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12/5/13 — 12/9/13
Resources:
HotStuff.ppt
Water relay: cups (5), buckets (2), short
rope (leg “shackle”)
Hot plate / hair dryer (forced convection)
Convection.avi / Flashlight
Radiation can / stryo cover (in Rm 12 drawer
47) / 3 temp probes & computer
Homework for 5/1:
Bell Work:
H. Heat (reprise)
1. Recap of temp vs heat
2. Law of Heat Transfer:
a. Heat is always transferred from a higher to a lower
temperature! (high KE to low KE)
I. Types of heat transfer
1. Conduction = “KE transferred by collisions”
a. requires contact
b. depends on carrier mobility & number of carriers
c. Hot Stuff labette
1) Mechanics:
a) Plot temp vs time
b) Draw best fit line & calc slope (slope formula) –
see HotStuff.ppt
2) KE is transferred by collisions — so materials have to
contact each other! (flame in contact with wire in
contact with water -- sketch)
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3) Carrier mobility: Which metal was better conductor?
Why?
a) Best conductor: molecules move easily / bang into
each other a lot / KE (heat) transferred to far end
quickly
b) Poor conductor: molecules move poorly / don’t
collide much / KE transferred slowly
c) Analogy: water relay (use cups to take water from
tub in front to tub in back) — one S normal, other
with ankles shackled
(i)
which molecules move more easily?
(ii)
which material will conduct heat better?
4) Number of carriers:
a) more carriers (electrons) = more heat flow
b) Analogy” Water relay w/ 4 Ss and wide aisle vs 1 S
and narrow aisle
c) So, thick wire has 4x the number of carriers, so
heat transferred 4x faster
2. Convection: movement of fluid (liquid or gas) that can then
carry KE (heat). “KE transferred by moving fluid”
a. Forced Convection
1) Fluid is forced to move (by fan for example)
2) EXAMPLES? (e.g. convection oven, thunderstorm, etc.)
3) hair dryer – demo:
a) Law of Heat Transfer: where is the higher T? Lower
T?
b) Transfer of KE: How does KE get from hair dryer to
hair?
b. Free Convection
1) Less dense fluid floats on more dense fluid
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2) Thermal expansion of a fluid (gas or liquid) causes
expansion with reduction of density, so warmer fluid
floats upward (blob sketch)
3) Remember:
a) Expansion only occurs because molecules are moving
faster and push each other away
b) Expansion only occurs if there is room to do so
c) Expansion isn’t very big (for liquids)
d) Hot fluid only floats if:
(i)
There is colder, denser fluid around it
(ii)
There’s gravity
4) Demo: Convection cell video
5) Demo: Projector beam (blank ppt slide) through rising
hot air over hot plate onto projector screen
6) As fluid rises, it transfers energy to surrounding,
cooler fluid and cools down
a) If it cools enough it sinks back down to be warmed
again
b) Generates column of rising fluid surrounded by wall
of falling fluid
c) REMEMBER: “HEAT” doesn’t rise! HEATED FLUID RISES
and takes KE with it!
7) Do Convection worksheet in class -- finish as homework
3. Radiation: Heat transfer by PHOTONS
a. Radiation can prelim:
1) which color can absorbs heat fastest?
2) which will radiate it fastest (and hence cool down
fastest)? – take vote
3) Start demo now and come back to @ end …
b. No contact needed – energy carried by electromagnetic
waves (helpful to think of photons — “energy bullets”)
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c. Hot object emits a photon which takes energy away (so hot
object cools) and carries energy until it hits another
object which will absorb the photon (and get warmer)
d. Can occur in a vacuum
e. Demo: Red-hot hot plate (lights out)
1) Hold sideways so S can feel on cheek
2) But! Convection is UP so how does heat go sideways?
Radiation!
f. Demo: Radiation cans (white, black & silver)
1) if heat absorbed best, heat will be radiated best
2) Look at temp record: check predictions
3) Q: which can would look brightest in IR scope?
Arrange as shown so tube can be displayed through projector.
Fill tube with water leaving a small void under fill tube.
Light burner and wait 10 sec or so (to start convection).
Add 1 drop of green food coloring.
After green nearly finishes cycle, add a tiny amount of permanganate.
(see convection.avi)