Heat & Thermodynamics
What is the Difference Between Heat
and Temperature?
Both are related to
energy but there’s a
big difference
Temperature
Measure of speed of particles (kinetic
energy)
Measured by thermometers
Work by expansion of a liquid
Other types use bimetallic strip
Digital Thermometers
Use “thermistors” - temperature dependant
semiconductor resistors
Temperature Scales
Fahrenheit
T(0F) = (1.8x 0C) + 32
Celsius (centigrade)
T(0C ) = [T(0F) –32]/1.8
Kelvin (Celsius + 273)
Examples
Zero degrees Celsius is what Kelvin?
Answer: 273o
What is the boiling point of water in degrees
Kelvin?
Answer: 373o
200 degrees Celsius is what in Kelvin?
Answer: 473o
Absolute Zero
0 degrees Kelvin = -273 Celsius
Lowest possible temperature
Molecular motion ceases
Courtesy Michigan State
University
Kinetic Theory of Heat
All matter is made of
tiny atoms and
molecules,
constantly in motion
Faster is hotter
solid
gas
Temperature and Kinetic Energy
In ideal gas
temperature is
proportional to
average kinetic
energy per molecule
Closely related in
liquids and gases
Temperature does not depend on
the amount
Heat does depend on the
amount
There is twice as much kinetic
energy of moving molecules in
two liters of water as in one
liter.
Analogy: Heat is like the total height of students
in this room, temperature is like their average
height.
Which has more heat?
A swimming pool full
of ice water?
A cup full of boiling
water?
Answer: the swimming pool, because it has so
much more water.
Heat – Energy Transferred
Definition: Energy that transfers
because of temperature difference
Heat flows governed by average
molecular kinetic energy difference
Heat always flows from
high energy to low
Cold- Absence of Heat
* does not exist!
Thermal Equilibrium
Objects at same
temperature are
at thermal
equilibrium – no
heat flows.
Measuring Heat
One calorie is the amount of heat
needed to raise the temperature of one
gram of water by one degree Celsius.
Kilocalorie raises the temperature of one
kg of water by 10 C (also called Calorie
or food calorie)
One calorie = 4.186 joules
One kilocalorie = 4186 joules
One kilocalorie= 1 Food Calorie
Density of Water
Density is mass per unit volume
D = M/V
One gram per cubic centimeter
One kilogram per liter
One thousand kg per cubic meter
Specific Heat Capacity
Different materials change their
temperature by different amounts when
they absorb the same amount of heat.
Some have more ways of storing
energy than others
Water has very high specific heat
(capacity)
Metals have much less
Q = mcDT
Q = mcDT expresses how heat absorption
works. C is specific heat
Question: A certain rock has a specific heat
of 0.25 (water is 1.0) How much heat will be
required to heat 5.0 kg rock from 20 to 800C?
Q = 5.0kg x 1000g/kg x 0.25 Cal/g 0C x 60 0C
Q = 75,000 C = 7.5 x 104 Calories
Calories
1 calorie raises the temperature of 1 gram of
water by 1 Celsius degree.
1 kilocalorie (kcal or Calorie) raises
temperature of 1 kg of water by 1 degree
Celsius
1 British Thermal Unit (BTU) raises
temperature of 1 pound of water by one
degree Fahrenheit
Application
If 10 calories of heat go into a gram of water,
how much will the temperature increase?
Answer : 10 degrees C
How much heat is needed to raise the
temperature of 10 grams of water by one
degree C?
Answer: 10 calories
How much heat is needed to raise the
temperature of 10 grams of water by 10
degrees C?
Answer: 100 calories
Specific Heat
How much does temperature rise when heat
is put into something?
It depends on the material as well as the
mass and the quantity of heat:
Q = m c Dt c is specific heat in calories/g
oC
Water has the highest specific heat of any
common material, 1 cal/g oC
Metals generally have low specific heats,
which makes them easy to cool or heat.
Temperature Change
Exothermic Reactions, are where heat
is released, this will cause the
temperature of the surrounding to
increase. (think a roaring fire to warm a
cold room) ∆T = Tf-Ti
Endothermic Reactions, are where
heat is absorbed, this will cause the
temperature of the surroundings to
decrease. (think ice melting in warm
drink)
∆T = Ti-Tf
Example
How much heat is required to raise the
temperature of 1000g water from room
temperature (20oC) to boiling (100oC)?
Q = m c Dt = 1000g x 1 cal/g oC x 80 oC
= 80,000 calories (or 80 kilocalories)
Fact: It would take about a tenth as much
heat to raise the temperature of an equal
amount of iron this much
Other Examples
If 2000 J of energy are added to .25 kg of
water at an initial temperature of 35⁰C
what will the final temperature be?
If 5000 cal are added to an unknown
substance and 200 g of it change
temperature by 34⁰C what is the
specific heat of that substance?
Mixtures
100 g of water at 50oC is added to 100g water
at 70oC. What will be the final temperature?
You guessed it: 60oC
Mix a liter of 20oC water with two liters of
30oC water. What is the final temperature of
the system?
Calorimeter
Energy device used to eliminate the loss of
energy to the surroundings.
Thermos and styrofoam cup work on same
principles.
High Specific Heat of Water
Makes it a good coolant (water also has high
conductivity although this is not the same)
Large bodies of water such as oceans
moderate climate
– Gives coastal communities relatively mild
summers and winters
Another peculiar fact about water. It’s highest
density (and smallest volume) is at 4oC.
– Water at bottom of frozen lake is always 4oC
Change of Phase
Heat of Fusion
Energy needed to melt 1 gram of a solid into
its liquid form.
Hf ice= 80 cal/g
Hf= Q/m
Q= Hf x m
m= Q/Hf
This is endothermic, absorbed into the
system, the same is true of the releasing,
just called Heat of Crystallization.
Heat of Vaporization
Energy needed to vaporize 1 g of some liquid
into a gas.
Hv water= 540 cal/g
Hv= Q/m
Q= Hv x m
m = Q/Hv
This is endothermic as well, again being
absorbed, the same is true of releasing just
called Heat of Condensation.
Phase Change Example
How many calories are required to raise the
temperature of 85 grams of – 20⁰C ice to
150⁰C?
(Think about all the different steps you must take
to solve this problem)
∆T of ice
Phase change (melt the ice)
∆T of water
Phase change (boil the water)
∆T of steam
Heat of Fusion Lab (Prove Hf= 80 cal/g)
Cook 125 ml of water to exactly 50⁰C. Record Ti.
Pour exactly 100 ml of water into sty. cup. Record Vi.
(DO NOT DESTROY STY. CUPS)
Add ice chips 2-3 at a time, carefully stirring with
thermometer, until you reach 0⁰C. Record Tf.
Carefully remove any ice chips. Record Vf.
Remember Dwater 1g= 1 ml
Calculate Q (released by water)= m x ∆T x c
Calculate the mass of melted ice= Vf-Vi
Calculate the Hf of ice = Q/m
Answer this question: Why did we start at 50⁰C?
Hint: Think about heat exchange and room temperature.
Thermal Expansion
Most materials expand when heated
Only exception is water between 00C
and 400C
Expansion joints in bridges, cracks in
sidewalks allow for expansion
Bimetallic Strip
How your thermostat
works
Don’t Let Your Car’s Engine Overheat
Aluminum expands
more than iron
Pistons made of
aluminum
Cylinder made of
iron
Mechanical Equivalent of Heat
Discovered by James Joule
Falling weight makes
paddle turn
4.186 x 103 J = 1 kcal
Interpretation:
HEAT IS ENERGY
TRANSFER
Courtesy W. Bauer
http://lecture.lite.msu.edu/~mmp/kap
11/cd295.htm
Joule’s Apparatus
Link to Joule’s
original article
Example
When digested a slice of bread yields 100
kcal. How high a hill would a 60 kg student
need to climb to “work off” this slice of
bread?
100
kcal x 4.186 x 103 J/kcal = 4.2 x 105 J
W = mgh
h = W/mg = 4.2 x 105 / (60 kg)(9.80 m/s2) =
714m = 7.1 x 102 m
If the body is only 20 percent efficient in transforming
the bread, how high need they climb?
Bullet in Block
When a 10 g bullet traveling 500 m/s is
stopped inside a 1kg wood block nearly all its
KE is transformed to heat. How many kcal
are released?
KE = ½ mv2 = 0.5 x 0.010 kg x (500)2 = 1250 J
1250 J x 1 kcal/4186 J = 0.30 kcal
Thermodynamics
Study of heat and its transformation into
mechanical energy
Based on conservation of energy
Explains how engines like car motors
work
First Law of Thermodynamics
Generally, when you add heat to a
system it changes into an equal amount
of some other form of energy
Heat added = increase in internal
energy + external work done by the
system
Work Done On and By
Compressing a gas
by pushing down on
a piston = work
done on
A gas expands by
pushing a piston up
= work done by
Questions
20 J of heat is added to a system that
does no work. What is the change in
internal energy?
Answer +20 J
20 J of heat is added to a system that
does 10 J of work. What is the change
in internal energy?
Answer +10 J
20 J of heat is added to a system that
does 30 J of work. What is the change
of internal energy?
Answer -10 J
20 J of heat is added to a system that
has 10 J of work done on it. What is the
change of internal energy?
Answer +30 J
Bicycle Pump
What do you think happens when you
operate the pump. Where does the
work you do go?
It goes to heat, some through friction,
some to adiabatic compression of the
air inside the pump
What does “adiabatic” mean?
Answer: No heat enters or leaves Q=0
Adiabatic Processes
Compression or expansion of a gas so that no
heat enters or leaves
Example: gas in cylinder of car or diesel engine
Why adiabatic? Because it happens too fast for
much heat to enter or leave.
In adiabatic compression, temperature rises.
– In diesel engine, enough to ignite gas without spark
plug
A process can also be adiabatic
if it happens inside a well
insulated conatiner.
Courtesy “How
Stuff Works”
Courtesy Shell
Canada
Adiabatic Expansion
Produces cooling
Example: blow on your hand first with
wide open mouth, then with puckered
lips
How do you explain the results?
The Chinook
What would you expect to happen if
cold air moves down the slopes of
mountains
Hint: it will be compressed by
atmosphere into smaller volume
Chinook wind is warm
Common in Rocky mountains
Second Law of Thermodynamics
Heat flows from hot to cold. By itself it
will never flow from cold to hot.
Question: Would it violate the First Law
of Thermodynamics (energy
conservation) if heat flowed from a cold
object to a warm object touching it?
Answer: No
Second Law Applied to Engines
It is impossible to build a heat engine
that changes heat completely into work.
Such an engine would
be 100% efficient!
Allowed by 1st law,
forbidden by 2nd law
Courtesy University of Oregon
Heat Engine
Some heat is converted to useful work
The rest is exhausted on at a lower
temperature (cause of thermal pollution)
Efficiency = useful work / heat input
– About 20-25% for gasoline engine
– About 35-40% for diesel engine
The energy exhausted is waste, cannot
be recovered
Ideal (Carnot) Engine
Ideal (Maximum possible efficiency) =
(Thot – Tcold)/Thot (Kelvin temperatures)
What is the efficiency of a steam turbine (assumed
ideal) operating between 400K (1270C) and 300K
(270C)?
(400 – 300)/400 = ¼ or 25%
What would be the efficiency if the turbine could
1/2
operate at 600K?
What would the exhaust temperature need to be for
an engine to be 100% efficient?
0K
Steam Turbine
Limits to Technology
What factors limit the efficiency of an
engine?
– Friction
– Temperature at which parts melt
– Carnot efficiency
What would be the advantages of a ceramic
engine? Disadvantages?
Can operate at 3000 degrees without cooling, is
light and doesn’t need much cooling, but…
Courtesy University of Colorado
Heat Engine Summary
Work done is
difference between
heat flow in at high
temperature and the
heat flow out at a
lower temperature
(conservation of
energy)
Order and Disorder
Useful energy tends to degenerate and
become less useful
Alternate statement of 2nd Law: Natural
systems tend toward disorder
Question: Could all the air molecules in
this room spontaneously concentrate at
the top of the room (more orderly
system)?
Entropy
A measure of how much change occurs
when energy spreads out according to
the second law.
More generally (and less accurately) a
measure of disorder
When disorder increases, entropy
increases
Mess to Neat?
Will this mess
become neat all by
itself?
No way!
Will this dish reassemble all by itself?
No Way, the Second Law of
Thermodynamics prohibits it
Entropy Summary
Entropy is a quantity that measures the
order or disorder of a system
This quantity is larger for a more
disordered system
The Second Law of Thermodynamics
says that entropy tends to increase
All real engines lose heat to their
surroundings
Courtesy California Science Standards in Physics
Global Warming
Courtesy University of Oregon
Calorimetry and Specific Heat
Heat and Temperature Basics
Temperature does not depend on the
amount
If two samples of identical material are at
the same temperature, the sample with
more mass has more thermal energy
(internal energy)
Heat is thermal energy transferred
Internal energy is thermal energy in
something
Which Contains More Thermal
Energy?
A cup of boiling water or a swimming pool
frozen solid?
Answer: the swimming pool. What it loses in
temperature it more than makes up in mass
This will become clearer as we learn more…
Hot Stuff
What would happen if 1 kg iron (specific heat
0.11 calories/ g oC) at 300 oC were placed in
200g water at 20 oC?
Heat lost by iron = heat gained by water
Let TW be initial temp. of water; TI that of iron;
TF final temp of both
mIcI(TI-TF) = mW cW (TF – TW)
mIcI TI – mIcITF = mW cW TF - mW cW TW