Thermal Conductivity

Tuesday, 29 May 2018

1:34 PM

Types of Heat Transfer

 

Conduction - direct transfer

Convection - mass motion of molecules

Radiation - Electromagnetism

 

Conduction

• Atoms/molecules/electrons gain energy, and collide with other particles, which also gain energy and collide
• This occurs when there is a temperature difference between particles.
• If - on a microscopic level - particles in different parts of an object have on average the same energy, then the net energy transfer is 0.

 

 

(Steady state - equal energy transfer rate into two states)

 

Compound slab

 

 

 

Convection

Bulk movement of a substance

• Natural convection, when the movement results from a difference in density
• Forced convection - Fan/pump/etc

 

Radiation

• All objects with a non-zero temperature emit energy continuously in the form of EM waves
• Dipole radiation //

 

Stefan-Boltzmann Law

Energy emitted per unit time by an object of temperature  is

 - power (W)

 - Stefan's constant -

 - surface areaa

 - emissivity (a ratio from 0 to 1)

 - temperature (K)

 

Emissivity

When , an object is considered a black body - it is a perfect absorber and emitter of energy

When , and object is considered a white body - it is a perfect reflector of energy

 

Kirchhoff's Law - a good absorber is a good emitter.