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Temperature and Heat

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Title: Temperature and Heat


1
Temperature and Heat
2
Temperature Scales
NOTE K0 is absolute zero, meaning (almost)
zero KE/molecule
14
3
Temp Scales Question
  • Two cups of coffee are heated to 100 degrees
    Fahrenheit. Cup 1 is then heated an additional
    20 degrees Centigrade, cup 2 is heated an
    additional 20 Kelvin. Which cup of coffee is
    hotter?
  • A) One B) Two C) Same

20
4
Thermal Expansion
  • When temperature rises
  • molecules have more kinetic energy
  • they are moving faster, on the average
  • consequently, things tend to expand
  • amount of expansion depends on
  • change in temperature
  • original length
  • coefficient of thermal expansion
  • L0 ?L L0 ? L0 ?T
  • ?L ? L0 ?T (linear expansion)
  • ?V ? V0 ?T (volume expansion)

23
5
Density Question
  • As you heat a block of aluminum from 0 C to 100 C
    its density
  • A. Increases
  • B. Decreases
  • C. Stays the same

26
6
Example concrete sidewalk
  • A concrete (a12x10-6 /K) sidewalk is 10m long
    when poured on a cold day (0o C) How much will
    the length of the sidewalk increase on a hot day
    (35o C)?

7
Example gas tank
  • A steel (ß36x10-6 /K) gas tank has a volume of
    75 lit. You fill it up with gasoline (ß950x10-6
    /K) early in the morning when it is cool During
    the day the temperature increases by 20o C. How
    much gas will spill out?

8
Differential Expansion Demo
  • A bimetallic strip is made with aluminum
    a16x10-6 /K on the left, and iron a12x10-6 /K
    on the right. At room temperature, the lengths of
    metal are equal. If you heat the strips up, what
    will it look like?

A B C
Aluminum gets longer, forces curve so its on
outside
29
9
Amazing Water
  • Water is very unusual in that it has a maximum
    density at 4 degrees C. That is why ice floats,
    and we exist!

30
10
Tight Fit
  • An aluminum plate has a circular hole cut in it.
    An aluminum 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 ?
  • A. The ball wont fit through the hole any more
  • B. The ball will fit more easily through the hole
  • C. Same as at room temperature

35
11
  • Why does the hole get bigger when the plate
    expands ???

Imagine a plate made from 9 smaller pieces. Each
piece expands. If you remove one piece, it will
leave an expanded hole
36
12
Stuck Lid Question
  • A glass jar (a 3x10-6 K-1) has a metal lid (a
    16x10-6 K-1) which is stuck. If you heat them by
    placing them in hot water, the lid will be
  • A. Easier to open
  • B. Harder to open
  • C. Same

Copper lid expands more, making a looser fit, and
easier to open!
38
13
Heat
  • Definition Flow of energy between two objects
    due to difference in temperature
  • Note similar to WORK
  • Object does not have heat (it has energy temp)
  • Units Joule
  • 4186 Joules 1 Calorie 1000 calories
  • calorie Amount of heat needed to raise
  • 1g of water 1ºC

10
14
Example Sears Tower
  • You decide to take the stairs to the top of the
    Sears tower (442m). If you have a weight of 670N
    and your body was 100 efficient in converting
    food into mechanical energy. How many Calories
    would you need to eat to replenish your body

Whopper with Cheese 696 Cal Big Mac with
Cheese 560 Cal
15
Specific Heat
  • Heat adds energy to object/system
  • IF system does NO work then
  • Heat increases internal energy. Q DU
  • Heat increases temperature!
  • Q c m DT
  • Heat required to increase Temp depends on amount
    of material (m) and type of material (c)
  • Q cm?T Cause inertia x effect (just
    like Fma)
  • cause Q
  • effect ?T
  • inertia cm (mass x specific heat capacity)
  • ?T Q/cm (just like a F/m)

15
16
Question
  • After a grueling work out, you drink a liter of
    cold water (0 C). How many Calories does it take
    for your body to raise the water up to body
    temperature of 36 C?
  • 1) 36 2) 360 3) 3,600 4) 36,000

1 liter 1,000 grams of H20 1000 g x 1
calorie/(gram degree) x (36 degree) 36,000
calories 36,000 calories 36 Calories!
18
17
Question
  • 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

Since aluminum has a higher specific heat than
copper, you are adding more heat to the water
when you dump the aluminum in the bucket
(qmcDT).
20
18
Specific Heat Question
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)
aluminum B) copper C) the same
?T Q/cm
23
19
Latent Heat L
  • As you add heat to water, the temperature
    increases for a while, then it remains constant,
    despite the additional heat!
  • Latent Heat L J/kg is heat which must be
    added (or removed) for material to change phase
    (liquid-gas).

28
20
Ice Question
  • Which will do a better job cooling your soda, a
    cooler filled with water at 0C, or a cooler
    filled with ice at 0 C.
  • A) Water B) About Same C) Ice

Latent Heat L J/kg is heat which must be
added (or removed) for material to change phase
(liquid-gas).
30
21
Example sweat
  • During a tough work out, your body sweats (and
    evaporates) 1 liter of water to keep cool (37 C).
    How much water would you need to drink (at 2C) to
    achieve the same thermal cooling? (recall CV
    4.2 J/g for water, Lv2.2x103 J/g)

33
22
Question
  • 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?

35
23
Example ice and water
  • How much ice (at 0 C) do you need to add to 0.5
    liters of a water at 25 C, to cool it down to 10
    C?
  • (L 80 cal/g, c 1 cal/g C , cice 0.5 cal/g C)

24
Example ice and water 2
  • 0.8kg of ice (at -10o C) is added to 1.4kg of
    water at 20o C, How much of the ice will melt?
    (Lf 33.5x104 J/kg, cwater 4186 J/kg C, cice
    2000 J/kg C)

25
Summary
  • Temperature measure of average Kinetic Energy of
    molecules
  • Thermal Expansion
  • ?L ? L0 ?T (linear expansion)
  • ?V ? L0 ?T (volume expansion)
  • Heat is FLOW of energy
  • Flow of energy may increase temperature
  • Specific Heat
  • Dt Q / (c m)
  • Monatomic IDEAL Gas CV 3/2 R
  • Diatomic IDEAL Gas CV 5/2 R
  • Latent Heat
  • heat associated with change in phase
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