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III. PHYSICAL PRINCIPALS -TERMINOLOGY
A. Temperature and Heat
1. Temperature•Intensity of Energy
•Units; Degrees C or F
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–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
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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
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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
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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
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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
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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
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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