Slide 1

Report
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

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