Chapter 10 Temperature and Heat
I.
Temperature
A.
Thermodynamics = study of heat and its effect on matter
1) One of the sub-fields of Physics
2) Part of Classical Physics (like Mechanics) developed by 1900
B.
Measuring Temperature
1) What is hot? What is cold?
2) We must compare it to some other object
3) Use a thermometer to assign a temperature
a) Sealed tube with a liquid inside (mercury or ethanol with a red dye)
b) Volume of the liquid changes faster than the length of glass
c) Volume increases until thermometer is same temperature as mouth
d) 101.6 oF is “hotter” than 98.6 oF
e) Must have a scale of the thermometer to compare temperatures
f) Could use other properties of matter than volume
Length of a rod, electrical resistance, color change, etc…
4)
5)
B.
Thermal Equilibrium = when touching objects no longer have changing
properties
Zeroth Law of Thermodynamics = Objects in thermodynamic
equilibrium have the same temperature
Temperature Scales
1) Must have a standard scale to compare temperatures
a) 1 meter defined by a metal rod of a certain length
b) Temperature standards use freezing point and boiling points of water
2) Fahrenheit (1686-1736)
a) Freezing point of water = 32 oF
180 Fo
b) Boiling point of water = 212 oF
c) Used primarily in USA
3) Celcius (1701-1744)
a) Freezing point of water = 0 oC
o
100
C
b) Boiling point of water = 100 oC
c) Used everywhere else and in science
4) 1 oC > 1 oF
180o F 9 o
100o C 5 o
o
o

F
per
C

C
per
F
100o C 5
180o F 9
5)
Conversion of Fahrenheit and Celcius Temperatures
a) 0 oC = 32 oF so there is a +/- 32 added into the conversion factors
9
TF  TC  32
5
b)
5
TC  (TF  32)
9
Convert 100 oC = ??oF
9
9
TF  TC  32  (100)  32  180  32  212o F
5
5
Convert 98.6 oF = ??oC
5
5
5
TC  (TF  32)  (98.6  32)  (66.6)  37 o C
9
9
9
d) Prove to yourself that –40 oC = -40 oF
c)
D.
Absolute Zero
1) 0 oC is based on freezing point of water, could use ethanol (-115oC)
2) Arbitrary point defined as zero, so we get negative numbers
3) What is the real lowest temperature = 0 ?
4)
Pressure of a gas depends on its Temperature (constant volume)
a)
b)
5)
At the lowest possible temperature, P = 0
Lowest possible temperature = -273.2 oC = Absolute Zero
Kelvin Temperature Scale: 0 K = absolute zero
a) TK = TC + 273.2
b) Room Temperature = 22 oC
TK = 22 + 273.2 = 295.2 K
c) Not degrees Kelvin, but simply Kelvins
d) Can’t ever reach absolute zero
II.
Heat
A.
Zeroth Law = Objects in contact will come to the same temperature
Hot
1)
2)
3)
B.
Cold
Hot object cools down
Cool object heats up
Early explanation was the flow of “caloric” from hot to cold
Amount of caloric in an object = temperature
Also would work for mixture of liquids
Heat = Q being transferred, not caloric, is responsible for equilibration
1) Heat is a form of Energy (unit = J)
2) Another unit is the calorie: 1 calorie = 4.19 J
3) 1 calorie = amount of heat needed to raise the temperature
of 1 g of water by 1 oC
4) Different materials require different amounts of heat to raise
their temperature by the same amount
5) Specific Heat Capacity = c = amount of heat needed to raise
the temperature of 1 g of a material by 1 oC (Units = cal/g oC)
6)
Water has a very large heat capacity, so it cools things very well
a) Small amount of water takes away a large amount of heat
b) Temperature of the water is not raised very fast
c) Coasts are cooler during the day, and warmer at night
7)
Q = mcDT is used to relate mass, heat, heat capacity, and Temperature
How much heat could 100 g of water and 100 g of steel take away if
there temperatures were raised 50 oC?
Qsteel  mcDT  (100 g )(0.11
Qwater
C.
cal
o
)(
50
C )  550cal
o
g C
cal
 mcDT  (100 g )(1.0 o )(50o C )  5000cal
g C
Heat and Temperature
1) Like caloric, Heat flows from Hot to Cold
a) Added heat increases temperature
b) Removed heat decreases temperature
c) Temperature tells us direction of heat flow
d) Temp depends on heat, mass, c
2)
3)
D.
Heat = energy flowing between objects of different temperatures
Temperature = quantity that indicates direction of heat flow. If no heat is
flowing, the objects are the same temperature.
Heat and Change of Phase
1) Phases of matter depend on how much heat they have (temperature)
a) Solid < Liquid < Gas
b) Ice < Water < Steam
2) Block of ice at –20 oC
a) Add heat: block warms up to 0 oC
b) Add more heat: ice melts to water at 0 oC
i. No temperature change
ii. Change in phase requires heat
3) Latent Heat of Fusion = Lf = amount of heat required to change the
phase of 1 g of a solid to a liquid = 80 cal/g for water
4) Latent Heat of Vaporization = Lv = amount of heat required to change
the phase of 1 g of a liquid to gas = 540 cal/g for water
How much heat is required to turn 15 g of liquid water at 100 oC to
steam at 100 oC?
Q  mLv  (15g )(540cal / g )  8,100cal
5)
Ice added to water
a) Ice is < 0 oC and the water is > 0 oC
b) Heat flows from water to ice
i. Heats the ice to 0 oC
ii. Then it starts melting the ice
c) If there is enough ice, eventually all of the water will be cooled to 0 oC
d) Both the ice and water will be at 0 oC = Thermal Equilibrium
e) Heat from the surroundings eventually melts the ice
f) After all ice melted, then the temperature will rise > 0 oC
6)
Boiling Water = 100 oC
1) Add more heat, change some of it into steam
2) No change in temperature, all the heat goes to Lv
3) High altitude lowers pressure needed to boil, less heat, b.p. = 96 oC at
an altitude of 1 mile (Denver)
7)
Q needed for 200 g ice (c = 0.5 cal/goC) at –10 oC to melt?
cal
QDT  mcDT  (200 g )(0.5 o )(10o C )  1000cal
g C
17,000 cal
Qmelt  mL f  (200 g )(80cal / g )  16,000cal