Unit 3: Heat Transfer - Conduction, Convection, and Radiation

Sensible heat

changing temperatures that can be perceived

Latent heat

changing forms

Short wave radiation

Radiation that comes from a main heat source (e.g., sun)

Long wave radiation

Infrared radiation that is emitted by an object absorbing short wave radiation

low e coating windows

Windows that reduce heat transfer by limiting the amount of radiant energy that is transmitted

incident radiation

radiation source.

What happens when radiation strikes a non opaque surface

incident radiation = 1. Reflected + Absorbed + Transmitted = 1

What happens when it strikes an opaque surface

incident radiation = 1. Reflected + Absorbed = 1. No shortwave is transmitted because the surface is opaque.

Re-radiated energy

Re-radiated energy can go either direction and its sum is equal to the absorbed shortwave, It is reradiated as long wave.

Kirchoff’s rule

What is absorbed = what is emitted

Greenhouse effect

Glass is not opaque to shortwave radiation, so it absorbs and re-radiates infrared radiation (longwave) and then the longwave radiation becomes trapped because glass is opaque to longwave radiation.

Emissivity and Absorption

Emissivity is correlated to absorption, so high emissivity means high absorption which means low reflection. Low emissivity means low absorption which means high reflectance.

Temperature of an object

Balance between SW gains and LW losses

black body

absorbs all, emits max

perfect collector

absorbs all, emits zero

perfect dissipater

absorbs zero, emits max (emits its own internal temperature)

white body

absorbs zero, emits zero

cool roofs

reflects more than it absorbs. This will result in the roof being cool to touch

Conduction

Heat transfer through direct contact

Convection

Heat transfer through movement of air or water

Radiation

Main heat source that radiates its heat