BOARD NOTES: THERMODYNAMICS
IDEA/ CONCEPT
NOTES
THERMODYNAMICS *Study of heat and transformation into mechanical energy
*Conservation of energy and how heat flows from hot to cold
(never other way around)
24.1 ABSOLUTE
ZERO
*Thermal motion of atoms increases as temperature
increases
*No upper limit of temperature, only lower limit
*As temperature decreases, kinetic energy decreases
ABSOLUTE ZERO
*No more energy can be extracted from a substance and
temperature cannot be lowered
*Absolute zero is -273 C
*No negative numbers on Kelvin scale
*Ice melts at 0 C or 273 K, water boils at 100  C or 373K
24.2 1st LAW OF
*Heat= energy, energy cannot be created nor destroyed
THERMODYNAMICS *Whenever heat is added to a system, it transforms to an
equal amount of some other form of energy
SYSTEM
What happens
when you put an
airtight can on a hot
plate & heat it up?
What about if the
can is fitted with a
moveable piston?
*any group of atoms, molecules, particles, or objects
*EXAMPLES: steam in an engine, Earth’s atmosphere, human
body
*We add energy to do work on external things. Added energy
can: 1. Increase internal energy of the system if it remains in
the system 2. Does external work if it leaves the system
*HEAT ADDED= increase in
external work
internal energy + done by system
The temperature inside the can increases. No external work is
done, so the heat added= the increase in internal energy of
the can.
Now the internal energy of the can is less than the heat added,
since work is being done by the piston.
*Heat is not the only way to increase internal energy of a
system, you can also add pressure
PRACTICE PROBS:
1. If 10 J of energy is added to a system that does no external
work, by how much will the internal energy of that system be
raised?
10 J
2. If 10 J of energy is added to a system that does 4 J of
external work, by how much will the internal energy of that
system be raised?
24.3 ADIABATIC
PROCESSES
ADIABATIC
10J – 4J= 6J
Process of compression or expansion of a gas so that no heat
enters or leaves a system
EXAMPLE: compression/ expansion of gases in cylinders of a
car (show video clip from youtube of cycle of a four- stroke
engine)
*compression & expansion occur in only a few 100ths of a
second (short period of time disallows for appreciable heat
energy to leave the combustion chamber)
*When work is done by adiabatically compressing it, gas
gains internal energy and becomes warmer. When a gas
expands, it does work on its surroundings and gives up
internal energy and becomes cooler
*Air temp. changes by + or – heat, changing pressure or both
*Change in temp ~ pressure change
*Adiabatic processes occur in large masses (blobs) of air
*As blobs flow up mountains, pressure decreases allowing it
to expand and cool
* reduces pressure = reduced temperature
*10  C decreases in temp. for each 1 km increase in altitude
1. If a blob of air initially at 0 C expands adiabatically while
flowing upward alongside a mountain a vertical distance of 1
km, what will its temperature be? When it has risen in 5 km?
PRACTICE
PROBLEMS:
At 1 km, the temp. will be -10 C, at 5 km, it will be -50 C
2. Imagine a giant bag full of air at a temp. of -10 C floating
like a balloon with a string hanging from it 6km above the
ground. If you were able to yank the bag to the ground, what
would it’s approximate temp. be?
50 C
BOARD NOTES: THERMODYNAMICS
24.4 2nd Law of
Thermodynamics
*Heat will never of itself flow from a cold object to a hot
object. (HIGH TO LOW IS THE WAY TO FLOW)
*Heat flows one way, downhill from hot to cold. In winter,
heat flows from inside a warm heated home to the cold air
outside. In summer, hot air from outside flows inside to a cool
house.
24.5 Heat Engines
and the 2nd Law
How can you easily
change work into
heat?
Rub your hands together and your hands become hot.
HEAT ENGINE
*Any device that changes internal energy into mechanical
work.
*The idea behind any engine (jet engine, steam engine,
internal combustion engine) is that mechanical work can be
obtained only when heat flow from high to low temps.
*In every heat engine, only some of the heat can be
transformed into work.
EFFICIENCY
What does
efficiency mean on
a basic level?
What you get out/ what you put in  decimal convert to %
Discussion Questions:
1. What is the highest efficiency possible?
2. Why is 100% the highest possible efficiency?
3. What does conservation of energy mean?
4. Where does the energy go if it is lost?
Example:
Simple Machines
Pulley
How much effort would a person using a pulley have to put in
to move a 40 kg box, ideally?
If it actually took him 50 kg of effort force to move the box,
what would be the efficiency of the pulley?
Example Problem:
1. If I can pull with 60 lbs of force and the pulley is 70%
efficiency, how much weight could I lift?
Efficiency and
Engines




Show figure 16.3 (pg 521) and explain the Otto Cycle.
Explain liter on cats (2.4 liter engine vs. 3.2 liter
engine), V-8/ V-6/ V-4 engines
Why do we care about the efficiency of a gas engine?
What could be a downside of making a very efficient
engine?
Calculating
efficiency of a gas
engine
*Gas engines are actually heat engines.
*To calculate the efficiency of a heat engine we care about the
heat going in and the energy coming out, both in terms of
Joules (J).
* Do we pump heat into our cars?
*How does this gas turn into heat? (refer back to figure 16.3).
*We can figure out the heat produced by the heat constant of
different fuels (briefly explain what constants are).
*The constant of gasoline (50,000 J/ g)
Practice Problem:
If I use 15g of gas and it produces 1,000,000 J of work, what is
the efficiency of the engine?
15g* 50,000J/ g= 7,500,000J
1,000,000/7,000,000= 14.3%