Download 1 - Pleasant Hill Schools

Physical Science
Ch. 6: Thermal Energy
Part I
Heat, Temperature, and Thermal Energy
• The Kinetic Theory of Matter
states that all matter is made up of
tiny particles (atoms or molecules)
that are in constant motion (either
vibrating in place, or actually moving around).
• When materials are heated up,
these particles move faster, and
when materials cool, the particle
speed decreases.
• Temperature is a measure of
the average kinetic energy of
the particles in a sample of
matter.
• So what is the relationship
between temperature and
particle speed?
• The SI unit for temperature is the Kelvin.
Although the Celsius and Fahrenheit
scales are more commonly used.
Absolute Zero
• As the temperature of a material decreases,
particle movement slows. If the temperature
decreases to a point where particle
movement stops completely, this is called
absolute zero. (0 Kelvin)
• Absolute zero is theoretical, in
that it has never been achieved.
Thermal Expansion
• The volume of an object will
increase as it's temperature
increases, and vice versa
(directly proportional)
Examples of thermal expansion:
Shrink Fitting
Heat
• Heat is energy
transfer from an
object at a higher
temperature to an
object at a lower
temperature.
• Heat flow from a warmer
object to a cooler object
will continue as long as
there is a temperature
difference between the
objects.
• Once the temperatures of
the objects are the same,
they are said to be in
thermal equilibrium.
Les was camping in
Colorado and ran out of
water. So he took some
water straight out of a
stream, but decided to
boil it first to kill any
pathogens. The problem
was, he only had a plastic
1 liter bottle to hold the
water. Is it possible to
boil the water in the
plastic bottle? Explain.
Explain why a
bimetallic strip
bends when it is
heated.
Energy Transfer
• As we have already learned, energy
transfer in the form of thermal energy will
travel from an object or location at a higher
temperature to one at a lower
temperature.
• There are 3 ways which this energy can
be transferred.
• Conduction is the transfer of energy through
matter (usually solids, but not always) by
direct contact between particles
• For conduction to occur the materials must
be: 1. In direct contact
2. At different temperatures
• In conduction the heat is transferred from
1 particle to the next, as they vibrate.
• Is it a very good idea to hold directly onto
the handle of an iron skillet?
• Why does heat always flow from a
warmer object to a cooler object?
• Different materials have
different levels of
conductivity.
• Good conductors: metals
• Poor conductors: wood,
glass, plastic, rubber, cloth,
air
• Convection is the transfer of
energy through the
movement of matter.
This occurs in fluids (liquids
or gases).
• Is convection the only way that a fluid
can transfer energy?
No!
A hot water bottle, for example, gains
heat from the warm water inside
through conduction (direct contact with
the warm water).
So if the heat is being transferred
through the motion of the fluid, then it
is convection.
If it is due just to direct contact, then
it’s conduction.
• Radiation is energy transfer through invisible
waves which do not require matter.
Examples:
-sunlight
-space heater
-radiator
-microwave oven
• Tell whether each of the following is primarily an
example of conduction, convection, or radiation.
1. ___An electric space heater in the corner of your room.
2. ___Having a VHS tape melt after being left in a hot car.
3. ___Car tires heating up as the car drives along.
4. ___Getting a sunburn.
5. ___Circulation of ocean currents.
6. ___The heat rising from a light bulb.
7. ___A warm breeze in your face.
8. ___Using a tanning bed.
9. ___Producing heat for your home with solar panels.
10. __Placing your hand in hot water.
• On a hot August day, Bill wants to
go fishing for muskie. So he calls
his friend Roland who tells him,
“Don’t you know that muskie won’t
bite if the water temperature is
over 80 degrees?”
“Trust me” said Bill, “there’s a way
around that”.
What was Bill’s secret?
Apu notices that some of
the dairy products in his
cooler at the Kwik-E-Mart
are turning bad before the
expiration date. He decides
to check the temperature of
his cooler to see if that is
the problem.
Does it matter where he
places his thermometer in
the cooler? Explain.
• Flower baskets with balloons
were given out to senior players
on Senior Night. The gifts were
purchased the day before and
put in a cool area so that the
flowers would not wilt.
However, the next day the
flowers looked fine, but the
balloons were all half flat.
Explain why this occurred and
how this problem could be fixed.
• Any material which is used to slow the flow
of heat is called insulation.
• Poor conductors make good insulators,
and vice versa
• R-value is a measure of the insulating
ability of a material.
The higher the r-value, the better an
insulator a material is.
• Why do you think some fiberglass
insulation is backed with a thin
layer of aluminum, if aluminum is
a metal and therefore a poor
insulator?
• Thermography is the process of using
special photographic film to show heat
loss.
• Thermography can be used to show heat
loss from a house, and may indicate
where more insulation or weatherproofing
is needed.
Specific Heat
• As you may know, different materials heat up
or cool down quicker than other materials.
• For example, on the first warm day of the
spring is it a good idea to go water skiing down
at the lake?
• The specific heat (Cp) of an object is the
amount of energy needed to raise the
temperature of 1 kg of a substance by 1
degree celsius.
• So, the higher the specific heat of a
material, does this mean that it will heat up
faster or slower than an object with a lower
specific heat?
Examples:
• Putting a pot full of
water in an oven
• Walking on a cold
concrete floor, or a
wooden floor
• Burning your feet on
the sand at the beach
• Two horseshoes (iron and
copper) are heated to the same
high temperature and then
plunged into separate buckets
of 40 degree water and left
there until thermal equilibrium
is reached.
• How could you predict which
system will have the higher
final temperature?
Thermal Energy
• Thermal energy is the total amount of
energy of the particles in a sample of
matter.
• This is similar to the mechanical energy of
an object, except this applies only to the
particles.
• Two identical tea kettles are heated until they
start blowing steam and whistling. One was
heated on a gas flame burner, while the other
was heated on an electric burner.
• Which one probably took longer to heat up,
and which one probably cooled down the
quickest after the heat was turned off?
Why?
• Bob leaves a pot of water outside in the shade
all day, with a thermometer in it. He also leaves
a thermometer sitting next to the pot. He
checks both temperatures every 15 min. for 24
hrs. Will the temperatures always be the same,
be different, or vary depending on the time of
day? Explain.
As Dwight and Jim were walking down the railroad
tracks, Dwight noticed that the point where 2 pieces of
track came together had about a ½” space between
them. “Wow, they did a pretty sloppy job connecting
these tracks”, Dwight commented.
“They’re supposed to be like that, genius”, replied Jim.
Explain Jim’s sarcastic response.
• Larry was putting new steel siding on his house,
and noticed that there were slots for the nails
instead of round holes. He also heard that he
needed to make sure not to hammer the nails in
tightly, but to leave a slight gap between the
nail head and the siding. Why?
• Three beakers containing the
same amount of water each
have 2 drops of red food
coloring dropped in. One
beaker is at 30 degrees, one
at 50, and one at 70. After
30 min. each beaker had
pink water. If the water was
still in each beaker, which
one turned pink first, or did
they turn at the same rate?
Explain.
• When choosing a cookware set, would it be
best to choose one made of materials with a
high specific heat or a low specific heat? Why?
• Andy was running some errands on a warm
summer day. He stopped by the grocery store,
and then went to the library to do some reading.
When he got home, he found that the rolls he
had purchased were now busted open and had
made a huge mess. Why did this happen?
• Tell which of the following does not belong,
and explain why:
A.
B.
C.
D.
An egg in a microwave
An aerosol can in a fire
Canned food suck the lid down tight
Running hot water over a jar lid
Material
Plastic Shot
Pipe Insulation
Wood chips
Water
Metal shot
Air
Change in
Temp.