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CONVECTION, CONDUCTION & RADIATION
Heat can travel in 3 ways, i.e. conduction, convection, and
radiation.
Conduction occurs mainly in solids when a heated group of
atoms speeds up and bumps into adjacent atoms, causing them to speed
up and perpetuating this domino-like process.
Convection occurs in fluids (liquids & gases) when a
group of atoms heats up, speeds up, expands, becoming less dense
and rises. Cooler fluid rushes in to replace the void and the pattern
repeats creating circulating weather-like patterns.
Radiation is the main source of heating that the earth recieves
from the sun or a camper receives from a fire. Atoms get so heated
up that during their collisions the electrons emit photons of infrared
radiation. These photons will travel through empty space until they
are absorbed by other atoms and heat up.
HEAT vs. TEMPERATURE
Heat is a measure of the total amount
of kinetic energy (atomic motion) contained
within a substance (solid, liquid or gas). Temperature
(°F, °C, K) is a measure of the average
kinetic energy in a substance. A 25°C glass of water
has the same temperature as a 25°C barrel of water
but less heat (Total Kinetic Energy). The 3 most common
temperature scales are compared to the states of water below:
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Fahrenheit |
Celsius |
Kelvin |
| Water boils |
212 |
100 |
373 |
| Water freezes |
32 |
0 |
273 |
| No Heat Left! |
-460 |
-273.15 |
0 |
KELVIN is the ABSOLUTE scale of temperature (zero actually
means zero heat).
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To convert °F to °C
Tc = 5/9(TF-32)
To convert °C to Kelvin
Tk = Tc + 273.15
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HEAT is measured in Joules or calories
(cal). 1 cal = 4.18 Joules ( 1 dietary Calorie
= 1000 cals! ) A calorie is metric and is defined
as the amount of heat required to raise the temperature of
1 gram of liquid water (1 ml, 1 cc) by 1 °C.
Different materials have different HEAT CAPACITIES
[ c ].
1 gram of liquid H2O requires 1 cal to raise its temperature
1°C, BUT 1 cal will raise the temperature
of 1 gram of aluminum by 4.5°C!
Aluminum has less capacity to store heat than water. Notice
that the heat capacity ( c ) of water varies with its state!
Even more interesting is that when water changes its state
it absorbs or gives off heat without changing its temperature!
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cals
= m x c x  T
HEAT Capacities (c)
of various materials
H20 (ice) = .5
H20 (liquid) = 1
H20 (gas) = .5
Aluminum = .22
Copper = .09
Mercury = .03 |
The heat required to raise the temperature
of 20 grams of water from -30°C to 150°C is 15,220
calories.
Using the graph at the right
20 x ( 15 + 80 + 100 + 540 +
25 )
20 = grams of water
15 = 30 degrees @ .5 cal/g°C
(from -30 to 0 °C)
80 = 80 cals/g for melting phase
change
100 = 100 degrees @ 1 cal/g°C
(from 0 to 100°C)
540 = 540 cals/g for boiling
phase change
25 = 50 degrees @ .5 cal/g°C
(from 100 to 150°C)
WOW! 80 calories just to melt 1 gram of ice! 540 calories
to boil 100°C water to 100°C steam!!
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Heat flows from areas of higher temperature
to areas of lower temperature.
If a 50g chunk of Aluminum at 100°C is placed in 100g
of water at 20°C. Knowing that the heat from the hotter
Aluminum flows into the water we can setup the following equation
and predict the final temperature of both (Tf = 29.8°C).
Many would assume wrongly that the final temperature would
be halfway between both but because each material has a different
heat capacity, the final temperature is closer to the original
temperature of the material with the greater heat capacity. |
m1c1T1
= m2c2T2
50(.22)(100-Tf) = 100(1)(Tf-20)
1100-11Tf = 100Tf -2000
3200 = 111Tf
28.8°C = Tf |
GENERAL GAS LAW (revisited)
The Pressure, Volume, and Temperature
of a contained ideal gas sample obey the General Gas Law
equation: PV/T = P'V'/T'
Pressure units: (atm, psi, mmHg, Pa, etc. )
Volume units: (gals, mL, L, cc, etc. )
Temperature: (MUST be in KELVIN!!!)
To simplify a problem, if P, V or T don't change
then that variable can be eliminated from the equation. If
Temperature doesn't change the equation is Boyle's Law. If
Pressure doesn't change it is Charle's Law. |
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