Transcript Slide 1

Heat and Thermal
Energy
Temperature!
• How hot or cold something is
with respect to some standard
Try this! What do you notice?
A standard is a frame
of reference that all
agree on. If I asked
you what temperature
it was in the room,
you’d all have
different answers! But
if I also gave you each
a thermometer…
Thermometers
• Thermometers
work through the
expansion and
contraction of a
liquid
• Usually mercury or
alcohol
In the U.S., we use Fahrenheit
Most other
countries use
Celsius
Where did the Fahrenheit
scale come from?
• Daniel Fahrenheit got water to the lowest
possible temperature. He called this 32.
• He then set the upper limit of his scale to
body temperature, or 96 degrees. Off a bit!
• Next he tested the strength of his
thermometer by placing it in boiling water.
He found it to read 212, thus the upper limit
Anders
Celsius
•Revised the
Fahrenheit scale
to be more
logical.
•Used freezing
and boiling
points of water
for the zero and
one hundred
markers.
So, what
does a
thermometer
do exactly?
So, how does
a
thermometer
work?
Temperature is a measure of the
AVERAGE KE of a substance
• When atoms within a substance
move faster, they have higher
temperatures!
This
pencil was
just
sharpened
Why does it have to be the
AVERAGE Kinetic Energy?
When you place a thermometer
in a substance, heat flows from
hot to cold until both have the
same average KE’s
The thermometer
measures her
temp. when heat
flows from her
head to the
thermometer!
Thermometers actually read
their own temperatures. Can
you explain why??
What makes
something hot?
How do you heat
something up?
What if I compressed the
box? What happens to the
temperature?
So what makes something
cold?
Cold
When air is cool, the
molecules of water
vapor move slowly.
Hot
Water vapor molecules in
warm air move faster.
They are less likely to join
together.
To change the temperature…
WAIT!!!!!!!!!!
• Not so fast! We forgot about
KELVIN!
So
what’s
Zero
Oh it
can get a lot
colderon
than the
this!
Kelvin scale?
• Zero on the Kelvin scale is
equal to ABSOLUTE ZERO
• That’s when all molecular
motion ceases
• Basically it’s the coldest
anything can get!
There is no theoretical limit on how hot something could get
Absolute Zero and temperature
• Check out this webpage…
• Is there an upper limit to
the temperature scale??
• What do YOU think??
How do you convert from one
scale to another?
www.fordhamprep.org/gcurran/sho/sho/review/rev29a.htm
Try some of these with
your partner!
How do you convert from one
scale to another?
C = (F – 32) 5/9
F = 1.8*C + 32
Pay attention to order of
operations!!
How do you convert from one
scale to another?
C = K - 273
K = C + 273
So what’s thermal energy?
• The sum of all the KE’s of the
individual particles of a substance
Who’s got more thermal
energy?
OK then, what’s HEAT?
• HEAT is when thermal energy
is transferred from one object
to another due to a temperature
difference between them
Heat always moves
from:
Heat
Units!
Joules
It takes 4.184 Joules to raise the
temperature of 1 g of water by
1 oC
Heat
Units!
1 calorie (cal) = amount of heat
needed to raise the temperature
of 1 g of water by 1 oC
The scientific "calorie" is
spelled with a "c".
One "calorie" = 4.184
Joules
------------------------------------The "dieter's" calorie is
spelled with a "C".
One "Calorie" = 1000
calories
• Because of the confusion
between Calorie and
calorie, joules is
commonly used for a unit
of thermal energy.
3000 calories of
thermal energy
enters each cup.
The temperature of
the water on the
left rises by 30
Celsius degrees.
By how much
does the
temperature of the
water in the cup
on the right rise?
U.S.A.
Specific Heat Capacity
The specific heat of a
substance is the
quantity of energy
needed to raise the
temperature of 1g of the
substance by 1°C.
The pans are of a similar type
but different size. Each pan is
filled with water. They are placed
on heaters having the same
power. In which pan would the
water boil first?
This beaker contains 100 grams of
water at 20°C.
Let's see how the temperature
changes as we add hot or cold water.
This beaker contains 100 grams of water
at 20°C. Let's add 100 grams of water at
60°C to our 100 grams sample at 20°C.
What temperature would you expect for
the mixture?
What temperature would you expect for the
mixture? 20, 30, 40, 50 ,60
http://www.chem.uiuc.edu/webFunChem/Gen
ChemTutorials.htm
•A penny,
marble and
foil are at the
same temp.
Substance
Marble
Specific Heat
0.90
Aluminum
0.215
Copper
0.092
•Which one
stays hot
longer after
removal from
the bath?
Specific Heats of Various
Substances
Substance
water (pure)
wet mud
Ice (0 °C)
sandy clay
dry air (sea level)
quartz sand
granite
Specific Heat Specific Heat
(cal/gram °C) (J/g °C)
1.00
0.60
0.50
0.33
0.24
0.19
0.19
4.186
2.512
2.093
1.381
1.005
2.95
2.94
Practice Problems!
• Example 1: How much energy does it take
to raise the temperature of 50 g of copper
by 10 C? (c of copper is 0.385 J/gC)
Example 2: If we add 30 J of heat to
10 g of aluminum, by how much will its
temperature increase? (c for aluminum
is 0.902 J/gC)
Example 3: A 4 g substance loses 23
Joules of heat energy when the
temperature falls from 35°C to
20°C. What is the specific heat of the
substance?
Here’s a website with a cool
demo and some practice
problems!
OK then, tell me why it
takes so long for water to
warm up in the summer??
Calorimeters
How do they work?
• Specific heat of water (c) is known
• Take a known mass of water at a known
temperature
• Heat a known mass of any substance
• Place the substance into the water in the
calorimeter and seal the lid
• Watch the change in temperature
• Use the equation
• Amount of heat lost by substance equals
amount of heat gained by water (Q)
• Calculate (c) for the substance
Online specific heat experiment
Let's measure the specific heat of a piece of brass, aluminum or plastic.
• http://www.chem.uiuc.edu/webFunChe
m/GenChemTutorials.htm
Click the material would you like to use.
• When heat is added and the
temperature increases, molecules
move faster and in general, move
apart.
Why should I know
about this?
Someone needs
to re-take
driver’s ed!
Thermal Expansion
Why do
railroad
tracks have
spaces in
between the
ties?
http://www.fotosearch.com/
bigcomp.asp?path=CSP/C
SP011/k0112770.jpg
The spaces
we are
discussing
are the small
spaces; not
the large ones
you see here.
Why do railroad tracks have
spaces in between them?
• On hot days, the tracks which are made of
metal expand. If there weren’t any
spaces, the metal would push on the
adjoining pieces as they expand and
cause a deformity in the track. When a
train travels along the track and hits the
problem area, it could derail, causing
injury and destruction.
Bimetallic Strips
•½ is made of brass, ½ is made of iron
•Can you tell which has the greater
rate of expansion? HOW?
•When the room
temp. changes,
the strip bends
•It will either turn
the switch on or
off, turning the
heat on or off
Who’s
weird??
When the molecules link up to
make ice, they form a crystal
So when it freezes,
it
Yes, way…
that’s why
ice floats on
top of liquid
water. Its…
LESS
DENSE
Sorry… couldn’t resist!
To sum up, when water
freezes:
• The molecules bond in a
crystalline structure
• Like Tinker Toys… when you
put them together, they take up
more space
• When you take them apart, they
take up less space!
What’s the deal with the whole 4
degrees Celsius thing?
• @ 5 C, if you heat water, it acts like
any other substance and expands
• BUT, if you cool it, the molecules
contract until 4 C when they are
the MOST dense (tightly packed)
• As you cool it further, they
EXPAND as polar bonds form and
become LESS dense
Why does this happen???
This is a GOOD thing… if water
sank when it froze, what would
happen to all of the fish?
How does a pond freeze?
• Water at the surface is cooled by
surrounding air
• Once it reaches 4 C, it sinks pushing
warmer water to the surface
• Then the next layer is cooled and then
sinks at 4 C
• The whole pond/ocean will not begin to
freeze until the whole body of water has
reached 4 C
• Called “turning over”
What does a solid “look”
like vs. a gas or a liquid?
As the
temperature
increases…
so does the
average KE!
Molecules
move faster,
preventing
bonds from
holding
together!
Specific heats of some metals
Aluminum
Brass
Copper
Nickel
Silver
Steel
Iron
Gold
0.9025
0.39212
0.38452
0.54
0.237
0.5
0.46
0.129
Conductivity of some metals
•
•
•
•
•
Copper .97
Aluminum .63
Brass .26
Steel .12
Nickel Silver .07