Transcript Understanding Heat Transfer, Conduction, Convection and

EQ: What does it mean for something to have
heat, or be hot?
Temperature & Heat
Temperature is related to the average kinetic energy
of the particles in a substance. KE = mv2
2
2. SI unit for temp. is the Kelvin
a. K = C + 273 (10C = 283K)
b. C = K – 273 (10K = -263C)
3. Thermal Energy – the
total of all the kinetic
and potential energy of
all the particles in a
substance.
Physical Standard for Temperature
“Kelvin”
Triple Point of Water
The temperature and pressure at
which water, water vapor, and ice
can coexist in equilibrium.
Temperature = 0.01C
Pressure = 4.58 mm Hg
Absolute Zero & the Kelvin Scale
The Kelvin scale is setup so that its zero point is the coldest possible temperature-absolute zero, at which point a substance would have zero internal energy. This is 273.15 °C, or -459.69 °F.
Absolute zero can never be reached, but there is no limit to how close we can get to
it. Scientists have cooled substances to within 10-5 kelvins of absolute zero. How do
we know how cold absolute zero is, if nothing has ever been at that temperature?
The answer is by graphing Pressure vs. Temperature for a variety of gases and
extrapolating.
P
A gas exerts no
pressure when at
absolute zero.
T (°C)
-273.15°C
0 °C
4. Thermal energy relationships
a. As temperature increases, so does
thermal energy (because the kinetic energy
of the particles increased).
b. Even if the temperature doesn’t change,
the thermal energy in a more massive
substance is higher (because it is a total
measure of energy).
5. Heat
Cup gets cooler while
hand gets warmer
a. The flow of
thermal energy from
one object to another.
b. Heat always
flows from warmer to
cooler objects.
Ice gets warmer
while hand gets
cooler
Thermal Equilibrium
Two bodies are said to be at thermal equilibrium if they are at the
same temperature. This means there is no net exchange of thermal
energy between the two bodies. The top pair of objects are in
contact, but since they are at different temps, they are not in thermal
equilibrium, and energy is flowing from the hot side to the cold side.
heat
hot
26 °C
cold
26 °C
No net heat flow
The two purple objects are at the same temp and, therefore are in
thermal equilibrium. There is no net flow of heat energy here.
Understanding Heat Transfer,
Conduction, Convection and
Radiation
Heat Transfer
• Heat always moves from a warmer place to
a cooler place.
• Hot objects in a cooler room will cool to
room temperature.
• Cold objects in a warmer room will heat up
to room temperature.
Question
• If a cup of coffee and a red popsickle were
left on the table in this room what would
happen to them? Why?
• The cup of coffee will cool until it reaches
room temperature. The popsickle will melt
and then the liquid will warm to room
temperature.
Heat Transfer Methods
• Heat transfers in three ways:
– Conduction
– Convection
– Radiation
Conduction
When you heat a metal strip at one end, the heat
travels to the other end.
As you heat the metal, the particles vibrate, these
vibrations make the adjacent particles vibrate, and so on
and so on, the vibrations are passed along the metal and
so is the heat. We call this? Conduction
Metals are different
The outer e______
lectrons of metal atoms
drift, and are free to move.
When the metal is
heated, this ‘sea of
inetic
electrons’ gain k_____
energy and transfer it
throughout the metal.
Insulators, such as w___
ood and p____,
lastic do not
have this ‘sea of electrons’ which is why they
do not conduct heat as well as metals.
Why does metal feel colder than wood, if they
are both at the same temperature?
Metal is a conductor, wood is an insulator. Metal
conducts the heat away from your hands. Wood
does not conduct the heat away from your hands as
well as the metal, so the wood feels warmer than
the metal.
Convection
What happens to the particles in a liquid or a
gas when you heat them?
The particles spread out and
become less dense.
What
A liquid
isfluid
aorfluid?
gas.
This effects
movement.
Fluid movement
Cooler, more d____,
ense fluids
sink through w_____,
armer less
dense fluids.
In effect, warmer liquids and
gases r___
ise up.
Cooler liquids and gases s___.
ink
Water movement
Cools at the
surface
Cooler
water sinks
Convection
current
Hot water
rises
Why is it windy at the seaside?
Cold air sinks
Where is the
freezer
compartment
put in a fridge?
It is put at the
top, because
cool air sinks,
so it cools the
food on the
way down.
Freezer
compartment
It is warmer
at the
bottom, so
this warmer
air rises and
a convection
current is
set up.
The third method of heat transfer
How does heat energy get
from the Sun to the Earth?
?
There are no particles
between the Sun and the
Earth so it CANNOT
travel by conduction or
by convection.
RADIATION
Radiation
Radiation travels in straight lines
True/False
Radiation can travel through a vacuum
True/False
Radiation requires particles to travel
True/False
Radiation travels at the speed of light
True/False
Emission experiment
Four containers were filled with warm water. Which
container would have the warmest water after ten minutes?
Dull metal
Shiny metal
Shiny black
Dull black
shiny metal container would be the warmest after ten
The __________
minutes because its shiny surface reflects heat radiation
_______ back
dull black container
into the container so less is lost. The ________
emitting heat
would be the coolest because it is the best at _______
radiation.
Absorption experiment
Four containers were placed equidistant from a heater. Which
container would have the warmest water after ten minutes?
Dull metal
Shiny metal
Shiny black
Dull black
dull black container would be the warmest after ten
The __________
radiation the best.
minutes because its surface absorbs heat _______
shiny metal container would be the coolest because it is
The _________
the poorest at __________
absorbing heat radiation.
Convection questions
Why does hot air rise and cold air sink?
Cool air is more dense than warm air, so the
cool air ‘falls through’ the warm air.
Why are boilers placed beneath hot water
tanks in people’s homes?
Hot water rises.
So when the boiler heats the water, and the hot water
rises, the water tank is filled with hot water.
Radiation questions
Why are houses painted white in hot countries?
White reflects heat radiation and keeps the house cooler.
Why are shiny foil blankets wrapped around marathon
runners at the end of a race?
The shiny metal reflects the heat radiation from the runner
back in, this stops the runner getting cold.
1.
Which of the following is not a
method of heat transfer?
A.
Radiation
B.
Insulation
C.
Conduction
D.
Convection
2. In which of the following are
the particles closest together?
A.
Solid
B.
Liquid
C.
Gas
D.
Fluid
3. How does heat energy reach
the Earth from the Sun?
A.
Radiation
B.
Conduction
C.
Convection
D.
Insulation
4. Which is the best surface for
reflecting heat radiation?
A.
Shiny white
B.
Dull white
C.
Shiny black
D.
Dull black
5. Which is the best surface for
absorbing heat radiation?
A.
Shiny white
B.
Dull white
C.
Shiny black
D.
Dull black
Chapter 8
Energy and Matter
2.3
Temperature Conversions
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
31
Temperature
Temperature
• is a measure of how hot or cold an object
is compared to another object.
• indicates that heat flows from the object
with a higher temperature to the object
with a lower temperature.
• is measured using a thermometer.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
32
Temperature Scales
Temperature Scales
• are Fahrenheit,
Celsius, and
Kelvin.
• have reference
points for the
boiling and
freezing points of
water.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
33
Learning Check
A. What is the temperature of freezing water?
1) 0°F
2) 0°C
3) 0 K
B. What is the temperature of boiling water?
1) 100°F
2) 32°F
3) 373 K
C. How many Celsius units are between the boiling and freezing
points of water?
1) 100
2) 180 3) 273
34
Solution
A. What is the temperature of freezing water?
2) 0°C
B. What is the temperature of boiling water?
3) 373 K
C. How many Celsius units are between the boiling and freezing
points of water?
1) 100
35
Fahrenheit Formula
• On the Fahrenheit scale, there are 180°F between the
•
freezing and boiling points and on the Celsius scale, there are
100°C.
180°F =
9°F =
1.8°F
100°C 5°C
1°C
In the formula for the Fahrenheit temperature, adding 32
adjusts the zero point of water from 0°C to 32°F.
TF
= 9/5 TC + 32
TF
= 1.8 TC + 32
or
36
Celsius Formula
• TC is obtained by rearranging the equation for TF.
TF = 1.8TC + 32
• Subtract 32 from both sides.
TF - 32
=
1.8TC ( +32 - 32)
TF - 32
=
1.8TC
• Divide by 1.8 =
TF - 32
1.8
°F - 32
1.8
=
TC
= 1.8 TC
1.8
37
Solving A Temperature Problem
A person with hypothermia has a
body temperature of 34.8°C. What
is that temperature in °F?
TF
= 1.8 TC + 32
TF = 1.8 (34.8°C)
exact
tenth's
+ 32°
exact
= 62.6 + 32°
= 94.6°F
tenth’s
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
38
Learning Check
The normal temperature of a chickadee is 105.8°F. What
is that temperature on the Celsius scale?
1) 73.8°C
2) 58.8°C
3) 41.0°C
39
Solution
3) 41.0 °C
TC
=
(TF - 32°)
1.8
=
(105.8 - 32°)
1.8
=
73.8°F
1.8°
=
41.0°C
40
Learning Check
A pepperoni pizza is baked at 455°F. What
temperature is needed on the Celsius scale?
1) 423°C
2) 235°C
3) 221°C
41
Solution
A pepperoni pizza is baked at 455°F. What temperature is
needed on the Celsius scale?
2)
235°C
TF - 32°
1.8
= TC
(455 - 32°)
1.8
= 235°C
42
Learning Check
On a cold winter day, the temperature is –15°C.
What is that temperature in °F?
1) 19°F
2) 59°F
3) 5°F
43
Solution
3) 5°F
TF = 1.8 TC + 32
TF = 1.8(–15°C) + 32°
= – 27 + 32°
= 5°F
Note: Be sure to use the change sign key on your
calculator to enter the minus – sign.
1.8 x 15 +/ – = –27
44
Kelvin Temperature Scale
The Kelvin temperature scale
• has 100 units between the freezing and boiling points of
water.
100 K = 100°C
or
1 K = 1°C
• is obtained by adding 273 to the Celsius temperature.
TK
= TC + 273
• contains the lowest possible temperature, absolute zero (0
K).
0K
= –273°C
45
Temperatures
TABLE
2.5
46
Learning Check
What is normal body temperature of 37°C in Kelvins?
1)
2)
3)
236 K
310. K
342 K
47
Solution
What is normal body temperature of 37°C in kelvins?
2) 310. K
TK = TC + 273
= 37°C + 273
= 310. K
48
Specific Heat
a. Some things heat up or cool down
faster than others.
Land heats up and cools down faster than water
b. Specific heat is the amount of heat required
to raise the temperature of 1 kg of a material by
one degree (C or K).
1) C water = 4184 J / kg C
2) C sand = 664 J / kg C
This is why land heats up quickly during the day and
cools quickly at night and why water takes longer.
Why does water have such a high
specific heat?
water
metal
Water molecules form strong bonds with each
other; therefore it takes more heat energy to break
them. Metals have weak bonds and do not need
as much energy to break them.
How to calculate changes in
thermal energy
Q
=
mC
ΔT
p
Q = m x T x Cp
Q = change in thermal energy
m = mass of substance
T = change in temperature (Tf – Ti)
Cp = specific heat of substance
c. A calorimeter is used to help measure
the specific heat of a substance.
First, mass and temperature
of water are measured
Knowing its Q value, its mass,
and its T, its Cp can be
calculated
T is measured for water to help
get its heat gain
This gives the heat lost
by the substance
Then heated sample is put
inside and heat flows into
water
Pause for a Cause:
Determine the specific heat of water if
8.8 x 105 Joules are lost when 3.00 Kg of
water is cooled from 80.0C to 10.0C.
Q = mCpΔT
Answer: 4190J
Kg ºC
Cp = __Q__
mΔT
Pause for a Cause:
A metal bolt with a mass of 8.50 x 10-2 kg and a temperature
of 85.0 ˚C is placed in a container of water. The mass of the
water is 0.150 kg, and its temperature is 25.0˚C. What is the
specific heat capacity of the bolt if the final temperature of the
bolt and water is 28.4˚C? (c = 4186 J/kg*˚C)
Water
mw= 0.150 kg
p
Ti = 25.0 x ˚C
Q = 0.15Kg * 4186 J (28.4 ˚C – 25 ˚C) = 2134.86 J
Tf = 28.4˚C
Kg ˚C
Cp = 4186 J/kg*˚C
Q = mC ΔT
C
__Q__
mΔT
Bolt
p=
mb= 8.50 x 10-2 kg
Ti = 85.0 x ˚C
Cp = ______2134.86 J__________ = 443 J
Tf = 28.4˚C
8.5x10-2 Kg (28.4 ˚C – 85.0 ˚C)
Kg ˚C
Cp = ?
Pause for a Cause: Q = CpmΔT
Find the final equilibrium temperature when 10.0 g of milk at 10.0 ˚ C is
added to 1.60 x 102 g of coffee with a temperature of 90.0 ˚C. Assume
the specific heats of coffee and milk are the same as for water (c = 4.19
J/g* ˚C), and disregard the heat capacity of the container.
Milk
mm= 10.0g
Ti = 10.0 ˚C
Tf = ?
Cpm = 4.186 J/g*˚C
Qm = Qc
Coffee
mc= 1.60 x 102 kg
Ti = 90.0 ˚C
Tf = ?
Cpc = = 4.186 J/g*˚C
CpmmΔT = -CpcmΔT
Cpmm(tf - ti) = -Cpcm(tf - ti)
Cpmm(tf - ti) = Cpcm(-tf + ti)
Cpm = Cpc
Milk
mm= 10.0g
Ti = 10.0 ˚C
Tf = ?
Cpm = 4.186 J/g*˚C
Solve for tf
Coffee
mc= 1.60 x 102 kg
Ti = 90.0 ˚C
Tf = ?
Cpc = = 4.186 J/g*˚C
Cpm = Cpc
Cpmmm(tf - tm) = Cpcmc(-tf + tc)
mm(tf - tm) = mc (-tf + tc)
mmtf - mmtm = -mctf + mctc + mmtm
mm + mmtm
mm
mctf + tf = -mctf + mctc + mmtm
mc
+mctf mc + mm
tf = mctc + mmtm
mc + mm
Milk
mm= 10.0g
Ti = 10.0 ˚C
Tf = ?
Cpm = 4.186 J/g*˚C
Solve for tf
Coffee
mc= 1.60 x 102 g
Ti = 90.0 ˚C
Tf = ?
Cpc = = 4.186 J/g*˚C
tf = mctc + mmtm
mc + mm
tf = (1.60 x 102 g * 90.0 ˚C) + (10.0g * 10.0 ˚C)
(1.60 x 102 g + 10.0g)
tf = 14400 g ˚C + 100g ˚C
170 g
tf = 14500 g ˚C = 85.3 ˚C
170 g
Thermal Expansion of Solids and
Liquids
• Thermal expansion of an object is
a consequence of the change in
the average separation between
its atoms or molecules due to the
increase of kinetic energy (heat).
Determining Thermal Expansion
Δl = α liΔt
Δl = (lf – li) Final length - initial length
α = coefficient of thermal expansion
Li = initial length
Δt = (tf – ti) Final temp - initial temp
Coefficient of Expansion
• Thermal expansion depends on the material
being heated or cooled.
• The coefficient of thermal expansion is
determined experimentally.
Pause for a Cause
• A steel railroad track has a length of 30.000m when
the temperature is 0˚C. Determine the coefficient of
thermal expansion for steel if the tracks expand to
30.013m after a hot day when the temperature is
40.0 ˚C.
α = (l – l )
f
i
li(tf – ti)
α = (30.013m – 30.000m)= 1.08 x 10-6 ˚C-1
30.000m(40.0˚C– 0˚C)
Pause for a Cause
• A copper bar changes in length by 1.1 meters with
a 150 degree Celsius change in temperature. What
is the bar’s original length?
Δl = α liΔt
li = _Δl_
α Δt
li = _______1.1m_____ = 431.4m
(17.0E-6 ˚C-1)(150˚C)
li = ˚C m
˚C
Latent Heat & Phase Change
• A transfer of heat energy from one
substance to another does not
always result in a temperature
change.
• For instance, ice water on a hot
summer day will remain 0˚ C until
all of the ice has melted before the
temperature begins to rise
• i.e. energy is lost due to phase
change
Latent Heat & Phase Change
• The energy needed to change the phase of a given pure
substance is called Latent heat.
• It is dependant on the nature of the substance and the
phase change.
Q = mL
Q = heat (J)
m = mass (Kg)
L = latent heat (J/Kg)
solid  liquid
333,000J/Kg
liquid  gas
2,263,300J/Kg
Latent Heat & Phase Change
Latent heat of Fusion: melting or freezing
Q = mLf
Latent heat of Vaporization: boiling or condensation
Q = mLv
• Latent heat is an experimental value
– Dependent on inter & intermolecular forces
Latent Heat of Water
Pause for a Cause:
How much heat must be added to a 25g ice cube at 0ºC to change it
to water at 0ºC if the latent heat of fusion for water is
3.33 X 10-5J/Kg? Q = mLf
Q = 25g * 1Kg * 3.33 X 105J = 8.3 X 103J
1000g * Kg