Convection

• Convection


  Transfer of heat happens when a fluid such as air or water is in motion
• Convection


  • Driven by temperature differences across that fluid
  • 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
  • The less dense portion of fluid then rises, while the denser portion of fluid sinks
  • The process repeats itself because the less dense fluids cool down as they move away from their heat source, making them sink, while the denser fluids heat up as they near the heat source, making them rise. This creates 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
• Actual black bodies don't exist
• 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
• Emissivity

  • 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 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
  • The sun hits the earth by electromagnetic waves
  • Half bodies emit heat waves
• Vacuum flask air is pumped out of the spaces between the walls