UB, Phy101: Chapter 12, Pg 1 Physics 101: Chapter 12 Temperature & Thermal Expansion l Textbook Sections 12.1 - 12.9 DEMO.

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


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



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




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



Not So Useful…Not So Useful…


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



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



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.


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”







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



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)



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


ExampleExample


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.



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



ExampleExample


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!!!


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”

UB, Phy101: Chapter 12, Pg 1 Physics 101: Chapter 12 Temperature & Thermal Expansion l Textbook Sections 12.1 - 12.9 DEMO.

Documents

correct slide

temperature thermal

thermostat slide

vg slide

useful slide

t linear expansion v

temp t

t volume expansion temp