Download Temperature difference Types of material

Thermal
conductivity
What factors affect how quickly thermal
energy travels through a substance?
Prior experience
We consider water that is
70F to be cold, but 70F air
to be comfortable.
You can burn your fingers if
you touch 250F metal but not
if you touch 250F bread.
Explain the difference.
Prior experience
We know we feel cold
when thermal energy
leaves our bodies quickly.
Why do additional layers of
clothes slow down how
quickly thermal energy
leaves your body quickly.
Prior experience
If you put an ice cube in a protector and
put the protector on a lukewarm water,
the ice cube melts slowly.
If you the protector in boiling water, the
ice cube melts much more quickly.
Explain the difference.
Extend your thinking
Sometimes it is
advantageous to have
things cool down quickly,
e.g., computer hard drives,
e.g., car engines,
e.g., back side of refrigerators or
air conditioners,
etc.
Extend your thinking
What characteristics do radiators,
computer heat sinks, and the back
of refrigerators have in common?
Propose an explanation for how
they transfer thermal energy
quickly.
Patterns
The rate at which energy is transferred seems
to depend on:
1) Area: cooling fins
2) Thickness: layers
3) Temperature difference
4) Types of material: water vs. air
Summarize relationships
As area gets larger,
energy transfers faster.
Area,
Rate,
𝐴
𝑄/𝑡
Summarize relationships, part 2
As substance gets thicker,
energy transfers slower.
Thickness,
Rate,
𝑙
𝑄/𝑡
Summarize relationships, part 3
As temperature difference gets
larger, energy transfers faster.
Technical vocabulary:
A difference based on position is
called a GRADIENT and is
symbolized with an inverted delta, 
Temperature
Rate,
gradient,
𝑄/𝑡
𝛻𝑇
Summarize relationships, part 4
Some substances transfer energy
more quickly than others.
To quantify how quickly different
substances transfer thermal
energy, carry out experiments
(or, easier, look it up).
Thermal conductivity constant
The thermal conductivity constant
tells you the rate at which thermal
energy flows through a one square
meter of a substance that is one meter
thick and has a 1C temperature
gradient between its sides.
It is symbolized with the Greek letter
kappa, 

Rate,
𝑄/𝑡
Mathematical models
Just as we created a conceptual
model to help explain how energy
is transferred, we can create a
mathematical model to predict
how quickly it transfers.
Mathematical models

𝑄/𝑡
𝐴
𝑄/𝑡
𝛻𝑇
𝑄/𝑡
𝑙
𝑄/𝑡
𝑄
 𝐴 𝛻𝑇
=
𝑙
𝑡
Application
To see how (and why!) to apply
this model, work through the
examples and practice questions
on the problem set.