iii.physical principals - terminology a. temperature and heat 1. temperature intensity of energy...

III. PHYSICAL PRINCIPALS -TERMINOLOGY

A. Temperature and Heat

1. Temperature•Intensity of Energy

•Units; Degrees C or F

–Heat (energy)•Capacity to work

•BTU (British Thermal Unit)- energy to raise temperature of one pound (2 cups) of water 1°F

•Calorie- energy to Raise temperature of one gram of water 10 C

2. Heat Transfer– Heat moving due to temperature difference

a. Conduction• Heat transfer from touching objects • Energy moves from high to low temperature

• Examples:

– Touching cold or hot surface–Metal spoon in boiling water–Heat loss through greenhouse covering

b. Convection•Transfer from surface to moving air or water

1) Forced convection–Examples• Fan blows air over warm

surface•Wind blows over black

driveway

2) Natural convectionHot air (less dense) rises, replaced by cooler air (more dense)

• Examples:

– Hot air rises from black driveway

– Air rising from hot pipe in greenhouse

Warm airCool air

c. Radiation• All surfaces emit energy• Energy moves from high to low temperature• Requires no air, water etc. between objects - transfer

better in vacuum

• Examples:–Two people–Person and building–Person and clear sky–Objects in greenhouse & clear sky

B. Relative Humidity1. Definition• % RH = Amount of moisture in air

divided by the Maximum amount of moisture air can hold at temp. x 100

2. Effect of Temperature• As air temperature increases air can

hold more moisture

4. Evaporation–Requires 1060 BTU to convert 1 lb (2 cups) of

H2O from liquid to vapor

5. Condensation–Moisture in the vapor form is converted to a

liquid–air cools below dew point condensation

occurs