Thermal Conductivity Challenge Problem

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Thermal Conductivity Challenge Problem

• AP Physics B
• Kernion

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H(Q/?t) kA?T/L
1 BTU 1054 J
1 cm 2.54 inches
180F Change 100C Change
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R-value of insulation

• R Resistance to heat flow
• The higher the R-value the greater the
  insulating power
• All insulation with the same R-value does the
  same job...regardless of the manufacturer.
• R L/k
• In the USA...
• L (thickness) is measured in feet
• k is measured in BTU/hrftF

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R-Value L/k
Note The Metric R-value unit is Km2/W)
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Home Insulation (in purple)
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Examples

• 6 inches of Fiberglass has R-19
• 2 inches of Polyurethane Foam has R-12
• R-values are additive
• Therefore 12 inches of Fiberglass has R-38
• 6 inches of Fiberglass backed with 2 inches of
  Polyurethane Foam has R-31

Polyurethane foam
Fiberglass Batting
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R-Value Data
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The Challenge Problem
 A slab of insulation material whose length and
width are both 2.6 m and whose thickness is 0.3 m
is made from a piece of Pink FOAMULAR rigid foam
insulation material manufactured by
the Owens-Corning Corporation.  This material is
used by homeowners to insulate the EXTERIOR walls
of the home.  The R-value of a 1.0 inch thick
piece of this material is 5.0.   If the
temperature on one side of the slab is 120ºC
while the temperature on the other side is only
30ºC, how many days will it take for 5.5 x 1012
Joules of heat to flow through the slab?

• CLUES
• Use the R-value to determine the thermal
  conductivity constant. To
• do this you MUST use many conversion factors.
• 2. Use the heat transfer equation to determine
  the time in seconds, then
• convert the time into days.