# Phy100: Heat transport Three basic forms of thermal heat transport i) Conduction; ii) Convection; iii) Radiation.

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Phy100: Heat transport Three basic forms of thermal heat transport i) Conduction; ii) Convection; iii) Radiation.

Heat conduction Conduction is heat transfer, or transfer of thermal energy within a substance or between different substances without motion of materials. It occurs when there is a temperature gradient within a substance or temperature differences between substances.

Examples of heat conduction

Conduction and collisions Microscopically, heat conduction occurs by means of collisions between molecules or atoms in crystals or diffusions of electrons in metals. The following pictures Illustrate such processes.

= heat transferred in time t = thermal conductivity of the barrier A=Cross section area T= temperature d = thickness Heat conduction rate or power d

SI units Q/t: J/s or Watts A: m^2 d: m T: Kelvin k: W m/K

Thermal conductivity table Copper 385 (in units of W/m K) Gold 314 Iron 79.5 Steel 50.2 Glass 0.8 Concrete 0.8 Brick, insulating 0.15 Brick, red 0.6 Wood 0.12-0.04 Fiberglass 0.04 Styrofoam 0.033 Polyurethane 0.02 Air (0c) 0.024

Q1 Which of the following does not reduce heat conduction between the hot and cold objects A) decrease the cross section A; B) increase the length L; C) use materials with lower thermal conductivity; D) increase the temperature of the hot object.

Q2 The heat conduction rate is doubled if A) The length L is doubled; B) The “hot” temperature is doubled ; C) The cross section area A is doubled; D) None of above.

R value This is a quantity usually used to specify house insulation properties in construction bussiness. R = d /k Q/t = A (T 2 - T 1 )/R R is defined in US units: (ft 2 ·°F·hr/BTU) BTU A British Thermal Unit (BTU) is the amount of heat energy needed to raise the temperature of one pound of water by one degree F. I BTU=1055 Joules 1 R = 0.093 m 2 ∙0.55 °C∙3600s/1055J = 0.17 m 2 ∙°K ∙s/J

Material R value(ft2·°F·hr/BTU) Hardwood siding (1 in. thick)0.91 Wood shingles (lapped)0.87 Brick (4 in. thick)4 Concrete block (filled cores)1.93 Fiberglass batting(3.5 in. thick)10.9 Fiberglass batting(6 in. thick)18.8 Fiberglass board (1 in. thick)4.35 Cellulose fiber(1 in. thick)3.7 Flat glass (0.125 in thick)0.89 Insulating glass(0.25 in space)1.54 Air space (3.5 in. thick)1.01 Free stagnant air layer0.17 Drywall (0.5 in. thick)0.45 Sheathing (0.5 in. thick)1.32

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