HEAT

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A priceless presentation brought to Ms. Johnsons
favorite class by Rajasree Roy and Philip Rembert!

• HEAT!
• Caution! Too Hot To Handle
• Latent Heat and Heat Transfer (Conduction)

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Latent Heat
What on Earth is it?

• Latent heat is the name given to the temperatures
  at which a certain substance undergoes a phase
  change
• Everything is solid at its coldest state, then
  it turns to liquid as heat is applied, then gas.

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Some things you probably didnt know before we
told you

• If a substance is receiving an amount of heat
  that is between two of its latent heats, the
  actual temperature of the substance will not
  change.
• For example (also on page 425)-
• If an amount of kcal of heat are applied to water
  between 200 and 720, the actual temperature of
  the water remains at 100 degrees Celsius. But if
  the amount of the heat being applied were to drop
  below 200, the actual temperature of the
  substance would begin to drop proportionally.

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Some names you ought to learn

• The heat required to change a substance from
  liquid to vapor is called its heat of
  evaporation
• The amount of heat required to change a substance
  from its solid state to its liquid state is
  called its heat of fusion
• The temperature at which a substance goes from
  being a liquid to being a gas is called its
  boiling point (you should know this already)

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More names you should know

• The temperature at which a substance goes from
  being a solid to being a liquid is called its
  melting point (again, very obvious)
• Solid-gas is called sublimation
• Gas-solid is called
• Liquid-gas is called evaporation
• Gas-liquid is called condensation

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Did you know that the temperature at which a
substance undergoes a phase change is
proportional to its mass?

• You do now!
• The heat required or given off by a reaction
  (denoted Q) is equal to the product of the mass
  (denoted m, obviously) of the substance and its
  latent heat (denoted by L)
• Thus QmL

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A Fact

• When the temperature is too high for a human,
  their body releases sweat. Since the evaporation
  of sweat requires energy, heat energy is absorbed
  through the process.

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Latent Heats Table

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EXAMPLE TIME!
Before jumping into a problem, Specific Heat
Should be reviewed c specific heat the amount
of heat required to raise a temperature of one
gram of a substance Q mc change in T Q is the
amount of heat required to change the temperature
of a given material.
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• Calculate the amount of heat required to
  completely convert 50 g of ice at 0 ºC to steam
  at 100 ºC. The specific heat capacity of water is
  4.18 kJ.kg-1.K-1. The specific latent heat of
  fusion of ice is 334 kJ.kg-1, and the specific
  heat of vaporization of water is 2260 kJ.kg-1.
• Answer
• 1. Heat taken up for converting iceat ºC to
  water at ºC
• mass of water x latent heat of fusion 0.050
  (kg) x 334 (kJ.kg-1) 16.7 kJ
• 2. Heat taken up heating the waterfrom 0 ºC to
  the boiling point, 100 ºC

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• mass of water x specific heat capacity x
  temperature change 0.05 (kg) x 4.18 (kJ.kg-1.
  K-1)x 100 (º K) 20.9 kJ
• 3. Heat taken up vaporizing thewater
• mass of water x latent heat of vaporization0.05
  (kg) x 2260 kJ.kg-1 113 kJ
• Sum 16.7 20.9 113 150.6 kJ (151 kJ)

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Heat Transfer Conduction

• Key Facts
• Heat is transferred from one place or body to
  another in 3 different ways
• conduction, convection, and radiation.
• Heat conduction in many materials can be
  visualized as the result of molecular collisions.
  In metals, the collision between free electrons
  are scrutinized.

A visual of conduction As the heated atom moves
faster, it bumps into the other atoms and make
them move faster and they get heated and it is
contagious..sort of like the domino effect. Ex
A hot panouch!
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• Conduction only takes place if there is a
  difference in temperature. The rate of heat flow
  through a substance is proportional to the
  difference in temperature between its ends. It
  also depends on the size and shape of the object
• The rate of heat flow by conduction is
• change in Q kA T1 T2
• change in t l
• A is the cross-sectional area of the object,
  l is the distance between the two ends, Ts are
  the temperatures and k is the proportionality
  constant (thermal conductivity).

Fourier's Law the minus sign is the heat
transfer to the colder temperature.
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Hmm.interesting. Highest heat flow at mid-ocean
ridges, lowest at old oceanic crust
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• Thus, the rate of heat flow ( J/s) is directly
  proportional to A and the temperature gradient.
  The k differs for various substances. (When the k
  is large for a substance, it is said to conduct
  heat rapidly metals good conductors.)
• Substances for which k is small are good
  insulators.
• Ex Tile (better conductor) vs. rug.
• Thermal Resistance l/k
• (for building materials)

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EXAMPLE TIME!

• Calculate the rate of heat flow through a glass
  window 2.0 m X 1.5 m in area and 3.2 mm thick, if
  the temperatures at the inner and outer surfaces
  are 15 degrees Celsius and 14 degrees Celsius,
  respectively.
• So.
• A 2.0 m X 1.5 m 3.0 m2,
• L 3.2 X 10-3, the k is picked from the chart
  .84 J/smC
• 790 J/s
• BOILING PASTA?
• Faster, but no increase in temperature,