Thermodynamics
1 – Mechanical Equivalent of Heat
 Looking at an object on a microscopic scale we see that the
molecules are in…
 As we heat the object the motion becomes…
 On a macroscopic scale if you touch an object with a high
temperature you perceive the ___________ of ___________ as a
sensation of warmth.
 What is the relationship between the temperature of an object
and the molecules that comprise the object?
o Temperature…
Whenever two objects with different temperatures come
into contact, energy...
The Zeroeth Law of Thermodynamics
We call this flowing energy ________________.
When two objects reach the same temperature they are
said to be in...
If you hammer a nail into a board what
happens to the temperature of the nail?
Ex: What is the mechanical energy expended by the mechanical apparatus?
Where does the heat energy come from?
Note that it takes ______ cal to increase _______ of water by _______.
Ex: How many calories of heat are required to raise the
temperature of the water by 1/100th of a degree Celsius,
given:
mwater = 468 g
c = 1 cal/goC
∆T = 0.01 oC
Q=
mc∆T
Ex: The mass of the nail is 5.0 g, the temperature change
of the nail is 10 oC and it takes an input of 0.11 calories of
heat per gram to raise the temperature of the nail 1 oC.
What is the work done by the hammer?
The mechanical equivalent of heat is 1 calorie = 4.19 Joule
Ex: Use ∆T, m, h and mfluid to determine the specific heat capacity of the fluid.
2 – Heat Transfer
(1) Heat Conduction: The transfer of heat from the ____ end of an
object to the _____ end of an object via...
The formula for heat transfer
∆Q = kA(T1 – T2)
t
l
Ex: A house has a 1cm thick concrete wall. The
temperature outside is 15 oC and inside is 20 oC.
How thick of an aluminum wall would have the
same rate of heat transfer?
Where:
Q=
t=
k=
A=
T=
l=
Heat Transfer is measured in ____________________
The higher the value of thermal conductivity, k, the _________ the
material is at conducting heat.
Metals are good conductors because they have a lot of __________
______________ which aid in heat transfer.
Ex: A wall on a house is 20 cm thick and composed of wood and an insulator. Calculate the thermal conductivity, k of the
unknown insulator.
(2) Convection: the transfer of heat by....
Convection and conduction both require
____________ with ___________ in
order to transfer heat.
(3) Radiation: Involves the ___________________ and __________________ of
______________________ radiation and therefore does not require a medium
for heat transfer.
Rate of Radiation is given by the Stephan-Boltzmann Equation
∆Q = eσAT4
t
Where: e =
σ = 5.67 x 10-8 W/m2K4
What type of object has an emissivity close to one?
If an object is a good emitter, then it is a __________ absorber.
A perfect absorber with an emissivity close to one is a __________ _________
Ex: What is the initial rate of heat loss of a cookie at 200 oC, an area of 0.005
m2 and an emissivity of 0.1?
Ex: If the diameter of the cookie is
doubled, what happens to the rate of
thermal radiation?
Most substances expand when heated. The expansion of an
object in one direction is called _________________
_____________________.
Ex: Suppose we have a thin strip of steel bonded to the
bottom of a strip of aluminum. If we heat the bimetallic
strip, what will happen to it?
The equation for linear expansion is
Explain.
∆L = αLo∆T
Where:
α=
Lo =
Since liquids and gases have no fixed shape they do not undergo linear
expansion. Any state of matter can undergo volume expansion as follows:
Ex: What happens to the diameter of a
ring when it is heated?
∆V = βVo∆T
What happens to the diameter of a circular
hole in an aluminum sheet when it is
heated?
Where: β =
In general the coefficients of volume expansion for gases and liquids are
__________________ than those for solids.
3 – Ideal Gases
The Gas Laws
All gasses exhibit similar properties at very low densities.
We call such gasses ____________ ____________.
Boyle’s Law
Charles’ Law
The important variables are:
m
P
V
T
From Boyle’s Law, at a
constant temperature:
Gay-Lussac’s Law
A plot of V vs. T for
an ideal gas gives a
straight line whose
theoretical zero is
_______________
____________.
From Charles’ Law, at a
constant pressure:
__________________ temperature units are always used in calculations involving ideal gasses and absolute temperatures.
The number of moles of a substance:
n=
m=
M=
The Ideal Gas Law
P measured in _______
T measured in _______
R =Ideal gas constant
=
n = measured in ______
V = measured in ______ = ___________ under standard conditions for ideal gasses
The number of moles of a gas:
We can simplify by defining a new
constant, Boltzmann’s Constant:
The Ideal Gas Law can now be written in terms of
the number of gas molecules:
Kinetic theory states that matter is made up of atoms and molecules that are in ______________
_______________ ________________.
The kinetic theory of gasses makes four assumptions.
(1) Gas is made up of ...
(2) Gas molecules are usually _________ ________ _____________ relative to their size.
(3) Gas molecules interact only by ______________________ and that these obey the laws
of Newtonian Mechanics.
(4) Collisions between molecules, or between molecules and the wall of the container are
____________________ (meaning no kinetic energy is lost).
The pressure that a gas exerts on the wall of a container is the
result of molecules...
 This provides us with an explanation of why molecules
making up a substance move faster when the
_________________ of the substance is increased.
By relating force to pressure and applying the ideal gas law we
can relate the temperature of a gas to its molecular properties.
We can calculate the average speed of particles by taking
the square root of the average the square of speed.
½mv2 = 3/2kT = Ek
This is called the root mean squared velocity.
4 – Thermodynamics
The 1st Law of Thermodynamics states:
For an ideal gas, the internal energy can be given by
And the average kinetic energy is given by
Therefore,
U = 3/2nRT
Ex: A gas is compressed by displacing a piston a
certain distance in the direction of the force. This is
done slowly to ensure that that gas remains in
equilibrium at a constant pressure. What is the
work done by the gas?
Note that the work done on the system is positive.
The above relationship doesn’t
work if the pressure changes.
However in the more general
case the work is equal to the
________ ________________
the P vs. V curve.
There are two kinds of processes that frequently occur in laboratory experiments:
Isothermal:
Adiabatic:
T=
For an adiabatic process:
Therefore, PV =
Therefore:
And ∆U =
Looking at a PV diagram for isothermal and
adiabatic processes, it is evident that for a given
change in pressure, an adiabatic process results in a
smaller change in ____________ than an isothermal
process. This is because during an adiabatic
compression _______________ changes as well.
Heat Engines
The Second Law of Thermodynamics
 A basic heat engine takes in energy from a ______ reservoir, converts some of
this heat into _________ and expels the remaining heat into a _____ reservoir.
“Heat does not flow...
 The process of converting heat energy into mechanical work requires that heat
flows from...
 A heat engine usually runs in a ___________, returning to its starting point
after each unit of work is done.
 Because the change in internal energy for each cycle must be zero, the net
work done by the engine is equal to...
For one cycle:
The efficiency of a heat engine is defined as:
Therefore:
Ex: An engine produces 80 000 J/s while operating at 100 cycles/s and
a heat input of 4000 J/cycle. What is the efficiency of the engine?
or
For an engine to operate at 100% efficiency, no heat can be lost to the cold reservoir, which is ________________.
(Another version of the) Second Law of Thermodynamics:
No cyclic process can...
A theoretical limit to the efficiency of a heat
engine can be determined from The Carnot
Cycle.
This cycle consists of 4 reversible processes
performed on an ideal gas in a piston device.
A reversible process
is one that follows an
_____________ ______
that can be retraced if
the process is reversed.
(1)
(2)
(3)
(4)
The Carnot Cycle
The Efficiency of the Carnot engine depends
only on the temperature of the hot and cold
reservoir between which it operates.
Ex: An engine claims an input of
5000 J/s at 400 K and a heat
output of 2000 J/s at 200 K. Are
these specifications believable?
The Carnot efficiency is a theoretical
_________ __________ that can never be
exceeded
Heat pumps and air conditioners operate in a manner
that is the ____________ of a heat engine. Each does
work to transfer heat from a ______ cold reservoir to a
______ reservoir.
Ex: A heat pump warms a home by extracting heat from the
cooler outside and delivering it to the warmer inside.
Calculate the coefficient of performance, power output and
operation costs of an ideal heat pump.
The Carnot cycle can be run in reverse as a heat pump
or air conditioner.
The theoretical coefficient of performance depends
only on the temperatures of the hot and cold reservoirs.
Entropy (S): As a system’s ability to do work decreases...
(Another) Second Law of Thermodynamics
Entropy can be thought of as a measure of...
For an ideal, reversible process ∆S
The more disordered a system the _________ its entropy.
For any real process ∆S