What happens when we place a hot bowl of soup in a cool room?
air trapped
air at 15 0C
from outside
at 25 0C
oC
water at 15 0C
ice water at 5
Why the difference in the two scenarios?
Different number of molecules involved in energy exchange
Once the temperature equalizes there is no further NET transfer of
thermal energy. This condition (no NET transfer) is called
Thermal Equilibrium
Thermal Equilibrium
make thermal contact
something
warm
something
two some things at an
cool
“intermediate” temperature
What happens over a period of time?
air = 25 C
soup = 95 C
There is a net transfer of thermal
from the soup to the air. As a result,
the temperature of the soup and the
temperature of the air become the
same. (i.e. reach Thermal Equilibrium)
more KinE
more momentum
air
soup
more KinE
more momentum
some KinE lost
some momentum lost
what can we say about
the average kE of the
soup molecules
compared to the air
molecules? ..how is
kE related to temp?
The zero th law of Thermodynamics: (i.e. the transitive property)
If A is in thermal equilibrium with B (Temp A = Temp B)
And B is in thermal equilibrium with C (Temp B = Temp C)
Then A is also in thermal equilibrium with C (Temp A = Temp C)
A
B
C
Thermal equilibrium
A
B
C
Consider once again the Bowl of Soup example……..
At first
less average
thermal
energy
Later that day
same average thermal
energy everywhere
more average
thermal energy
In which situation is the energy of the system (bowl of soup + air in room)
more organized ?
At First
you could easily select a molecule with larger
thermal (kE) energy……. just scoop out some
of the “hot” soup
Later that
day
molecules of various energies
evenly distributed throughout
the soup and air…who knows
where to find one with a lot of
energy
As a general rule, the disorganization of the energy of the Universe
NEVER DECREASES as a result of a process. And, the disorganization
of the energy of the Universe could only stay the same as a result of
an IDEAL process (which never actually occurs)
NOTE: Over time, things run down…but the universe does not lose energy!!
Remember the track and ball example:
“start”
“later”
“IDEAL CASE”
hE
hA
At the “start” the energy is in an organized state, at point E the energy is still all in just as organized a state
“start”
hA
all organized
“later”
h E < hA
“REAL CASE”
not all
organized
We use the word Entropy to describe the quantitative measure of
disorganization of energy.
1. The soup and the air have higher entropy after coming to
the same temperature than before
2. The ball and the track along with the surroundings have more
entropy when the ball is at point E than at point A in the
“REAL CASE”
The 4 laws of Thermodynamics:
zero: If two systems are in thermal equilibrium with a third system, then
then they are also in thermal equilibrium with one another
one: energy can neither be created nor destroyed, only transformed into
another type of Energy
two: Entropy NEVER decreases and only stays the same in IDEAL processes
three: there is a lowest possible temperature (= -273 C) it is not attainable
due to a number of irreversible energy transfer
play
Flow of thermal energy is essential to heat engines.
somewhat
disorganized
Tin
measure these
in Kelvin, K,
K = C + 273.15
organized
2nd Law: Any
process that uses
thermal energy to
do work must also
have a thermal
energy output or
exhaust. In other
words, heat engines
are always less than
100% efficient.
3rd Law: It is not
possible for Tex to
be at or lower than
absolute zero (0 K)
Tex
highly disorganized
“exhaust”
“waste”
Summary:
1. Every heat engine has at least some thermal energy as
output.
2. even an “ideal” heat engine (which does not exist) is less
100% efficient
The Ideal Efficiency (IE) of a heat engine is computed using:
Tin Tex
IE  
 Tin

  100%

Note: Tin and Tex must be expressed in Kelvin (K)
For any real heat engine, the Actual Efficiency is less than the
Ideal Efficiency:
AE < IE
Exponential Growth
thermodynamics: 1. Energy quality runs down as time proceeds
2. Useful energy is needed to do work and thus
support human civilization
history: Human populations grow at nearly an exponential rate
problem: Will we run out of high quality energy resources?
Solution: 1. Renewable energy resources?
2. Slow down the exponential population growth?
Linear Growth
Start a new job and work for free the 1st day with the agreement
that you get a fixed $10,000 raise each day.
Exponential Growth
Start a new job and work for $1 the 1st day with the agreement that
you get a fixed 1% raise each day.
Which job would you take (assuming each involves the same set of
responsibilities?
A. job #1 – Linear
B. job #2 – Exponential
Does the length of the contract affect your decision?
Linear
Exp
50000000
Exponential Growth
at 1% /day
Daily Wage
40000000
30000000
Linear Growth
at $10,000/day
20000000
10000000
0
0
500
1000
Days
1500
2000
8
7
Daily Wage
double
6
5
4
double3
2
double
1
0
0
70
T = 70/P
140
T = 70/P
Days
210
T = 70/P
Population of Basset Hounds
20
18
16
Exponential growth
at 2% / year
14
12
10
8
Exponential growth
at 1%/year
6
4
2
0
0
35
T = 70/2% = 35 years
70
Years
105
140
T = 70/1% = 70 years
For exponential growth at rate P%, the population DOUBLES in
a time:
70
T 
P%
If P% is P%/year then T is in years
If P% is P%/day then T is in days…etc…
NOTE: this approximation only works for P% < 10%!!