Understanding Heat Transfer, Conduction, Convection and Radiation

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Understanding Heat Transfer, Conduction,
Convection and Radiation
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Heat Transfer

• Heat always moves from a warmer place to a cooler
  place.
• Hot objects in a cooler room will cool to room
  temperature.
• Cold objects in a warmer room will heat up to
  room temperature.

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Question

• If a cup of coffee and a red popsicle were left
  on the table in this room what would happen to
  them? Why?
• The cup of coffee will cool until it reaches room
  temperature. The popsicle will melt and then the
  liquid will warm to room temperature.

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Heat Transfer Methods

• Heat transfers in three ways
• Conduction
• Convection
• Radiation

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Conduction

• Conduction is transfer of heat through direct
  contact no matter is transferred.
• On a molecular level, hotter molecules are
  vibrating faster than cooler ones.
• When they come in contact, the faster moving
  molecules bump into the slower moving molecules
  and heat is transferred!
• This is how heat is transferred to your finger if
  you touch a hot stove!

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Thermal conductors

• Think of a frying pan
• The bottom heats up first, but even though the
  handle never touches the flame it gets hot. Why?
• Because the pan is made of metal which is a good
  thermal conductor (conducts thermal energy well)
• Tile floors feel colder to bare feet then wooden
  floors because they conduct the heat away from
  your feet.

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Thermal Insulators

• If one left a wooden spoon in a pan of sauce you
  would be able to pick that up, for the spoon
  remains cool. Why?
• Wood is a poor conductor, but a good thermal
  insulator (conducts thermal energy poorly).
• Air is another great insulator. Double paned
  windows contain air to slow down the release of
  heat in the winter and the input of heat in the
  summer.
• Other Examples

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Convection

• Transfer of thermal energy when particles of a
  fluid move from one place to another.
• What are fluids?
• Gases and liquids
• When fluids are heated, currents are created.
• This is because the individual molecules that
  come in contact with a hot surface expand, become
  less dense, and rise. (this is how hot air
  balloons work!)
• When this happens, the cooler molecules circulate
  down and take their place, and a cycle is
  established.
• This circulation is called a convection current
  (when a fluid circulates in a loop as it
  alternately heats up and cools down)

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• An example of this can be observed in the air
  currents that are created in a room with a
  radiator against one wall.
• The air in contact with the radiator rises, moves
  across the ceiling to the far wall, sinks, and
  then comes back to the radiator across the floor.
• Convection currents are important in many
  naturals cycles.
• Ocean currents, weather cycles, and movements of
  how rocks.

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Why is it windy at the seaside?
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Cold air sinks
Where is the freezer compartment put in a fridge?
Freezer compartment
It is warmer at the bottom, so this warmer air
rises and a convection current is set up.
It is put at the top, because cool air sinks, so
it cools the food on the way down.
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Radiation

• Radiated heat energy travels as an
  electromagnetic wave through space.
• Electromagnetic waves travel at the speed of
  light, which is 300,000,000 meters per second.
• Sometimes these waves are in the visible part of
  the spectrum, like when something is red hot.
  You can see how hot it is, but you can also feel
  it from a distance, as your skin absorbs the
  energy.
• Examples Feeling the heat from a charcoal grill,
  heat lamps, and of course the sun.
• ALL OBJECTS RADIATE ENERGY. AS ITS TEMPERATURE
  INCTREASES THE RATE OF RADIATION INCREASES

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The third method of heat transfer
How does heat energy get from the Sun to the
Earth?
There are no particles between the Sun and the
Earth so it CANNOT travel by conduction or by
convection.
RADIATION
?
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Thermodynamics

• The study of conversions between thermal energy
  and other types of energy
• The study of thermodynamics is about energy flow
  in natural systems
• The Laws of Thermodynamics describe what is known
  about energy transformations in our universe

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First Law of Thermodynamics

• States that energy is conserved.
• Law of conservation of energy
• Energy is neither created nor destroyed but can
  be transformed from one form to another
• If energy is added into a system it can increase
  thermal energy or it can do work to the system,
  but energy is being conserved.

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Second Law of Thermodynamics

• Thermal energy flows spontaneously only from
  hotter to colder objects
• Thermal energy CAN flow from colder objects to
  hotter objects only if WORK is done on the system

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• Heat engine converts heat into work-NEVER 100
  efficient
• Creates waste heat that is put into the
  environment
• Spontaneous changes always make a system less
  orderly. Disorder is called entropy.
• Every energy transformation or transfer results
  in an increase in the disorder of the universe

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Third Law of Thermodynamics

• Absolute Zero cannot be reached
• Complete absence of all movement is absolute
  zero.
• Never been reached even in controlled experiments