Unit 9 : Heat - LCHS Physical Science LCHS Main SiteSD271 Main site
Objectives: The learner will...
...understand the differerence between heat and temperature and that heat is evidenced by random motion and vibration of atoms
...relate molecular motion to changes in state
...work problems involving heat of fusion, heat of vaporization, and specific heat
...work heat problems involving the General Gas Law
...understand the meaning of entropy
Vocabulary:
heat : measure of the total amount of kinetic energy (atomic motion) contained within a substance
temperature :
is a measure of the average kinetic energy in a substance.
calorie :
the amount of heat to raise 1 gram of water by 1 degree Celsius
Kelvin :
an absolute temperature scale
heat capacity :
an objects ability to store heat rather than change temperature
 

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. Metals conduct heat very well.

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.

Conduction & Convection thermal energy transfer requirs matter.

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 (like the sun warming the earth) 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 , measured in Fahrenheit, Celsius, and Kelvin (°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). As the temperature of an object rises, so does the thermal energy. The 3 most common temperature scales are compared to the states of water below:
  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).




To convert °F to °C
Tc = 5/9(TF-32)
To convert °C to Kelvin
Tk = Tc + 273.15


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!


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

PHASES
Depending on the temperature a substance can assume one of 5 phases
SOLID - LIQUID - GAS - PLASMA or Bose/Einstein Condensate
Solid:
     particles are tightly packed, more orderly, locked in position
     definite shape, definite volume, lower energy
     
Liquid:
     definite shape, NO definite volume, medium energy

Gas:
     NO definite shape, NO definite volume, higher energy
     


PHASE CHANGES
When heat is applied or removed from matter it can go through a physical change called a phase change. What is strange is that during the actual phase change, heat is flowing into or out of the matter but the temperature does NOT change!

SOLID >> LIQUID is melting       LIQUID >> SOLID is freezing
LIQUID >> GAS is vaporization   GAS >> LIQUID is condensation
GAS >> SOLID is deposition        SOLID >> GAS is sublimation


The heat required to raise the temperature of 20 grams of water from -30°C to 150°C is 15,200 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!!


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
...again
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.

Heat engines (combustion, sterling) will always be less than 100% efficient due to frictional losses, which is itself a form of heat that leaks from the cylinder instead of pushing the piston.

 

 
Practice: Help:
PB1
65 degrees Celsius equal how many Kelvin?
Tk = Tc + 273.15
PB2
How many calories to raise 79 g of Copper
from 27 degrees to 42 degrees Celsius?
cal=mcT
cal=79x.09x(42-27)
PB3
What is the heat capacity (c) of a material if 7600 calories
raise 47 g of it by 63 degrees Celsius?
cal=mcT
7600=47xcx63
PB4
78 g of Aluminum at 49 degrees C are added to 8 g of water at 83 degrees C.
What is the final temp (C)?
mcT=mcT
78(.22)(Tf-49)=8(1)(83-Tf)
solve for Tf
good luck!
PB5
17 g of water is heated from -31 C to 89 C. How many calories of heat are required?
17x(15.5+80+89)
PB6
52 g of water is heated from 20 C to 60 C. How many calories of heat are required?
52x(40)
PB7
83 g of water is heated from 17 C to 173 C. How many calories of heat are required?

83x(83+540+36.5)

PB8
16 g of water is heated from -62 C to 187 C. How many calories of heat are required?
16x(31+80+100+540+43.5)
PB9
At constant volume, a gas at 2.7 atm and 160 K changes its temperature to 460 K,
what is its new pressure (atm)?
P/T=P'/T'
2.7/160=P'/460
PB10 (HINT: EQN 8)
At constant pressure, 61 L of gas at 54 Kelvin changes its volume to 88 L.
What is the new temp (Kelvin)?
V/T=V'/T'
61/54=88/T'
answer bank:
54738.5
338.15
106.65
2080
2.5667
59.81081
3136.5

77.90164
12712
7.7625

Scientist Spotlight:
Richard Feynman 1918-1988
American physicist who was born in New York City on May 11, 1918.


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