1. Dimensions and Units Ashfaq M Ansery ChE Deptt, BUET

2. 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

3. 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

4. Units & Dimensions Cont…. We'll break them up this way: ValueUnitDimension 110 mgmass 24 handlength 5 galVolume (length 3 )

5. Dimensions  Any physical quantity can be characterized by dimensions  A "dimension" can be measured or derived

6. Types of Dimensions  Primary or fundamental dimensions (length, time, mass, temperature)  Secondary or Derived dimensions (velocity, volume, density, etc.)

7. 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)

8. 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

9. Fundamental dimension and their units in SI DimensionUnit Lengthmeter (m) Masskilogram (k) Timesecond (s) Temperaturekelvin (K) Electric Currentampere (A) Amount of lightcandela (cd) Amount of mattermole (mol)

10. Standard Prefixes in SI MultiplePrefixMultiplePrefix 10 1 deca, d10 -1 deci, d 10 2 hecto, h10 -2 centi, c 10 3 kilo, k10 -3 milli, m

11. 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

12. 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

13. 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.).

14. 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

15. 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

16. Multiplication and Division  Values may be combined; units combine in similar fashion