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Laws of Thermodynamics0th Law: If Ta=Tb and Tb=Tc then
Ta=Tc
1st Law: ΔU=Uf - Ui=W+Q (Conservation of Energy)The increase in internal energy of a
system is determined by the heat of the system and the work added to it.
The idea behind the Heat Engine
Isobaric- Constant PressureQ
Adiabatic- No Heat Exchange (also no change in entropy)
Q= 0 & ΔS=0 =constant
A PV diagram can be used to represent the state changes of a system, provided the system is always near equilibrium.
1st Law & Thermodynamic ProcessesProcess ΔU Q W
Isobarric nCvΔT nCpΔT - PΔV
Adiabatic nCvΔT 0 ΔU
Isovolumetric nCvΔT ΔU 0
Isothermal 0 - W
GeneralnCvΔT ΔU - W
(PV Area)U=
Table 12.2 for Monatomic Ideal Gas
2nd Law: A system can be defined by its entropy. In a closed system entropy tends to increase.Entropy: A measure of the disorder
(randomness) of the system3rd Law: Temperature and Entropy
are absolute scales.At some point no temperature or
entropy exists.T→0 K and S→S0
Heat Engines Engines allow heat energy to be
transformed into work or mechanical energy.Work or Energy Heat increases
no big deal. Friction does this.Heat Work or Energy
is a big deal. Heat is easy to move around. You could just bring heat wherever you needed work done and “Boom!” you wouldn’t have to do the work, a machine could.
Work or Energy Heat decreases is also a big deal. Making food cold preserves it and
allows it to be moved readily. Less spoilage means less disease.
Any heat engine works on the same properties. A hot reservoir is the source of the
energy. Both words mean something. Hot means
that there is plenty of heat energy and reservoir means that if heat is removed the temperature doesn’t drop much.
There is also a need for a cold reservoir. Again, both words mean something. Cold because it is at a lower temperature
than the hot reservoir and reservoir because it must be large enough that you can dump heat into it without appreciably raising the temperature.
What happens if we put a hot and cold reservoir in contact? Thermal Equilibrium is
not the answer! Heat transfer (or flow) is the answer.
Remember that these are reservoirs so it would take a long time for them to come into thermal equilibrium.
This is great, but we don’t get any work out of it.
We need to “steal” some of the energy leaving the hot reservoir and make it do work for us.
Unfortunately we don’t get an even trade!
We lose energy to randomness/EntropyThis is the 2nd Law of Thermodynamics
Automobile engines are only about 15% efficient. That means for every 100J of heat energy, 15J worth of work is done on the piston and 85J of heat are discarded. Still, this is the source of energy for most of our transportation.
h
c
h
ch
h
net
QQQ
QWeff
1
The efficiency of an engine is defined by
𝑒𝑓𝑓=𝑁𝑒𝑡𝑊𝑜𝑟𝑘𝑑𝑜𝑛𝑒𝑏𝑦 h𝑡 𝑒𝑒𝑛𝑔𝑖𝑛𝑒𝐻𝑒𝑎𝑡 𝑖𝑛𝑝𝑢𝑡
If a steam engine takes in 2.254 x 104 kJ of heat and gives up 1.915 x 104 kJ of heat to the exhaust, what is the engines efficiency? How much work is done?
h
c
1eff
15%or 10 x 2.25410 x 1.915
4
4
15.
1
kJkJeff
Carnot Engine Maximum efficiency
In theory could run backwards
All temperatures in Kelvin What is the efficiency of an ideal steam
engine with steam at 685 K and exhaust at 298 K?What is Qhot if Qcold is 450J?
h
ch
TTT
inputHeatoutputWorkeff
This requires Energy/WorkThis is why your refrigerator must be
plugged in.It is constantly dumping heat into your
kitchenDue to the 2nd Law of
Thermodynamics more heat is dumped than is removedIf you left the refrigerator door open you
would heat up the kitchen
EfficiencyWnet=Qnet=Qhot-Qcold
or Eff=Wnet/Qhot=(1-Qcold/Qhot)
Ex: A steam engine absorbs 1.98 x105J and expels 1.49 x105J in each cycle. Assume that all the remaining energy is used to do work.a. What is the engine’s efficiency?b. How much work is done in each
cycle?
More 2nd Law Entropy
S=entropyQ=Heat (Joules)T=Temperature (In Kelvin)
Entropy in a system must increase or at least stay the same!!!!!!!!!!!!
TQS
An engine has a hot reservoir at 1000 K and uses the atmosphere at 300 K as the cold reservoir. You take 2500 J from the hot reservoir to do 1900 J of work.A. How much heat goes into the
atmosphere?B. Is this engine possible? (Does the
entropy increase?)