Rate in Thermal Systems

1. Rate in Thermal Systems 3.4

2. Heat Flow Rate • Movement of heat—instead of a substance • Q = Q/Δt (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 material—how 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 k—making it a good insulator, not a conductor • Q = -kAΔT/Δx (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 transfer—within the same body

5. Convection • transfer of heat by movement of a fluid • natural convection—winds • forced convection—fan or pump creates a pressure difference in a heated or cooled fluid—blood in our bodies

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