Physics 101: Lecture 26 Conduction, Convection Radiation

1
Physics 101 Lecture 26 Conduction,
ConvectionRadiation

• Todays lecture will cover Textbook Chapter
  14.4-14.9

• Only two more lectures check your gradebook
• Hour Exam 3
• Next Monday, Dec. 3, 7pm
• Lectures 17-23
• Sign up online
• Review, Dec 2, 8pm, 141 Loomis, Spring 2006 HE3
• Conflicts Prof. Ben Wandelt, bwandelt_at_uiuc.edu
• Rooms different from last two exams. Check web
  page.
• Extended office hours Dec 2,3

2
Suggested Practice Problems for Exam III
Chapter 9 Examples 9.1, 9.2, 9.3, 9.7, 9.9,
9.11 Problems 9.5, 9.9, 9.11, 9.13, 9.15, 9.17,
9.23, 9.29, 9.35, 9.39, 9.41, 9.49, 9.51, 9.73,
9.75, 9.88, 9.89, 9.93, 9.97 Chapter
10 Examples 10.6, 10.7, 10.8, 10.9 Problems
10.27, 10.29, 10.35, 10.43, 10.45, 10.49, 10.53,
10.57, 10.59, 10.63, 10.71, 10.75, 10.81, 10.87,
10.97 Chapter 11 Examples 11.1, 11.3, 11.7,
11.8 Problems 11.3, 11.9, 11.11, 11.13, 11.19,
11.21, 11.23, 11.25, 11.29, 11.31, 11.47, 11.51,
11.53, 11.57, 11.67, 11.69, 11.75, 11.77,
11.79 Chapter 12 Examples 12.1, 12.2, 12.3,
12.5, 12.6, 12.7, 12.8, 12.9 Problems 12.1,
12.3, 12.9, 12.11, 12.15, 12.19, 12.21, 12.23,
12.25, 12.29, 12.31, 12.33, 12.35, 12.37, 12.41,
12.57, 12.59, 12.65
05
3
Review

• Heat is FLOW of energy
• Flow of energy may increase temperature
• Specific Heat
• DT Q / (c m)
• Latent Heat
• heat associated with change in phase
• Today Heat
• Conduction
• Convection
• Radiation

07
4
Heat Transfer Conduction

• Hot molecules have more KE than cold molecules
• Fast molecules on left collide with slow
  molecules on right
• energy transferred to lower-speed molecules
• heat transfers from hot to cold
• I rate of heat transfer Q/t J/s
• I k (A/L) (TH-TC)
• Q/t k A ?T/L
• k thermal conductivity
• Units J/s-m-C
• good thermal conductorshigh k
• good thermal insulators low k
• R L/(Ak) thermal resistance Then I DT/R

L
TH Hot
TC Cold
Area A
rods LN2 demo
13
5
ACT

• Touch base of chair, and desk top, which feels
  colder?
• A) Base B) Same C) Desk

Both must be the same temperature (room
temperature), but metal feels colder because it
conducts heat better/faster.
10
6
Preflight

• On a cool night you make your bed with a thin
  cotton sheet covered by a thick wool blanket. As
  you lay there all covered up, heat is leaving
  your body, flowing though the sheet and the
  blanket and into the air of the room. Compare the
  amount of heat that flows though the sheet to the
  amount of heat that flows through the blanket.
• 1. More heat flows through sheet than through the
  blanket.
• 2. More heat flows through blanket than through
  the sheet.
• 3. The same amount of heat flows through sheet as
  the blanket.

80 9 11
The heat will flow through both fabrics so the
amount of heat flowing through them should be the
same
The blankets are right on top of eachother so
the heat flowing through the cotton sheet goes
straight to the wool blanket
15
7
Conduction w/ 2 layers ACT

• Compare the heat flow through material 1 and 2.
• A) I1 gt I2 B) I1I2 C) I1 lt I2
• Estimate t0 the temperature between the two
• A) 2 C B) 12.5 C C) 20 C

R L/kA I DT/R
L1 0.02 m A1 35 m2 k1 0.080 J/s-m-C L2
0.075 m A2 35 m2 k2 0.030 J/s-m-C
19
8
Conduction w/ 2 layers

• Find T0 and I
• Key Point Continuity (just like fluid flow)
• I1 I2 I
• k1A(T0-TC)/L1 k2A(TH-T0)/L2
• solve for T0 (k2L1THk1L2TC)/(k1L2k2L1)
• then solve for I1 or I2
• T02.27 C , I318 Watts

L1 0.02 m A1 35 m2 k1 0.080 J/s-m-C L2
0.075 m A2 35 m2 k2 0.030 J/s-m-C
22
9
Conduction ACT / Demo

• Which marbles will fall last?
• 1) Copper 2) Steel 3) Aluminum

24
10
Heat Transfer Convection

• Air heats at bottom
• Thermal expansiondensity gets smaller
• Lower density air rises
• Archimedes low density floats on high density
• Cooler air pushed down
• Cycle continues with net result of circulation of
  air
• Practical implications
• heater ducts on floor
• A/C ducts on ceiling
• stove heats water from bottom
• riding the thermals

convection demo
27
11
Heat Transfer Radiation

• All things radiate electromagnetic energy (light)
• Iemit Q/t eA?T4
• e emissivity (between 0 and 1)
• perfect black body has e1
• T is temperature of object in Kelvin
• ? Stefan-Boltzmann constant 5.67 x 10-8
  J/s?m2?K4
• Electromagnetic waves (no medium required)
• All things absorb energy from surroundings
• Iabsorb eA?T04
• T is temperature of surroundings in Kelvin
• good emitters (e close to 1) are also good
  absorbers

match lighting demo
35
12
Heat Transfer Radiation

• All things radiate and absorb electromagnetic
  energy
• Iemit eA?T4
• Iabsorb eA?T04
• Inet Iemit - Iabsorb eA?(T4 - T04)
• if T gt T0, object cools down
• if T lt T0, object heats up

38
13
Preflight

• One day during the winter, the sun has been
  shining all day. Toward sunset a light snow
  begins to fall. It collects without melting on a
  cement playground, but it melts immediately upon
  contact on a black asphalt road adjacent to the
  playground. How do you explain this.

Black tends to absorb heat and since the sun was
out the asphalt has maintained a higher
temperature than teh cement therefore the snow
melts on the asphalt and not on the cement.
Black attracts more heat and thus when the snow
falls the temperature of the asphalt causes it to
melt.
45
14
Prelecture 23 / ACTconvection, conduction, or
radiation?

• 1. You stir some hot soup with a silver spoon and
  notice that the spoon warms up.
• 2. You stand watching a bonfire, but cant get too
  close because of the heat.
• 3. Its hard for central air-conditioning in an
  old house to cool the attic.

Conduction
Radiation
Convection
29
15
Earth Homework

• The Earth has a surface temperature around 270 K
  and an emissivity of 0.8, while space has a
  temperature of around 2 K. What is the net power
  radiated by the earth into free space?
• (Radius of the Earth is Re 6.38106 m)

Inet Iemit - Iabsorb eA?(T4 - T04)
42
16
Summary

• Conduction contact
• Convection fluid motion
• Radiation EM waves

50