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Physics 101: Chapter 12 Temperature

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Temperature & Thermal Expansion Textbook Sections 12.1 - 12.9 DEMO Not So Useful Chapter 12, Preflight As you heat a block of aluminum from 0 C to 100 C its density 1. – PowerPoint PPT presentation

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Title: Physics 101: Chapter 12 Temperature


1
Physics 101 Chapter 12Temperature Thermal
Expansion
  • Textbook Sections 12.1 - 12.9

DEMO
2
Internal 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

3
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4
Temperature Scales
DEMO
NOTE K0 is absolute zero, meaning (almost)
zero KE/molecule
5
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6
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7
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 ? L0 ?T (volume expansion)

Expansion DEMO
Use expansion in thermostat
8
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9
Not So Useful
10
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

11
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

FB rlVg
12
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

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.
13
  • 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
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15
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16
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18
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

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20
How To Change the Temperature of a System
  • Add heat
  • Q heat that flows into a system.
  • Q cm?T
  • 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 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)

21
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22
Examples of Specific Heat Capacity(see Text,
Table 12.2)
Substance c in J/(kg-C) aluminum 900 copper 3
87 iron 452 lead 128 human body
3500 water 4186 ice 2000
?T Q/cm
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
23
Example
24
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

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.
25
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26
Latent Heat
water temp rises
T
DEMO
Q added to water
Latent Heat L J/kg amount of heat needed to
add to or remove from a substanceto change the
state of that substance
27
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28
Example
29
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?

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!!!
as the water evaporates off the body, it takes
with it the latent heat of vaporization, which
cools you down.
30
Vapor Pressure Pvapor(not on exam)
vapor
water
  • 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 mountain
  • boils at Tlt100C
  • Boiling water in pressure cooker
  • boils at Tgt100C
  • Boiling by cooling
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