Transcript Alfano 1 Heat Propagation - monica

Heat Propagation
Liceo Alfano
Concept Question
What is heat?
A) A measure of how hot or cold something is.
B) A basketball team from Boston.
C) Energy transferred due to a temperature difference.
D) A quantity measured in degrees Kelvin.
Answer
What is heat?
A) A measure of how hot or cold something is.
B) A basketball team from Boston.
C) Energy transferred due to a temperature
difference.
D) A quantity measured in degrees Kelvin.
How does heat travel?
•
Heat energy always travels from HOT to COLD
•
When the sun shines on the Earth, does energy
travel from the sun to the Earth or from the
Earth to the sun?
•
If I burn my hand by touching the stove, does
energy travel from the stove to my hand or from
my hand to the stove?
•
If I take a cold shower to cool down, does
energy travel from the water to my body or
from my body to the water?
Kinds of Heat
Propagation
•
3 types
•
Heat Propagation Rap
•
Conduction
•
Convection
•
Radiation
Conduction
•
What happens to the roof of your mouth
if you eat a piece of pizza before it cools
down?
•
If you have coffee in a mug does the
outside of the mug feel hot or cold?
•
Two objects touching = CONDUCTION
•
Two objects = pizza and your mouth,
two molecules in mug
Conduction
•
Different materials conduct heat differently
•
Which will get hotter, a metal spoon or a wooden
spoon in a pot of boiling water?
•
Thermal conductivity – How well a material
can conduct heat
vs.
Conduction
•
Thinking about if something is
a good thermal conductor
•
Ask, “if I put one end of this
in a boiling pot, how long
would it take for the other
end to get hot?”
•
Short time = good conductor
•
Long time = poor conductor
(also called insulator)
Concept Question
Which of the following are good thermal conductors?
1.
Copper
Yes (thermal conductor)
2.
Plastic
No (insulator)
3.
Cloth
No (insulator)
4.
Wood
No (insulator)
5.
Gold
Yes (thermal conductor)
Concept Question
•
Would you use a material with high or low
thermal conductivity in the walls of your house to
keep heat in during the winter?
A)
High thermal conductivity
B)
Low thermal conductivity
Concept Question
•
Would you use a material with high or low
thermal conductivity in the walls of your house to
keep heat in during the winter?
A)
High thermal conductivity
B)
Low thermal conductivity
Conduction
•
The thermal conductivity of a material is
measured in W/m*C (ex: for aluminum,
k = 250 W/m*C)
•
We can measure how much heat has gone
through a bar based on the following equation:
•
Q = k*A*ΔT*Δt/L
•
A = cross-sectional area, T = temperature, t =
time, L = length
Conduction
•
Q = k*A*ΔT*Δt/L
•
ΔT = T1 – T2
•
Δt = elapsed time
•
Q = amount of heat transferred
Concept Question
•
Would aluminum or wood have a higher value
for k, thermal conductivity?
A)
Aluminum
B)
Wood
Concept Question
•
Would aluminum or wood have a higher value
for k, thermal conductivity?
A)
Aluminum
B)
Wood
Concept Question
•
If I increase the cross-sectional area of a bar, the
amount of heat transferred will
A)
Increase
B)
Decrease
Concept Question
•
If I increase the cross-sectional area of a bar, the
amount of heat transferred will
A)
Increase
B)
Decrease
Concept Question
•
If I increase the length of a bar, the amount of
heat transferred will
A)
Increase
B)
Decrease
Concept Question
•
If I increase the length of a bar, the amount of
heat transferred will
A)
Increase
B)
Decrease
Concept Question
•
If I increase the change in temperature across a
bar, the amount of heat transferred will
A)
Increase
B)
Decrease
Concept Question
•
If I increase the change in temperature across a
bar, the amount of heat transferred will
A)
Increase
B)
Decrease
Concept Question
•
If I increase the amount of time elapsed, the
amount of heat transferred will
A)
Increase
B)
Decrease
Concept Question
•
If I increase the amount of time elapsed, the
amount of heat transferred will
A)
Increase
B)
Decrease
Practice Problems
•
A 2 m copper rod with a cross-sectional area of
0.1 m2 has one end at 80C and the other at 50C.
The thermal conductivity of copper is
370 W/m*C. How much heat is transferred in 10
seconds?
•
How much heat is transferred in an hour through
a 5 m long copper rod with a square crosssectional area, where each side measures 0.1 m?
One end of the rod is at 300K and the other is at
320K
Convection
•
Which way does a flame point on
a candle?
•
Right after you burn yourself,
what should you do?
•
Fluids flowing = CONVECTION
•
Fluid = liquid (i.e. water), air
Convection
•
Two parts to convection:
1)
Hot air always travels UP (ex: hot air balloon)
2)
Flowing liquids transfer heat (ex: run
something frozen under warm water to heat it
up)
•
Cool convection video!
Convection
•
Remember, FLUIDS MOVING (air,
liquid) = CONVECTION!
Radiation
•
Where do you normally hear the
word “radiation?”
•
Cell phones, X-rays, nuclear power
plants, etc.
•
What do all these have in common?
•
Two objects that DON’T touch
•
Energy transferred through
electromagnetic waves
Radiation
•
If you light a campfire and sit a few feet
from it, what do you feel?
•
If you put something in a microwave,
does it come out hotter or colder?
•
In heat propagation, radiation means
heat transfer between two objects that
DON’T touch
•
•
Heat energy is transferred through
electromagnetic waves
Radiation experiment!
Radiation
•
We can predict how much heat energy an object
will give off based on its temperature!
•
The Stefan-Boltzmann Law gives us the
amount of heat energy radiated per unit surface
area per second (flux)
•
Think: would a higher temperature mean an
object gives off more or less heat energy?
Stefan-Boltzmann Law
•
•
THE EQUATION: F
= σT4
•
F = heat propagation flux (energy radiated per unit
surface area per second)
•
σ = Stefan-Boltzmann constant
(5.67 * 10-8 J/s*m2*K4)
•
T = temperature (but be careful! Temperature must
be measured in Kelvin, not degrees Celsius or
Fahrenheit)
Does higher temperature mean more or less heat
propagation?
Practice Problems
•
Remember: F = σT4
•
A star’s temperature is 5000K. According to the
Stefan-Boltzmann law, how much energy is
radiated into space per second by each square
meter of the star’s surface?
•
If the star is a perfect sphere with radius
640000000 m, what is the total power output of
the star?
Examples from earlier
•
Conduction, Convention, Radiation
•
Sun shining on Earth
•
•
Burning my hand on the stove
•
•
Radiation
Conduction
Taking a cold shower
•
Convection
Heat transfer applet
•
http://www.teachersdomain.org/asset/lsps07_i
nt_heattransfer/
More Examples
•
For each of the following examples, what kind of
heat transfer is it (and why)?
•
Hot water rises in a boiling pot
•
An incandescent light bulb feels warm when your
hand is a few centimeters away
•
A chocolate bar melts in my hand on a hot day
•
A firefighter uses a hose of water to put out a fire
•
A crazy man walks across burning coals
More Examples
•
I ride on a waterslide to cool down on a hot day
•
You feel the coolness of a breeze on a warm day
•
The cold outside makes the inside of the
windows feel cold
•
Venus is much hotter than Saturn
•
You stir fry vegetables in a pan
•
You put your finger in a cup of hot coffee