HEAT

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HEAT TemperatureChap.10

• (This is a hot presentationneed I say more?)

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10-1 Heat Energy

• 1798 - Count Rumford (Ben Thompson) Drilling out
  cannon barrels causes water in them to boil.
  Ie. Drilling heat
• Drilling work done energy heat
• Thus, heat is a kind of energy
• 40 yrs. later, J.P. Joule - heat is related to
  motion.
• Amount of heat is related to amount of motion.
• Example rubbing your hands together generates
  warmth.and the faster you rub the warmer they
  are.

Energy heat are related.
(ability to do work)
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• Scientists at the time of Joule knew that energy
  is needed for motion. They also knew matter is
  made of particles (molecules)so they correctly
  decided that heat is a kind of energy, and is
  somehow related to motion of particles.
• Important random note Cold is just the absence
  of heat energy.
• Cold appears to transfer into one place from
  another because heat drains OUT of it.
• Heat may be transferred from one place to another
  in three waysConduction, Convection, and
  Radiation.

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Molten Steel flows from a crucible in a steel
plant...

• Heat energy by radiation, conduction,
  convection???

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Temperature and Heat

• Temperature - not the same thing as heat.
• Temp. is a measure of average kinetic energy.
  (The higher the temp., the faster the molecules
  are moving, on average).
• Thermometer - instrument for measuring
  temperature.
• As temp. rises, liquid is forced by
  increased molecular motion to expand.
• It then must move up the scale and
  mark the temperature for the reader.

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Scales of temperature Celsius, Kelvin and
Fahrenheit (0F). (F. is not used often in
science)
0C to K 273 K to 0C -273
(-273 deg. C)

• Kelvin is another scale of temp. used in science.
• Meas. in Kelvins (K).
• Same degrees as 0C, but zero moved down 273
  degrees.
• 0 kelvin (0 K) is lowest possible temp.
  (absolute zero) where all molecular motion
  stopsno energy.

• Celsius scale of temp. is most common.
• Measured in degrees Celsius ( oC ).
• Zero deg. Celsius is the freezing/melting pt. of
  water
• 100 deg. Celsius is the boiling pt. of water.

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Temperature Conversions

• Celsius to Kelvin
• add 273 to the Celsius temperature
• Kelvin to Celsius
• subtract 273 from the Kelvin temperature
• Formula for both of these
• T t 273
• (T is Kelvin temp., t is the Celsius temp.)

• Celsius to Fahrenheit
• TF 9/5t 32
• (where TF is Fahrenheit temperature and t is the
  Celsius temperature)
• Fahrenheit to Celsius
• t 5/9(TF - 32)

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• Example The highest atmospheric temperature ever
  recorded on earth was 570C. What is this in 0F
  and K?
• 0C to 0F gtgtgt
• TF 9/5t 32
• (9/5 x 57.8) 32
• 104 32 1360F

0C to K gtgtgt T t 273 57.8 273
331K
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• When you touch an object warmer than you, the
  molecular faster movement is transferred to
  youyou feel the change in energy level. You
  feel nothing (temp-wise) when touching your own
  skin.
• Heat, then, is the transfer of particle motion
  energy from one object to another.
• Heat may be transferred from place to place in 3
  ways Conduction, Convection, and Radiation.

Heat Transfer 10-2
For heat energy to be transferred, there must be
a difference in energy level (temp.). The
greater the difference the more energy will
transfer as heat.
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Conduction - (solids, liquids, gases)
Transfer of energy as heat between particles as
they collide within a substance or between two
objects in contact with each other. Insulators
dont conduct very well, conductors do.

• Heat is moved by direct contact between
  molecules.
• Faster moving molecules bang into slower ones and
  give them energy (speed them up).
• Example you grab a metal spoon to stir your soup
  and burn your hand. Heat is transferred directly
  from soup to spoon to hand b/c they are in direct
  contact.
• Heat conductors - move heat well (metals,silver,
  copper)
• Insulators - dont cond. easily (glass, wood,
  plastic, rubber)

Where is conduction, or the lack of it,
important here?
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Convection - (liquids, gases)

• Molecules move in currents.
• Warmer part of liquid/gas expands, becomes less
  dense, and thus rises.
• This creates currents that carry heat.
• Example Air currents in Earths atmosphere are
  caused by convection currents, and...
• Air balloons rise because the heated air in them
  is lighter than the surrounding air.

Transfer of energy by the movement of fluids with
different temperatures.
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Radiation

• Heat energy is transferred through space via
  electromagnetic waves, directly.
• Example our Suns rays heat the earth with
  radiant energy. There is no need for molecules
  in between to transfer the heat energy.

Heat from the sun is transferred directly thru
space via infrared rays.
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Intermission
Abandon all hope, ye who enter here. -Dante
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Measuring Specific Heat

• Heat and Temp. arent the same thing.
• Temperature is a level of energy heat is an
  amount
• Heat increase is shown by increase in temp., a
  decrease shows a removal of heat.
• Unit of heat is the Joule or kJ. Specific heat
  is in J/kgK
• Amount of heat to change to a given temp will
  depend on mass and type of material.
• Specific heat - amount of energy transferred as
  heat or ability to absorb heat...see p.336energy
  to raise the temp. of 1kg of a substance by 1K.

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Energy (mass) x (change in temp) x (specific
heat)
(Heat Gain/loss)

• Example Find heat lost by 10g copper if its
  cooled from 35oC to 21oC.
• 35 - 21 -14 deg. C. or change of 14 K
• Ht. Lost (10g) (-14K) (385)
• Heat loss 53,900 kJ of energy
• Thus with the specific heat of a substance (kind
  of the capacity of something to hold heat), we
  can find exact amounts of heat energy given a
  mass and change of temperature.

Specific Heat from table on p.336.
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Thermal Expansion - expansion of a substance due
to heat increase.

• Most substances (for solids, liquids, and gases)
  expand when heated and contract when cooled.
• Exception 4 deg. C to 0 deg. C water
  expandsand ice is less dense than water (floats)
• Generally, as heat/temp.increase, molecules have
  more energy move faster get farther apart.
• Cooling has an opposite effect.
• Thermal exp. allows thermostats to work - 2
  metals (bimetallic) in a strip expand at
  different rates, causing the strip to bend
  (changing switch). (p.326)

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Bridges have expansion links...
Sidewalks have seams to prevent buckling
because of expansion in hot weather.
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10-3 Heating/Cooling systems

• Heating system-device or process to move energy
  to a substance to raise temperature.
• Work is done, moving heat energy or being
  tranformed into heat energy.
• Ex. Food energy into heat energy in blood.,
  heated water or air tranfers heat energy in
  central heating systems (from burning fuel, sun,
  etc. Solar heating works similarly). See
  p.340-342 for pictures.
• Useable energy is generally lost in these
  processes (not 100 efficient).
• Use of insulation (p.343 for R-values) can slow
  unwanted heat transfers (less heat/cooling
  needed).

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• Cooling systems-device that transfers energy as
  heat out of an object to lower its temperature.
• Evaporating substances cool things, so used to
  absorb heat energy in the object.
• Carried in refrigerant to outside of object/area
  and condensed (releasing heat to surroundings.
• Refrigerators, heat pumps, air conditioners use
  this principle (see next slide and p.339,344-345)
  to remove heat from an area and dump it in
  another, thereby cooling the area.
• P.346, Energyguide federal labels on
  heating/cooling appliances give an idea of their
  efficiency (and thus cost) of operation.

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Inside building
Fan
Gaseous refrig.
Outside
Liquid refrigerant
fan
Something that easily evaporates and condenses is
used in A/C to move energy to the outside. When
liquid evap., it absorbs heat energy from air
inside. When gas forced to cond. it releases heat.
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The End is Near...
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The End