Download Thermodynamics: the study of thermal energy

The Laws of
Thermodynamics
The Zeroth Law !
If Object 1 is in thermal equilibrium with Object 2 and
Object 2 is in thermal equilibrium with Object 3, then
Object 1 is in thermal equilibrium with Object 3.
Well, duh…
The First Law of Thermodynamics
The change in the internal energy of a substance is the
sum of the heat transferred plus the work done.
“internal energy” is associated with the random motion of
all of the molecules that make up an object- both the
kinetic energy and the potential energy. The more
molecules there are, the more internal energy is possible.
Which has more internal
energy: a hot coal or a
frozen lake???
1st Law: The change in the internal energy of a substance
is the sum of the heat transferred plus the work done
D Internal Energy = Q (in or out) + W (in or out)
1st Law: The change in the internal energy of a substance
is the sum of the heat transferred plus the work done
The First Law of Thermodynamics is yet another way to
state that energy is neither created nor destroyed, but may
be transferred or transformed-
The Law of Conservation of Energy
Heat Engines
A “heat engine” takes in heat and
with that heat energy performs
work. It then gives off a lesser
amount of heat at a lower
temperature.
QH = QC + Wout
A heat engine is an example of
the first law of
thermodynamics:
D Internal Energy = Q + W
If we rearrange things we get:
Q (in) = D Internal Energy + W (out)
Heat Engines
Even the very best engines are
not able to transform all the heat
energy into work. Some of it is
ALWAYS “wasted”.
The “ideal” engine is called a
“Carnot Engine”- a perfect,
theoretical (but not physically
possible) design.
Even the Carnot Engine would
not have an efficiency of 100%.
Examples of Heat Engines
Steam Engines produce motion which can be
used to do work or generate electricity.
What is another well-known “heat engine”???
Think of something most of us use every day that
converts heat into motion (with some waste heat).
closed
closed
open
closed
four-stroke internal combustion engine
Internal Combustion Engine Simulator
intake stroke: intake valve is open,
allowing gas and air to enter the
cylinder. The piston moves downward
compression stroke: valves are closed as
the piston moves upward, creating high
pressure in the fuel-air mixture. At the
top of the stroke, the spark plug sparks,
which ignites the fuel-air mixture.
power stroke: the explosion pushes the
piston down. The piston’s rod turns the
crankshaft, which provides the torque to
turn the wheels.
exhaust stroke: the piston moves back
upward. The exhaust valve opens to
allow exhaust gases to leave the
cylinder.
Of course, much of the energy released by
the combustion of fuel does not produce
the mechanical energy of motion in the
car.
In what form does much of that released
chemical potential energy appear???
HEAT !
The 2nd Law of Thermodynamics
The Law of Entropy:
Natural processes always increase entropy.
Entropy: disorder
What does “entropy” have to do
with heat??
… Go back to the Kinetic-Molecular Theory:
The faster atoms are moving, the more
“disorder” or ENTROPY they have.
So, if you increase the temperature of a
substance, you also increase its ENTROPY.
Absolute Zero is the temperature at which
entropy would reach its minimum value.
2nd Law: Natural processes always increase
entropy.
Alternative statement of the 2nd Law:
Heat flows spontaneously, “naturally”, from a hotter
substance to a cooler substance
…the “heat” in the cooler substance will NOT flow
out of that cooler substance into the warmer
substance to make it even more warm!!
That nice “orderly” cooler substance is naturally
going to become more “disorderly”
The Second Law of Thermodynamics
Heat won't pass from a cooler to a
hotter.
You can try it if you like,
But you far better notta,
cause the cold in the cooler
Will get hotter as a ruler,
And that's a physical law!
[Michael Flanders and Donald Swan]
Just like water flows naturally from high elevation
to low elevation, heat flows naturally from
warmer temperature to cooler temperature.
You can make water move from low elevation to
high elevation if you use a water pump, which
requires an input of energy.
You can make heat flow from a cooler object to a
warmer object if you use a heat pump, which
requires an input of energy.
Heat pumps are used in your refrigerator and in
the heating/air conditioning system in houses!
Different kinds of heat pumps:
Cooler inside air
Cooler inside air
Direction of heat
flow with the
spontaneous,
“natural”process
Warmer outside air
Direction of heat
flow with a heat
pump
Warmer outside air
What is “Absolute Zero”?
For ALL gases, as the temperature drops, the
pressure within the gas drops in a direct
relationship.
Graphing pressure vs temperature for many gases
and then EXTRAPOLATING the graphs to a
pressure of ZERO (which is impossible) yields the
same temperature for every gas:
-273 C = 0 Kelvins = Absolute Zero
3rd Law: It is not possible to lower the
temperature to absolute zero.
Since absolute zero is the temperature at
which a gas would exert zero pressurewhich is impossible, reaching 0 Kelvins is
impossible also.
c) (6) E,F,G, TAKS I.6B
Other changes due to heat flow…
Gases: the relationship
between temperature,
pressure and volume:
If the volume of a gas is held constant,
as the temperature increases, the gas
pressure increases as well.
(Gay-Lussac’s Law)
If the pressure is held constant, as the
temperature increases, the volume will
increase.
(Charles’ Law)
Thermal Expansion
Generally, as temperature increases, the
lengths and/or volumes of substances,
including liquids and solids, also increase.
Examples:
concrete sidewalks expand and crack in hot
weather,
The air in a “hot air” balloon expands and
fills out the balloon
Metals expand when heated and contract
when cooled.
Gasoline expands in your gas tank on a hot
summer day.
Bimetallic strips:
used in thermostats
– How much will the length or volume
change?
DL = LoaDT
a - the coefficient of linear expansion
DV = VobDT
b- the coefficient of volume expansion
Example
1. An overpass bridge is about 300 m long.
How much will it expand if the temperature
changes by 30º C (asteel = 12 x 10-6 )
Lo = 300
DT = 30
asteel = 12 x 10-6
D L = Loa DT = 300 (12 x 10-6) 30 =
0.108 m
As the temperature of
water drops its
volume decreases
until 4°C is
reached.
Then its volume
expands as the
temperature
continues to drop
to 0°C.
Volume
One exception to the rule of
thermal expansion is water
Temperature
Result: ice is less
dense than water
so that ice floats.
This protects
underwater life in
cold climates