Transcript Slide 1
Dimensions and Units
Ashfaq M Ansery
ChE Deptt, BUET
System
The application of thermodynamics
to any real problem starts with the
identification of a particular body of
matter as the focus of attention
This body of matter is called the
system
Its thermodynamic state is defined
by a few measurable macroscopic
properties
Units & Dimensions
Many people aren't sure of the
difference. Let's try and get a set
of definitions we can use. Consider
110 mg of sodium
24 hands high
5 gal of gasoline
Units & Dimensions Cont….
We'll break them up this way:
Value
Unit
Dimension
110
mg
mass
24
hand
length
5
gal
Volume
(length3)
Dimensions
Any physical quantity can be
characterized by dimensions
A "dimension" can be measured or
derived
Types of Dimensions
Primary or fundamental dimensions
(length, time, mass, temperature)
Secondary or Derived dimensions
(velocity, volume, density, etc.)
Units
To quantify any dimension we need a
scale of measure for the dimension
A specific magnitude (chosen
arbitrarily) of the dimension is
assigned to the scale to be used as
unit
Example:
meter (symbol m)
kelvin (symbol K)
Unit Systems
English System (also known as
United States Customary System,
USCS)
SI (also known as International
System)
Many different units can be used for a
single dimension, as inches, miles,
centimeters, furlongs, and versts are
all units used to measure the
dimension length
Fundamental dimension and
their units in SI
Dimension
Unit
Length
meter (m)
Mass
kilogram (k)
Time
second (s)
Temperature
kelvin (K)
Electric Current
ampere (A)
Amount of light
candela (cd)
Amount of matter
mole (mol)
Standard Prefixes in SI
Multiple
Prefix
Multiple
Prefix
101
deca, d
10-1
deci, d
102
hecto, h
10-2
centi, c
103
kilo, k
10-3
milli, m
Few points to note
The degree symbol is dropped from
the absolute temperature unit
All unit names are to be written
without capitalization even if they
were derived from proper names
The abbreviation of a unit is to be
capitalized if the unit was derived
from a proper name
Few points to note
The full name of a unit may be
pluralized, but its abbreviation
cannot
For example: the length of an object
can be 5 m or 5 meters, not 5 ms or
5 meter
Few points to note
No period is to be used in unit
abbreviations unless they appear at
the end of a sentence
For example: the proper
abbreviation of meter is m (not m.).
Units and Calculations
It is always good practice to attach
units to all numbers in an engineering
calculation. Doing so
attaches physical meaning to the
numbers used,
gives clues to methods for how the
problem should be solved, and
reduces the possibility of accidentally
inverting part of the calculation
Addition and Subtraction
Values MAY be added if UNITS are the
same.
Values CANNOT be added if
DIMENSIONS are different
EXAMPLES:
different dimensions: length, temperature - so cannot be added
same dimension: length, different units -can be added
Multiplication and Division
Values may be combined; units
combine in similar fashion