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UB, Phy101: Chapter 12, Pg 1
Physics 101: Physics 101: Chapter 12 Chapter 12Temperature & Thermal ExpansionTemperature & Thermal Expansion
Textbook Sections 12.1 - 12.9
DEMO
UB, Phy101: Chapter 12, Pg 2
Internal Energy and TemperatureInternal Energy and Temperature
All object have “internal energy” (measured in Joules) random motion of molecules
» kinetic energy collisions of molecules gives rise to pressure
Amount of internal energy depends on temperature
» related to average kinetic energy per molecule how many molecules
» mass “specific heat”
» related to how many different ways a molecule can move translation rotation vibration
» the more ways it can move, the higher the specific heat
UB, Phy101: Chapter 12, Pg 3
UB, Phy101: Chapter 12, Pg 4
Temperature ScalesTemperature Scales
Water boils
Water freezes
212
32
Farenheit
100
0
Celcius
273.15
373.15
Kelvin
273-K C 273 C K
32-F9
5 C 32 C
5
9 F
NOTE: K=0 is “absolute zero”, meaning (almost) zero KE/molecule
DEMO
UB, Phy101: Chapter 12, Pg 5
UB, Phy101: Chapter 12, Pg 6
UB, Phy101: Chapter 12, Pg 7
Thermal ExpansionThermal Expansion
When temperature risesmolecules have more kinetic energy
» they are moving faster, on the averageconsequently, things tend to expand
amount of expansion depends on…change in temperatureoriginal lengthcoefficient of thermal expansion
» L0 + L = L0 + L0 TL = L0 T (linear expansion)V = L0 T (volume expansion)
Temp: T
Temp: T+T
L0
L
Expansion DEMO
Use expansion in thermostat
UB, Phy101: Chapter 12, Pg 8
UB, Phy101: Chapter 12, Pg 9
Not So Useful…Not So Useful…
UB, Phy101: Chapter 12, Pg 10
Chapter 12, Preflight Chapter 12, Preflight
As you heat a block of aluminum from 0 C to 100 C its density 1. Increases 2. Decreases 3. Stays the same
T = 0 C
M, V0
0 = M / V0
T = 100 C
M, V100
100 = M / V100
< 0
CORRECT
UB, Phy101: Chapter 12, Pg 11
Chapter 12, Preflight Chapter 12, Preflight
Not being a great athlete, and having lots of money to spend, Gill Bates decides to keep the lake in his back yard at the exact temperature which will maximize the buoyant force on him when he swims. Which of the following would be the best choice? 1. 0 C 2. 4 C 3. 32 C 4. 100 C 5. 212 C
The answer is 4 C, because water has its greatest density at 4 C. Since bouyant force is equal to the weight of the displaced fluid or density*volume, when the density is the largest, the bouyant force is maximized.
999.55
999.60
999.65
999.70
999.75
999.80
999.85
999.90
999.95
1000.00
0 2 4 6 8 10
Density
CORRECT
FB = lVg
UB, Phy101: Chapter 12, Pg 12
Chapter 12, Preflight Chapter 12, Preflight
An aluminum plate has a circular hole cut in it. A copper ball (solid sphere) has exactly the same diameter as the hole when both are at room temperature, and hence can just barely be pushed through it. If both the plate and the ball are now heated up to a few hundred degrees Celsius, how will the ball and the hole fit ? 1. The ball wont fit through the hole any more 2. The ball will fit more easily through the hole 3. Same as at room temperature
CORRECT
The aluminum plate and copper ball both have different coefficients of thermal expansion. Aluminum has a higher coefficient than aluminum which means the aluminum plate hole will expand to be larger than the copper ball's expansion and allow more space for the ball to pass through.
UB, Phy101: Chapter 12, Pg 13
Why does the hole get bigger when the plate expands ???
Object at temp TSame object at higer T:
Plate and hole both get larger
Imagine a plate made from 9 smaller pieces. Each piece expands. If you remove one piece, it will leave an “expanded hole”
UB, Phy101: Chapter 12, Pg 14
UB, Phy101: Chapter 12, Pg 15
UB, Phy101: Chapter 12, Pg 16
UB, Phy101: Chapter 12, Pg 17
UB, Phy101: Chapter 12, Pg 18
Chapter 12, Preflight Chapter 12, Preflight
You measure your body temperature with a thermometer calibrated in degrees Kelvin. What do you hope the reading is (assuming you are not trying to fake some sort of illness) ? 1. 307 K 2. 310 K 3. 313 K 4. 317 K
correct
310 273C K
37 32)-F(9
5 C
98.6 F
UB, Phy101: Chapter 12, Pg 19
UB, Phy101: Chapter 12, Pg 20
How To Change the Temperature of a How To Change the Temperature of a System:System:
Add heat Q: heat that flows into a system.
Q = cmT Q = amount of heat that must be supplied to raise the temperature of mass m by an
amount T » Units of Q: Joules or calories
1 cal = 4.186 J 1 kcal = 1 Cal = 4186 J (this is what you read on the label)
c = specific heat capacity: Heat required to raise 1 kg by 1oC.
Q = cmT : “Cause” = “inertia” x “effect” (just like F=ma) cause = Q effect = T inertia = cm (mass x specific heat capacity)
T = Q/cm (just like a = F/m)
UB, Phy101: Chapter 12, Pg 21
UB, Phy101: Chapter 12, Pg 22
Examples of Specific Heat CapacityExamples of Specific Heat Capacity(see Text, Table 12.2)(see Text, Table 12.2)
Substance c in J/(kg-C)aluminum 900copper 387iron 452lead 128human body 3500water 4186ice 2000
Suppose you have equal masses of aluminum and copper at the same initial temperature. You add 1000 J of heat to each of them. Which one ends up at the higher final temperature
a) aluminumb) copperc) the same
correct
T = Q/cm
UB, Phy101: Chapter 12, Pg 23
ExampleExample
UB, Phy101: Chapter 12, Pg 24
Chapter 12, Preflight Chapter 12, Preflight Suppose you have two insulated buckets containing the same amount of water at room temperature. You also happen to have two blocks of metal of the same mass, both at the same temperature, warmer than the water in the buckets. One block is made of aluminum and one is made of copper. You put the aluminum block into one bucket of water, and the copper block into the other. After waiting a while you measure the temperature of the water in both buckets. Which is warmer? 1. The water in the bucket containing the aluminum block 2. The water in the bucket containing the copper block 3. The water in both buckets will be at the same temperature
correct
Aluminum has a greater specific heat capacity than copper, so it can retain more heat. Since aluminum has a greater heat capacity, when added to water, it will warm up the water more than copper.
UB, Phy101: Chapter 12, Pg 25
UB, Phy101: Chapter 12, Pg 26
These are BIGnumbers !
Latent HeatLatent Heat
T
Q added to water
water temp rises
Latent Heat L [J/kg]:
amount of heat needed to add to or remove from a substanceto change the state of that substance
Substance Lf (J/kg) Lv (J/kg)
water 33.5 x 104 22.6 x 105
water changesto steam
(boils)
steam temp rises
100oCDEMO
UB, Phy101: Chapter 12, Pg 27
UB, Phy101: Chapter 12, Pg 28
ExampleExample
UB, Phy101: Chapter 12, Pg 29
Chapter 12, Preflight Chapter 12, Preflight Summers in Phoenix Arizona are very hot (125 F is not uncommon), and very dry. If you hop into an outdoor swimming pool on a summer day in Phoenix, you will probably find that the water is too warm to be very refreshing. However, when you get out of the pool and let the sun dry you off, you find that you are quite cold for a few minutes (yes...you will have goose-bumps on a day when the air temperature is over 120 degrees). How can you explain this?
as the water evaporates off the body, it takes with it the latent heat of vaporization, which cools you down.
The water is evaporating off of your skin. This means that enough heat (or energy) is entering the water drops to break bonds between water and allow them to evaporate. Where is this heat coming from? Your body! Heat flows from your body to the drops of water, making you feel cooler. When the water is gone, no more heat will flow from your body and you will get hot once again. However, this whole situation may be avoided by NOT GOING OUTSIDE WHEN IT IS 125 DEGREES!!!
UB, Phy101: Chapter 12, Pg 30
water
Vapor Pressure PVapor Pressure Pvaporvapor
(not on exam)(not on exam)
In equilibrium, the “Vapor Pressure” of a liquid depends only on temperature. (We will discuss partial pressure more in a week or so when we do Ideal Gas law) .
Boiling occurs when pvap psurrounding
Boiling water on mountainboils at T<100C
Boiling water in pressure cookerboils at T>100C
Boiling by cooling
vapor
T
P
83C 100C
1 atm
.5 atm
“Phase Diagram”