IPC 07: Heat

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IPC 07 Heat

• Temperature measures the average kinetic
  energy of the atoms in an object. (a measurement
  of an objects hotness)

TAKE
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Thermometer

• A thermometer uses expansion and contraction of a
  liquid, usually colored alcohol or mercury to
  measure temperature.

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Fever Rhyme

• The following rhyme still scares people101 the
  fevers begun, 102 you're feeling quite
  blue,103 you get wobbly at the knee,104 you are
  at death's door,105 you're more dead than alive,
  106 you are crossing the Styx, 107 you are an
  angel in heaven!
• Again, these are old tales.

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Temperature is measured in degrees Celsius,
degrees Fahrenheit, Kelvin Celsius is used by
most of the worlds citizens.
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Temperature Conversions

• ºC?K Kelvin ºC 273
• K?ºC ºC Kelvin 273
• ºF?ºC ºC (ºF 32) ? 1.8
• ºC?ºF ºF (1.8 x ºC) 32

ºF?ºC?K
K?ºC?ºF
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Example 1 Convert ºF to ºC

• A meteorologist forecasts a high temperature of
  28 ºF. What is the predicted high temperature in
  degrees Celsius?

ºC (ºF 32) ? 1.8 (28-32)/1.8 -2.22 ºC
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Example 2 Convert ºC to ºF

• A thermostat is set to keep the temperature of a
  room at 20 ºC. What is the thermostat setting in
  degrees Fahrenheit?

ºF (1.8 x ºC) 32 (1.8 x 20) 32 68 ºF
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Example 3 Convert ºF to K

• The temperature outside is 87 ºF. For scientific
  purposes, what is the temperature in Kelvin?

Takes 2 steps ºC (ºF 32) ? 1.8 (87-32)/1.8
30.6 ºC K ºC 273 30.6 273 303.6 K
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Temperature and Bridges

• Most solids expand due to an increase in
  temperature.
• Ex a 100 meter long bridge could be up to 10 cm
  longer on a hot day than on a cold day.
• Therefore, civil engineers use expansion joints
  to prevent damage to bridges.

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Heat

• Heat is the flow of thermal energy from one
  object to another.
• Thermal Energy Units
• SI unit is Joule. (1 calorie 4.184 J)
• Calorie -- the amount of thermal energy that must
  be added to water to change the temperature of
  one gram of water by one degree Celsius .
• Btu (British Thermal Unit)-- the amount of
  thermal energy that must be added to water to
  change the temperature of one pound of water by
  one degree Fahrenheit.

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Thermal energy always flows from the hotter
object to the colder object, as kinetic energy is
transferred when particles collide.
Which direction does the thermal energy flow in
each picture below?
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Interpreting a Heat Graph
50ºC

• Starting Temp?
• How long did it remain constant?
• Final Temp?
• How long was the substance heated before it
  reached 600ºC?
• How much did the temperature rise between 125 and
  200 seconds?

120 s
1220ºC
150 s
1100º - 50º 1050ºC
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Specific Heat

• The specific heat of a substance is the amount of
  heat required to raise the temperature of a unit
  mass of a substance by one degree.
• Water has a high specific heat of 1cal/gC which
  is why it is used as a coolant. It takes more
  energy to increase the temperature of water than
  for other substances.

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Specific Heat of Water is 1cal/gC
energy
cal
Specific heat of water
1
g C
Unit of temp.
mass
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Formula for Specific Heat

• Heat transferred (cal) objects mass (g) x
  temperature change (C) x specific heat (cal/gC)
• QH m ?T Cp
• Can also be Joules

QH
?T T2 T1
m
?T
Cp
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Ex. 1 Tea Kettle

• A tea kettle holds 500 grams of water. How much
  heat is needed to increase the temperature of the
  water from 20 ºC to 100 ºC? (specific heat of
  water is 1 cal/gºC).

Q m ?T Cp (500)(100-20)(1) 40,000 cal
40,000 cal x 4.184J/cal 167360 J
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Ex. 2 Specific Heat

• What is the specific heat of an unknown substance
  that absorbs 5382 calories of heat when a 750 g
  sample increases the temperature from 30C to 76
  C?

Cp Q ? (m ?T ) ?T 76-30 46 C Cp 5382 ?
(750 x 46 C ) Cp 0.16 cal/gC Unknown is IRON.
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Methods to transfer heat

• Conduction heat transfer by direct contact.
• Convection heat transfer by movement of fluid.
  (liquid gas)
• Radiation heat transfer by electromagnetic
  waves.

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Heat Energy Transfer
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What type of heat transfer is occurring and why?
Radiation- (heat)
Conduction- (contact)
Convection- (fluids)
Insulation- (cloth)
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Conductors Insulators Video
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Which conducts heat better?Gases, Liquids, or
Metals?

• Gases are poor conductors of heat because their
  molecules are so far apart resulting in low
  collision rates.
• Liquids some nonmetallic solids are better
  conductors of heat than gases because their
  molecules are closer together.
• Metals make the best conductors of heat because
  they have free electrons which are able to move
  quickly.

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Insulators

• Insulators are substances that do not conduct
  heat easily.
• Examples Styrofoam and cloth

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Convection is a transfer of heat by movement of
fluid.

• Two bodies with different temperatures separated
  by fluid.
• Warm fluid rises, the cool fluid sinks.
• The result is a circulation of fluid and transfer
  of energy due to temp. diff.

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Lava Lamps

• The "lava" is basically a specially compounded
  wax. When heated from below by a 40-watt bulb, it
  expands until it becomes less dense than the
  liquid above, causing it to rise. When it gets to
  the top of the globe, the wax cools and starts to
  sink again, and the cycle repeats. Convection
  currents in the liquid presumably add to the
  effect.

Click on Lava Lamp!
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Convection Video
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Natural Convection

• It takes place in the Earths atmosphere and is
  what causes convection currents and wind.
• The sun heats the Earths surface, energy is
  transferred from the surface to the air, warm air
  rises, and cool air falls.

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Atmospheric Convection

• Atmospheric Convection
• Warm air rises and cool air falls.

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Forced Convection

• It uses a fan or pump to create a pressure
  difference in a heated or cooled fluid, which
  forces the fluid to circulate.
• Ex. The human body when you exercise The heart
  is the pump that circulates the blood. When you
  workout, your heated muscles heat the surrounding
  blood, the warmed blood is pumped to the skins
  surface which is now cooled by convection.

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• Forced Convection
• Hot room air is forced outside, while cooler air
  replaces it.

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Radiationis a transfer of energy by
electromagnetic waves.

• All objects radiate energy in the form of
  electromagnetic radiation.
• Radiation transfers energy from one body to
  another through empty space it does NOT use a
  medium.

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A hot object radiates more energy per unit time
than an identical cooler object.
Radiation is the most common means of energy
transfer in the universe. Ex. Energy from Sun to
planets
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Absorption and Emission

• Objects that are good absorbers of radiation are
  also good emitters of radiation.
• Ex A black road (will cool quickly)
• Ex A white sandy beach (will cool slowly)

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Radiation Video
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Global Warming

• Because our atmosphere is relatively thin, it
  allows much of the radiation to be reflected back
  into space.
• Global warming occurs when a thickened atmosphere
  reabsorbs too much radiation causing an increase
  in the earths temperature.
• Due to the high amounts of CO2 (released from
  the combustion of fossil fuels) which traps heat.

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The Oceans Effect Weather

• During the daytime, the oceans keep the Earth
  cool, and at night, they keep it warm. The
  difference in specific heat means the variation
  in temperature over land is much larger than the
  variation in temperature over water.
• Typically, the change in temperature near the
  water between day and night is only 10F. This is
  also why temperatures tend to vary less in
  coastal areas compared to inland areas.