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Physics and Physical Measurement

Physics and Physical Measurement. Topic 1 Physical quantities and units. Learning outcomes. (a) show an understanding that all physical quantities consist of a numerical magnitude and a unit

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Physics and Physical Measurement

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  1. Physics and Physical Measurement Topic 1Physicalquantities and units

  2. Learning outcomes • (a) show an understanding that all physical quantities consist of a numerical magnitude and a unit • (b) recall the following SI base quantities and their units: mass (kg), length (m), time (s), current (A), temperature (K), amount of substance (mol) • (c) express derived units as products or quotients of the base units and use the named units listed in this syllabus as appropriate • (d) use base units to check the homogeneity of physical equations • (f) use the following prefixes and their symbols to indicate decimal submultiples or multiples of both base and derived units: pico (p), nano (n), micro (μ), milli (m), centi (c), deci (d), kilo (k), mega (M), giga (G), tera (T) • (g) make reasonable estimates of physical quantities included within the syllabus

  3. For your notes • All physical quantities have a magnitude and a unit • Base units are: • mass (kg) • length(m) • time (s) • current (A) • temperature (K) • amount of substance (mol) • Express derived units in terms of the base units • Use the SI units

  4. Standards of Measurement • SI units are those of the Système International d’Unités adopted in 1960 • Used for general measurement in most countries • Scientists and engineers need to make accurate measurements so that they can exchange information • To be useful a standard of measurement must be • Invariant, Accessible and Reproducible

  5. Fundamental dimensions • There are seven fundamental dimensions. • A base unit is accurately defined for each quantity. • They are: • Length metre m • Mass kilogram kg • Time second s • Electric current ampere A • Thermodynamic temp Kelvin K • Amount of a substance mole mol • Luminous intensity candela Cd

  6. Derived Quantities • Speed, force , energy etc. are not base quantities. • Derived units allow us to measure any quantity. • Derived quantities are combinations of the fundamentals • e.g. speed • average speed = distance/time • So the derived unit is meters per second • We write derived units in the form ms-1 not m/s

  7. Figuringoutderivedunits • Writeoutanequationforthequantityyouwanttoknowtheunits of. ( Thedefiningequationisbest) • egPressure = F/A • Rewritetheequationsubstitutingunitsforquantities • Eg [Pa]=[N] / [m2] or [N] [m-2] • Checkifunits are base units • m-2 • Repeatprocessforanynone base units • F=ma [N] = [kg] [m] [s-2] all base units • So…….. [Pa]= [N] [m-2] = [kg] [m] [s-2] [m-2] • Simplified [Pa]= [kg] [s-2] [m-1]

  8. Practise • Try these • Velocity • Density • Work • Power • Resistance

  9. Writing Units two examples • A car has a velocity of 10ms-1 • Notes • Do not use m/s • Meters and seconds are SI units • A metal has a density of 5Kgm-3 • Notes • Kg not g is the SI unit • Per m3 is written m-3

  10. Some Derived Units • Acceleration ms-2 • Angular acceleration rad s-2 • Momentum kgms-1 or Ns • Others have specific names and symbols • Force kg ms-2 or N • Pressure kgm-1s-2or Pa • Resistance kgm2A-2s-3 or Ω • Of course there are so many things out there to measure there are many more.

  11. Summary questions • What are the base quantities? • What are the base units? • Give 5 derived units • What is a volume of 1m3 in cm3? • What is a pressure of 1Ncm-2 in Pa? • State 2 problems with this reading and rewrite it correctly: Speed= 39 mph

  12. Can you? • State the fundamental units in the SI system. • Distinguish between fundamental and derived units • Give examples of derived units. • Convert between different units of quantities. • State units in the accepted SI format.

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