Transcript Da Procida 2 Heat Propagation - monica

Heat Propagation
Liceo Da Procida
Reminder
• Last time we learned about kinetic theory of matter
• Which state of matter has the fastest moving
particles?
• How do we make the particles move even more
quickly?
• Today, we will explore the different ways to transfer
heat to a substance
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.
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
Practice Problems
•
Q = k*A*ΔT*Δt/L
•
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