Rate in Thermal Systems

1
Rate in Thermal Systems

• 3.4

2
Heat Flow Rate

• Movement of heatinstead of a substance
• Q Q/?t (Heat flow rate heat transferred
  divided by the change in time)
• Heat transfer can be measured in any energy
  unit (calories, joules, British thermal units or
  foot pounds) divided by seconds, minutes, or
  hours.

3
Heat Conduction

• The transfer of thermal energy arising from a
  temperature difference between adjacent parts of
  a body (touching)
• For heat conduction to take place, there must be
  a temperature difference within an object.
• Conductivity depends on materialhow far apart
  molecules are and how free the electrons are to
  move about in the material

4
Heat Conduction

• Thermal conductivity(k) of a material is a
  measure of its ability to conduct heat
• metals have a large value of k
• Wood has a low value of kmaking it a good
  insulator, not a conductor
• Q -kA?T/?x (heat conduction rate thermal
• conductivity X cross sectional area X change in
  temperature all divided by the change in
  thickness of the material)
• Q is used for heat transfer rate and heat
  conduction because heat conduction is a special
  case of heat transferwithin the same body

5
Convection

• transfer of heat by movement of a fluid
• natural convectionwinds
• forced convectionfan or pump creates a pressure
  difference in a heated or cooled fluidblood in
  our bodies

6
Radiation

• All objects radiate energy in the form of
  electromagnetic radiation
• Rate depends on the objects temperature, surface
  area, and material composition
• Transfers through empty space