Transcript Physics 106P: Lecture 1 Notes

Physics 101: Lecture 28
The Transfer of Heat

Today’s lecture will cover Textbook Chapter 13
The Transfer of Heat
» Convection
» Conduction
» Radiation
Physics 101: Lecture 28, Pg 1
Concept Question
Which of the following is an example of convective, conductive and
radiative heat transfer?
1. You stir some hot soup with a silver spoon and notice that the spoon
warms up.
2. You stand watching a bonfire, but can’t get too close because of the
heat.
3. Its hard for central air-conditioning in an
old house to cool the attic.
Physics 101: Lecture 28, Pg 2
Heat Transfer: Convection

Convection: heat is transferred by the bulk movement of a gas or
liquid.
Example:
Fluid is sitting on a burner is heated from below.
The fluid right above the flame is getting hot and thus
expands: V increases => density decrease
Thus, the hotter fluid experiences a net force upward (buoyant force)

FB = rcold V g > rhot V g
Archimedes Principle: low density floats on high density
=> warmer fluid moves upward and is replaced by cooler fluid ->
fluid is warmed -> moves upward -> and so on
 Cycle continues with net result of circulation of fluid that carries
heat.
 Practical aspects:
heater ducts on floor
A/C ducts on ceiling
Physics 101: Lecture 28, Pg 3
stove heats water from bottom
Heat Transfer: Conduction

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
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Conduction: Heat is transferred directly trough a
material (bulk motion does not play a role).
Atoms/molecules of hotter materials have more KE than
atoms/molecules of cooler materials.
Gas/fluids: high-speed atoms/molecules collide with
low-speed atoms/molecules:
 energy transferred to lower-speed atoms/molecules
 heat transfers from hot to cold
Metals: electrons can “freely” move and can transport
L = Dx
T
energy
H
P = rate of heat transfer = Q/t [J/s] Hot
Area A
 Q = k A (TH-TC)/L
 k = “thermal conductivity”
» Units: J/(s m C)
» good thermal conductors…high k (e.g. metals)
» good thermal insulators … low k (e.g. wood)
TC
Cold
Physics 101: Lecture 28, Pg 4

Example with 2 layers: find P=Q/t in J/s
Key Point: Continuity (just like fluid flow)
» P1 = P2
» k1A(T0-TC)/Dx1 = k2A(TH-T0)/Dx2
» solve for T0 = temp. at junction
» then solve for P1 or P2

answers: T0=5.8 C P=265 Watts
P1
P2
Inside: TH = 25C
Outside: TC = 4C
T0
Dx1 = 0.019 m
A1 = 35 m2 k1 = 0.080 J/s-m-C
Dx2 = 0.076 m A2 = 35 m2 k2 = 0.030 J/s-m-C
Physics 101: Lecture 28, Pg 5
Heat Transfer: Radiation
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Radiation: All things radiate electromagnetic energy.
Pemit = Q/t = eAT4
Surroundings at T0
» e = emissivity (between 0 and 1)
 perfect “black body” has e=1
Hot stove
T
» T is Kelvin temperature
»  = Stefan-Boltzmann constant = 5.67 x 10-8 J/(s m2K4)
No “medium” required

All things absorb electromagnetic energy from surroundings.
 Pabsorb = eAT04
» good emitters (e close to 1) are also good absorbers
Physics 101: Lecture 28, Pg 6
Heat Transfer: Radiation

All things radiate and absorb electromagnetic energy.
Pemit = Q/t = eAT4
Pabsorb = eAT04
Pnet = Pemit - Pabsorb = eA(T4 - T04)
Surroundings at T0
Hot stove
T
» if T>T0, object cools down if T<T0, object
heats up
Physics 101: Lecture 28, Pg 7
NASA’s Thermal Imaging System
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http://mars.jpl.nasa.gov/
During day time the sun heats the surface of Earth or
Mars. At night the materials on the surface emit this
thermal energy again in form of radiation in the infrared.
Since different materials emit differently, a whole
spectrum of infrared radiation is measured which
is used to identify the material.
Physics 101: Lecture 28, Pg 8
Concept Question
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 (asphalt) absorbs electromagnetic waves (radiation) more readily
than white (cement) does (emissivity is larger). Hence, the black has
more radiation to emit because it has absorbed more. As a result, it
releases more radiation into the snow, causing the snow to heat up, and
melt.
Physics 101: Lecture 28, Pg 9