth is only 287K (14°C)
• CO2 is the 4th most abundant constituent when re-radiation is called greenhouse effect
• Infrared waves are the most radiation transfer
• Visible radiation is possible appearing red hot
• Radiation heat transfer


  • Process where heat waves are emitted that may be absorbed, reflected or transmitted through a body
  • Sun hits the earth by electromagnetic waves
  • Half bodies emit heat waves
• Emissivity


  • Defined as the ratio of energy radiated from the surface of a material to the radiated from a perfect emitter
  • A black body has an emissivity of 1
  • 0 = not emitting radiation
• The emission spectrum of a black body was first fully described by Max Planck
• Radiation energy per unit time from a black body is proportional to the fourth power of the absolute temperature and can be expressed with Stefan-Boltzmann Law