ERT 206/4 THERMODYNAMICS
SEM 2 (2011/2012)
The name thermodynamics stems from the
Greek words therme (heat) and dynamis
(power).
Energy:
Conservation of energy principle:
During an interaction, energy can change from
one form to another but the total amount of
energy remains constant.
The ability to cause changes.
The first law of thermodynamics:
Energy cannot be created or destroyed; it
can only change forms.
The second law of thermodynamics:
It asserts that energy has quality as well as quantity,
and actual processes occur in the direction of
decreasing quality of energy.
Heat flows in the direction of
decreasing temperature
The first law of thermodynamics
Power plant
Aircraft and spacecraft
Car
Human body
Wind turbines
Refrigeration systems
Air conditioning systems
Boats
Solar hot water systems
Any physical quantity can be
light
characterized by dimensions
The magnitudes assigned to the
light
dimension are called units
L of
Electric
current
AmountL of light
Amount of matter
Primary/
fundamental
light
dimensions
Temperature
light
Length
light
Mass
light
Time
light
English
light
system
Primary dimensions and
their units in SI
International
light
system (SI)
Secondary/
derived
light
dimensions
Velocity
light
Standard prefixes in SI units
Volume
light
Energy
light
Force
light
Mass,
lightm
Total volume,
Vt
light
No. oflight
moles, n
Represent the size
of alight
system
m  nM
Mass
light
No. of moles
Molecular weight
No. of moles
Mass
m
n light
M
Molecular weight
Specific volume:
Vt
V
m
or
V t  Vm
Molar volume:
Vt
V
n
or
V t  Vn
m=1 kg
a=1 m/s2
m=32.174 lbm
Definition:
F light
ma
light
a=1 ft/s2
Mass
F=1 N
F=1 lbf
Acceleration
light a mass of 1 kg or
Force required to accelerate
32.174 lbm at a rate of 1 m/s2 or 1 ft/s2.
Mass
Definition:
W  mg
light
Weight is gravitational
force
applied to a body
Local gravitational acceleration
light
g = 9.807 m/s2
= 32.174 ft/s2
English system
light
Pound-force
(lbf)
Unit in many European
countries
light
Kilogram-force
(kgf)
1 N = 1 kg. m/s2
light 2 = 4.44822 N
1 lbf = 32.174 lbm.ft/s
1 kgf = 9.807 N
SI
light
Newton
(N)
• System: A quantity of matter or a region
in space chosen for study.
• Surroundings: The mass or region
outside the system
• Boundary: The real or imaginary surface
that separates the system from its
surroundings.
• The boundary of a system can be fixed or
movable.
• Systems may be considered to be closed
or open.
• Closed system
(Control mass):
A fixed amount of
mass, and no
mass can cross its
boundary.
• Open system (control volume): Both mass
and energy can cross the boundary of a
control volume.
• Device: compressor, turbine, or nozzle.
• Control surface: The boundaries of a control
volume. It can be real or imaginary.
An open system (a control volume)
with one inlet and one exit.
8
Commonly measured with liquid-in-glass thermometer,
wherein the liquid expands when heated
Boiling point of pure water at
light pressure
standard atmospheric
Freezing point of water saturated
with air at standard
light atmospheric
pressure
Lower limitlight
of temperature
Relations among
light
temperature scales
Comparison of
light
magnitude
of various
temperature units
Electric power generation by a wind
turbine
A school is paying $0.09/kWh for electric power. To reduce its power bill,
the school install a wind turbines with a rated power of 30 kW. If the turbine
operates 2200 hours per year at the rated power, determine the amount of
electric power generated by the wind turbine and the money saved by the
school per year.
a) Determine the total energy
b) Determine the money saved
a) Determine the total energy
Total energy = (Energy per unit time) (Time interval)
= (30 kW) (2200 h)
= 66, 000 kWh
b) Determine the money saved
Money saved = (Total energy) (Unit cost of energy)
= (66,000 kWh) ($0.09/kWh )
= $5940
Convert your answer of total energy in kJ.
Total energy = 66,000 kWh
3600 s
1 kJ/s
1h
1 kW
=
2.38 X 108 kJ
The weight of one pound-mass
Using unity conversion ratios, show that 1.00 lbm weighs 1.00 lbf on earth.
W = mg =
1.00 lbm
32.174 ft/s2
1 lbf
32.174 lbm.ft/s2
= 1.00 lbf
Expressing Temperature Rise in
Different Units
During a heating process, the temperature of a system rises by 10 ⁰C.
Express this rise in temperature in K, ⁰F and R.
Δ T(K) = Δ T(⁰C) = 10 K
Δ T(R) = 1.8 Δ T(K) = (1.8) (10) = 18 R
Δ T(⁰F) = Δ T(R) = 18 ⁰F
A normal force exerted by a fluid per unit area
light
Some basic pressure gages.
•
Absolute pressure: The actual pressure at a given position. It is measured relative to
absolute vacuum (i.e., absolute zero pressure).
•
Gage pressure: The difference between the absolute pressure and the local
atmospheric pressure. Most pressure-measuring devices are calibrated to read zero in
the atmosphere, and so they indicate gage pressure.
•
Vacuum pressures: Pressures below atmospheric pressure.
The pressure of a fluid at rest
increases with depth (as a result of added weight).
In a room filled with a
gas, the variation of
pressure with height is
negligible.
Pressure in a liquid at
rest increases
linearly with distance
from the free surface.
The pressure is the same
at all points on a
horizontal plane in a given
fluid regardless of
geometry, provided that
the points are
interconnected by the
same fluid.
Pascal’s law:
The pressure applied to a confined fluid increases the pressure throughout by the same amount.
light
Lifting of a large weight by a
small force by the application of
Pascal’s law.
It is commonly used to measure small and moderate pressure
differences. A manometer contains one or more fluids such as
mercury, water, alcohol, or oil.
The basic
manometer.
Measuring the
pressure drop
across a flow
section or a flow
device by a
differential
manometer.
In stacked-up
fluid layers, the
pressure change
across a fluid
layer of density
 and height h
is gh.
•
Atmospheric pressure is measured by a device called a barometer; thus, the atmospheric
pressure is often referred to as the barometric pressure.
•
A frequently used pressure unit is the standard atmosphere, which is defined as the pressure
produced by a column of mercury 760 mm in height at 0°C (Hg = 13,595 kg/m3) under standard
gravitational acceleration (g = 9.807 m/s2).
The basic barometer.
Absolute Pressure of a Vacuum
Chamber
A vacuum gage connected to a chamber reads 40 kPa at a location where
the atmospheric pressure is 100 kPa. Determine the absolute pressure in
the chamber.
Pabs = Patm - Pvac
= 100 - 40 = 60 kPa
Measuring Pressure with a
Manometer
A manometer is used to measure the pressure in a tank. The fluid used has
a specific gravity of 0.85, and the manometer column height is 55 cm, as
shown in figure. If the local atmospheric pressure is 96 kPa, determine the
absolute pressure within the tank.
ρ  SG(ρ H 2O )  (0.85)(100 0kg/m 3 )  850 kg/m 3
P  Patm  ρgh
= 96 kPa + 850 kg/m3 9.81 m/s2
= 100.6 kPa
Determine the gage pressure in the tank.
0.55 m
1N
1 kPa
1 kg.m/s2
1000 N/m2
Measuring Pressure with a
Multifluid Manometer
The water in a tank is pressurized by air and the pressure is measured by a
multifluid manometer as shown in the figure. The tank is located on a mountain at
an altitude of 1400 m where the atmospheric pressure is 85.6 kPa. Determine the
air pressure in the tank if h1 = 0.1 m, h2 = 0.2 m and h3 = 0.35 m. Take the densities
of water, oil and mercury to be 1000 kg/m3, 850 kg/m3 and 13600 kg/m3,
respectively.
Ans: P1 = 130 kPa
Measuring Atmospheric Pressure
with a Barometer
Determine the atmospheric pressure at a location where the barometric reading is
740 mmHg and the gravitational acceleration is g = 9.81 m/s2. Assume the
temperature of mercury to be 10 ⁰C, at which its density is 13570 kg/m3.
Ans: in unit kPa
Ans: 98.5 kPa
Work, energy and heat will be covered in other chapter!
Work = Force  Distance
1 J =light
1 N∙m
1 cal = 4.1868 J
1 Btu = 1.0551 kJ