Heat Transfer

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Heat Transfer

• Grade Nine Science
• by
• Rodney Hodder (B.Sc, B.Ed, M.Ed IT)

Text Science Directions 9, Unit Three
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? Rodney Hodder (2005)
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1.Heat as a form of Energy (p.118, 122 123)
Heat is a form of the energy associated with the
motion of particles within a particular amount of
substance.

• Describe heat as a form of energy.

Heat is a form of energy called thermal energy
that is transferred from one substance to
another. Heat is measured in Joules or Calories
by a device called a calorimeter. Temperature is
a measure of how fast the particles in a
substance are moving. Temperature is measured in
Degrees Celsius or Farenheit by a thermometer.
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2. Heat Transfer (p.122-123)
A. Heat travels from substances with high energy
to substances with low energy.
Heat can be transferred from one substance to
another.

• Heat always moves from warmer objects to colder
  objects.
• In essence, colder objects suck the heat energy
  from warmer objects.

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3. Heat Conduction (p. 124-127)
Conduction is one means of heat transfer. It is
the process of heat passing through a substance
by direct collision of particles.

• Define Conduction (refer to diagram p.124)
• When heat travels through a substance by direct
  collision of particles similar to a domino effect.

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Heat Conduction Cont.

• Conduction is a point-by-point process of heat
  transfer. If one part of a body is heated by
  direct contact with a source of heat, the
  neighboring parts become heated successively.
  Thus, as shown in the diagram, if a metal rod is
  placed in a burner, heat travels along the rod by
  conduction.
• This may be explained by the kinetic theory of
  matter (click on Behavior of Matter Three
  Phases). The molecules of the rod increase their
  energy of motion. This violent motion is passed
  along the rod from molecule to molecule.

• Conduction works best in solids, not so well in
  liquids of gases due to the lack of contact
  between the particles.
• Materials that allow heat to pass through easily
  are called heat conductors. Materials that
  resist the movement of heat are called insulators.

View Animation
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Conduction Cont.

• In considering the flow of heat by conduction, it
  is sometimes helpful to compare the flow of heat
  to the flow of electricity.
• The temperature difference can be thought of as
  the pressure, or voltage, in an electrical
  circuit. The ability of a substance to transfer
  heat (its thermal conductivity) can be compared
  to electrical conductivity.
• When the temperature difference (or voltage)
  between two points is great, the driving force to
  move heat (or current) is high. The quantity of
  heat (or current) transferred will depend upon
  the temperature difference (or voltage
  difference) and the resistance to the flow of
  heat (or current) offered by the conductor.

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3. Heat Conduction Cont.

• Measure thermal (heat) conductivity of various
  substances.
• Lab Conductometer (p.121)
• Relate the thermal conductivity of a material to
  its function.
• Thermal Conductivity refers to the ability of a
  substance to conduct heat. (refer to chart
  p.126)
• Generally speaking, metals are good heat
  conductors whereas non-metals are good heat
  insulators.

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4. Convection (p.128-134)

• Define Convection
• Convection is the transfer of heat through the
  circulating motion of particles

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4. Convection Cont.

• Convection Currents
• In liquids and gases convection currents will
  occur if the cooler substance is above the warmer
  substance.

Convection in a liquid or gas.
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4. Convection Cont.

• Examples of Convection

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5. Radiation of Heat (p.136 -140)

• Radiation is defined as the transfer of heat in a
  wave-like form.

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5. Radiation Cont.

• Radiation of heat can travel through empty spaces
  (vacuums) and transparent materials without
  heating the materials in between.

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5. Radiation Cont.

• Properties of Radiant Energy
• May travel in a vacuum (empty space).
• This is unlike conduction and convection which
  require the presence of molecules to carry the
  energy.
• Ex The suns energy reaches the Earth through
  empty space.
• Travels at extremely high speeds.
• For example, light travels in a vacuum at 300
  million metres/sec.
• Only travels in a straight line.
• For example, flashlights only shine in a straight
  line and not around corners.

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5. Radiation Cont.

• Examples of radiant heat energy.
• Sunlight
• X-rays
• Radio waves

• Infrared (heat waves)
• Ultra-violet (may cause skin cancer)
• Lasers (CD/DVD players)

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5. Radiation Cont.

• Lab Effect of Radiation on light vs dark
  objects. (p. 137)
• Effects of reflection, transmission, and
  absorption on materials. (p.139)

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5. Radiation Cont.

• Reflection of Heat.
• Heat reflects off smooth/shiny surfaces and the
  energy bounces off without being absorbed.
• Ex mirror, shiny metals

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5. Radiation Cont.

• Absorption of Heat
• When the radiant energy is transferred to the
  particles in the object resulting in a
  temperature increase in the object.
• Ex Pavement, food in a microwave oven

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5. Radiation Cont.

• Transmission of Heat
• When the radiant energy passes right through and
  the material does not heat up.
• Ex Glass, transparent plastics

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5. Radiation Cont.

• Radiation Emitters vs. Absorbers
• Dark objects are good absorbers of radiant
  energy and are therefore good emitters of radiant
  energy.
• Ex Black T-Shirt on a hot summer day.
• Light objects are not good absorbers of radiant
  energy because they reflect the energy, therefore
  they are not good emitters either.
• Ex Light T-Shirt on a hot summer day.

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6. Controlling Heat Transfer (p.141-149)

• Heat transfer can be slowed by the use of
  insulating materials

• Insulating materials reduce the flow of heat.
• Thermal insulation consists of materials that
  conduct heat poorly by decreasing the flow of
  heat from a hot region to a cooler one.
• For example, insulation around hot-water heaters
  or steam pipes cuts the heat loss to the
  surrounding room, while insulation in the walls
  of a refrigerator reduces the flow of heat from
  the room into the cold space in which the food is
  stored.
• There is no perfect insulator, but a thin layer
  of air resists the flow of heat about 15,000
  times better than a good metallic conductor of
  the same thickness. Many good insulators are made
  of nonmetallic materials filled with tiny air
  spaces. These air pockets must be small
  otherwise movement of the air by convection
  currents may transport heat across the space.
  This tends to occur when an air layer becomes
  thicker than about 1/4 inch (0.6 centimeter).

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6. Controlling Heat Transfer Cont.

• Insulators
• Insulators are materials that help to prevent
  heat transfer.

http//multimedia.climatechange.gc.ca/oee/Insulati
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6. Controlling Heat Transfer Cont.

• Lab Measuring the effect of materials on heat
  transfer.
• Graph the results of the lab.
• Examples of insulating materials. (P. 144 in
  text.)
• Fibreglass, polyurethane, drywall (gypsum)

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6. Controlling Heat Transfer Cont.

• Importance of Insulation
• In homes insulation keeps the heat in by slowing
  the conduction and radiation of heat through the
  walls.

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http//multimedia.climatechange.gc.ca/oee/Moisture
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http//multimedia.climatechange.gc.ca/oee/AirSeali
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http//multimedia.climatechange.gc.ca/oee/Renovati
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6. Controlling Heat Transfer Cont.

• RSI Values
• R Resistance to heat transfer
• SI System International (universal system of
  measurement)
• RSI values refers to an international system that
  describes a materials resistance to heat
  transfer.
• Good insulators have high RSI values and poor
  insulators have low RSI values.

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6. Controlling Heat Transfer Cont.

• Calculating RSI values
• RSI value RSI/cm x Thickness (cm)
• note the RSI/cm for each material can be found
  in table 3-2 on page 144 of the text.
• Ex What is the RSI value of 5cm of
  polystyrene blue insulation?
• RSI 0.35 x 5cm
• (5 cm poly blue) 1.75

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6. Controlling Heat Transfer Cont.

• Calculating RSI values
• Ex 2 Calculate the total RSI value for a wall
  that contains 8cm of brick, 3cm of vermiculite
  and 1.5cm of gypsum board.
• RSI 0.014 x 8 0.112
• (brick)
• RSI 0.16 x 3 0.48
• (Verm)
• RSI 0.035 x 1.5 0.0525
• (Gypsum)
• total 0.6445 or 0.64

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Review

• Concept Sheet
• Read text sections indicated in notes.
• Read focus on p.164
• P.164165 1, 2, 3, 4, 5, 6, 7, 8, 14, 18, 21
• Review Sheet Heat
• Review terms from word search puzzle and
  crossword.
• Review your labs