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PHY430 – Lecture 1

PHY430 – Lecture 1. Physical Quantities & Units Base & Derived Quantities & SI Units Dimensional Analysis Unit Conversion. Base quantity A quantity which is not a combination of other physical quantities. Must be defined in terms of a standard.

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PHY430 – Lecture 1

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  1. PHY430 – Lecture 1 Physical Quantities & Units Base & Derived Quantities & SI Units Dimensional Analysis Unit Conversion

  2. Base quantity A quantity which is not a combination of other physical quantities. Must be defined in terms of a standard. Units for base quantities are base units Derived quantity A quantity which is a combination of two or more physical quantities. Units for derived quantities are derived units Base quantity & Derived quantity

  3. 1 meter • The meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.

  4. 1 kilogram • A particular platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris.

  5. 1 second • One second is defined as the time required for 9,192,631,770 periods of radiation emitted by cesium atoms.

  6. Effective technique of Unit Conversion 1. Write the unit in traditional form and not the index form. eg. • Remember or look up the conversion factor. • Multiply the initial quantity with the proper conversion factor so that only the required units are left, the others cancelled off.

  7. Conversion factor to remember • 1 km = 1000 m • 1 m = 100 cm • 1 cm = 10 mm • 1 kg = 1000 g • 1 h = 60 min • 1 min = 60 s • 1 h = 3600 s • Remember the values for the prefixes. • The rest – look up the conversion table

  8. Unit Conversion • Convert • (a) 110 km/h to m/s • (b) 340 m/s to km/h • (c) 300 m2 to cm2 • (d) 20 m3 to cm3 • (e) 1000 kg/m3 tog/cm3 • (f) 15 nm to m • (g) 25 A to A • (h) 240 MW to kW

  9. Dimensional Analysis • Dimensions of a quantity are base quantities or base units that make up the quantity • Dimensional Analysis is a useful technique to check if a relationship is incorrect • Add & subtract quantities only if they have the same dimensions • [LHS] = [RHS]

  10. Symbol for Dimension

  11. Eg. 1 Dimensional Analysis • Show that v = u + at is dimensionally correct.

  12. Rule All non zero digits in a number are sf Zero in between two non zero digits are sf For any whole number, zero at the end of a number can be a sf or not a sf. It depends on the of precision of the reading. Examples (i) 3421 : 4 sf (ii) 62.5 : 3 sf (i) 503 : 3 sf (ii) 1.006 : 4 sf (i) 63 000 : 2 sf if the precision is to the nearest thousand (ii) 63 000 : 3 sf if the precision is to the nearest hundred Significant Figures (sf)

  13. Rule For a decimal number less than 1, zero placed before any non-zero digit is not a sf. For a decimal number, zero placed after a non-zero digit is a sf. Examples (i) 0.0028 : 2 sf (ii) 0.0902 : 3 sf 5. (i) 7.40 : 3 sf (ii) 0.020 : 2 sf Significant Figures (sf)

  14. Significant Figures : multiplication & division • The final results of multiplication or division should have only as many digits as the number with the least number of significant figures used in the calculation. • Eg. 11.3 x 6.8 = 76.84 = 77 (round off to 2 sf)

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