PAGE EDITOR(S): Georges Cuissart de Grelle, David Iparraguirre, Javier Henriquez, Najib Wahab

TEMPERATURE AND ...

Miami_Heat_bandana.jpg
















Good Thermal conductor will transfer two things:
1) Temperature: a measure of the average kinetic energy of the molecules in a substance
2) Heat (Q): a transfer of energy due to a temperature difference (The heat is also a super awesome Miami basketball team)
*Heat energy is measured in joules.

Three Main Methods of Energy Transfer as Heat
1) Conduction: A transfer of energy as heat through physical contact between two substances at different temperatures.
2) Convection: A transfer of energy as heat due to the bulk movement of a fluid.
3) Radiation: A transfer of energy as heat due to electromagnetic waves.



Examples of each:
1) Conduction: Heating up a pan on a stove, like if you were boiling water. The stove heats up and since the pan is resting on the stove, through conduction (physical contact), the pan heats up.
2) Convection: Cooking pasta in boiling water. The stove heats up the pasta by boiling the water, through means of convection.
3) Radiation: A microwave...

Rate at which energy transfers as heat through conduction:
Heat=
hcon1.gif



Cross multiply the time (t) so you get:
Heat (Q) = Q/t = KAexternal image delta.gifTt/L

k=Thermal conductivity constant
A=Area
external image delta.gifT= Change in temperature t= Time
L=Length
htcd1.gif










Heat flows from hot to cold. The faster moving molecules excite the slower one, getting them to move faster...


nw0554-nn.jpg
Heat is energy in transit from hot to cold.
(a) Heat flows from the hotter coffee cup to the colder hand.
(b) Heat flows from the warmer hand to the colder glass of ice water.


























Notice the Heat transfer from Dwayne Wade to the crowd through means of conduction, convection, and radiation, all at the same time... The physical contact between the players, the hot air from D-Wade's body and from the screaming Heat fans, and the radiation from Dwayne for being awesome...





Thermal Expansion

Coefficients of Thermal Expansion for Solids and Liquids


Coefficient of Thermal Expansion (C°)-1
Substance
Linear (α)
Volume (β)
Solids


Aluminum
23 × 10-6
69 × 10-6
Brass
19 × 10-6
57 × 10-6
Concrete
12 × 10-6
36 × 10-6
Copper
17 × 10-6
51 × 10-6
Glass (common)
8.5 × 10-6
26 × 10-6
Glass (Pyrex)
3.3 × 10-6
9.9 × 10-6
Gold
14 × 10-6
42 × 10-6
Iron or steel
12 × 10-6
36 × 10-6
Lead
29 × 10-6
87 × 10-6
Nickel
13 × 10-6
39 × 10-6
Quartz (fused)
0.50 × 10-6
1.5 × 10-6
Silver
19 × 10-6
57 × 10-6
Liquids


Benzene

1240 × 10-6
Carbon tetrachloride

1240 × 10-6
Ethyl alcohol

1120 × 10-6
Gasoline

950 × 10-6
Mercury

182 × 10-6
Methyl alcohol

1200 × 10-6
Water

207 × 10-6



Linear Expansion
-
Over small temperature ranges, the fractional thermal expansion of uniform linear objects is proportional to the temperature change
- The linear expansion of a solid depends on its initial length, temperature change, and the type of material it is made from.
- The coefficient of linear expansion is different for different materials

Equation:

external image u4a2_2.gif

external image delta.gifT= change in temperature Lo= initial length
external image delta.gifL= change in length
external image alpha.gif= coefficient of linear expansion


Area Expansion
external image aexpan.gif
or

ΔA = A0 2α ΔT
or

external image texp6.gif




Volume Expansion

external image vexpan.gif
or

ΔV = βVT

or


external image texp8.gif



* 3α=β


Thermal Expansion Practice Problems

  1. A steel bridge is built in several segments, each 20 m long. The gap between segments is 4 cm at 18 ℃. What is the maximum temperature that the bridge can manage before buckling?
  2. How much taller is the Eiffel Tower on the hottest day of the summer (25 ℃) than the coldest day of the winter (2 ℃)? The tower is 324 m tall measured from the top of the flagpole. Assume the tower is built of structural steel.

external image ExpansionJoint.gif
Most bridges are built with gaps that allow the bridge to expand without cracking the material.



SOURCES
Paul Natland
http://hyperphysics.phy-astr.gsu.edu/HBASE/thermo/heatra.html

http://www.sasked.gov.sk.ca/docs/physics/u4a2phy.html
The Physics Hypertextbook