Transcript PPT - CEProfs

Lec 1: Introduction, conservation of energy,
dimensions and units, systems, properties
Mt St Helens, 1980
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• ENGR 212-503
• For next time:
– Read: §1-1 to 1-10
– HW 1 --- book not available; postponed to September
8, 2003
• Outline:
– Introduction
– What is thermodynamics?
– New concepts
• Important points:
– Systems (open/closed)
– Properties (intensive/extensive)
– Units
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Laws of thermodynamics
• First law of thermodynamics is same as
law of conservation of energy--deals with
quantities of energy
•Energy can change forms:
from potential energy to
kinetic energy, for
example--but the total
amount never changes.
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Laws of thermodynamics
• Where a system such as a human body
may undergo an energy change, the first
law is written:
E in  E out  E
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Units
• Two systems will be used heavily:
– The International System (known as the
SI system from its French title--see
text).
– The US Customary System (USCS).
(The USCS is also known as the inchpound (IP) system and the English
Engineering System).
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Newton’s 2nd Law
F  ma  const
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SI system constant
• In the simpler SI system the units are
chosen so that the constant is unity. The
primary dimensions are mass (kg), length
(m), and time (s or sec).
• The equation F  ma  const that
contains four units is not over-determined
because force F is then a derived or
defined unit.
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Force is a defined unit in SI
1 N  1 kg  1 m / s2
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USCS
• This unfortunate system has four
applicable primary dimensions for the
equation which has only four dimensions.
– Mass is lbm
– length is ft
– time is s (sec)
– force is lbf
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USCS
• The system is over-determined
• We get around this by introducing gc, the
gravitational constant, so that
ma
F
gc
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Gravitational constant
• In other words, the const in F=maconst
is:
1
const 
gc
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Gravitational constant
lb m  ft
g c  32.174
2
lb f  sec
•Please memorize this.
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Gravitational constant
• Your text gets around the problem of
over-determination by defining:
lb m  ft
1 lb f  32.174
2
sec
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Advice which will gain you points
• 1. Write out all units in all equations to
the bitterest of detail.
• 2. Use a horizontal line in all equations,
not a slash.
e.g. Write
ma
F
gc
; Not F = ma/gc
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To gain you points
• Remember that a goal is to communicate
that you know how to do the
assignments--including obtaining the
correct answer.
• Keep steps in order--work from top to
bottom on the page--do not jump
erratically to left over open space for key
steps, which may then be out of order.
Circle or box key numbers and results.
• Make it easy for your reader, not hard.
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Notes on expressing units
• The º symbol is not used with absolute
temperatures K or R.
• Unit names are not capitalized even if
they are proper names (for example,
1,000 watts).
• Abbreviations of units are capitalized if
they are from proper names (1,000 W or
1.2 kWh).
• Abbreviations are never plural (4 joules
but 4 J; 4 kilojoules but 4 kJ).
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TEAMPLAY
WHEN A TEAM PLAY EXERCISE APPEARS,
IT IS TO BE SOLVED FIRST BY YOU, THEN
BY A PARTNER, AND FINALLY AS A GROUP
OF FOUR.
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TEAMPLAY
In the old movie Armageddon, actor Bruce
Willis flew to an asteroid:
• What was the force on Bruce Willis, whom
we shall assume weighs perhaps 200 lbm,
when he stepped on the asteroid where
the acceleration was an assumed 1.8
ft/s2?
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System
• A system is a quantity of matter or region
of space we want to study.
• Two types of systems (closed and open).
• The first is known as a closed system (or
control mass)--because it involves a fixed
amount of matter and is closed to matter
(mass) transfer across the system
boundary.
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System
•Closed system--no mass crosses boundary
indicated by dashed line.
•Mass indicated by gray is system to study.
•Boundary can move (piston could go in and
out)
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System
• A system is a quantity of matter or
region of space we want to study.
• The second is known as an open system
(or control volume)--because it involves
a fixed region in space and is open to
matter (mass) transfer across the
system boundary.
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System
Fluid velocity=V0
•Open system--boundary generally does
not move (it could) and mass crosses the
boundary.
•We are just interested in the region
bounded by the dashed lines.
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System
System boundaries are often defined by
dashed lines, as in the previous pipe flow
example. The dashed lines define a control
surface.
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TEAMPLAY
A bottle of Coke is placed in
a refrigerator that is at 3C:
•Should the bottle of Coke
be treated as a closed or
open system?
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Definitions
• Property--A characteristic of a system to
which numerical values can be assigned to
describe the system.
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Definitions
• State--Condition of a system defined by its
properties.
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TEAMPLAY
• What are some properties that define the
liquid in the previous Coke example of a
system?
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Definitions
• Extensive properties--properties which can
be counted, and for which their value for
the whole system is the sum of the value
for subsystems into which the overall
system might be divided.
• They depend on the extent of the system.
• Often symbolized with capital letters.
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Definitions
• Intensive properties are independent of
the size (mass or volume) of the system.
• Often symbolized by lower case letters.
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Energy
• Energy = sum of internal energy, kinetic
energy, and potential energy.
m V2
E  U  KE  PE  U 
 mgz
2
• (Note energy E here is an extensive
property).
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Energy
• On a unit mass basis, or intensive basis
2
V
e  u  ke  pe  u 
 gz
2
• Every term in the previous equation has
been divided through by the mass m.
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Other units
• Volume V will be in m3 and ft3
• Density  is in kg/ m3 or lbm/ ft3
• Specific volume v is new and powerful;
the inverse of density, it is in units of
ft3/lbm or m3/kg.
1
v
ρ
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Specific gravity
• Also known as relative density--it is the
ratio of the density of a substance to some
reference density.
• Reference density is usually water at 4 ºC.
• Sometimes, for gases, “gas gravity” is
used, and in that case the reference
density is for air at 60 ºF and 14.696 psia.
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TEAMPLAY
Sketch possible system boundaries for the
following and classify them as open or
closed:
•
•
•
•
•
Bicycle tire deflating.
Kettle of water heating (but not boiling).
Kettle of water boiling.
Jet engine in flight.
Pentium IV, 2.4 Ghz chip in operation.
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