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Measurement

Measurement. Lecture #2 June 18 2013. Role of measurement in Science. Our understanding of the physical world depends on measurement. Even though a correspondence between the Nature and mathematics exist, the ultimate test of any theory still rests on experiment.

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Measurement

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  1. Measurement Lecture #2 June 18 2013

  2. Role of measurement in Science Our understanding of the physical world depends on measurement. Even though a correspondence between the Nature and mathematics exist, the ultimate test of any theory still rests on experiment.

  3. Orbital perihelion precession of Mercury

  4. Apparent position of star due to bending of starlight by Sun’s gravity 0.00049 degrees = Angle subtended by a coin from 2 miles away

  5. Measurement is essentially a mapping or assignment of a number to a physical quantity

  6. Measurement is Magnitude +Unit +Uncertainty

  7. Magnitude of a measurement: Scientific notation and significant figures Speed of light: 299 792 458 m/s Gravitational constant: 0.0000000000667384 m3/kg.s2 Electron mass: 0.00000…(20 zeros)…00000910938291 kg

  8. What’s significant in significant figures? Significant numbers are those digits in a measurement that is reliably known. Ex. 400.0m versus 400 m

  9. Units of Measurement • Units are introduced to identify the physical quantity being measured. • Units are specified to arrive at a common understanding of the magnitude of the quantity involved.

  10. Units of effective radiation levels • 360 millirem/year: average effective radiation per year absorbed by a man • Conversion factor: 1rem = 0.01 Sievert • 6 millirem: chest x-ray

  11. Chernobyl Disaster 300 Sieverts/hour

  12. SI standards of measurement

  13. Fundamental versus derived units

  14. Why SI? SI units are the product of an attempt to standardize units of measurement and make them more precise. 1 meter equals: • One/ten million of the distance from the Earth’s equator to the geographic North Pole (1790) • Platinum-iridium bar kept at International Bureau of Weights and Measures in Paris (1889-1960) • Distance travelled by light in vacuum for

  15. Dimensional analysis Technique of exploiting the relationship between physical quantities • to facilitate unit conversion • to check the validity of an equation during problem solving

  16. Stoichiometry as dimensional analysis How many number of H atoms are there in 47.25g of C8H18?

  17. How many minutes does it take for sunlight to reach the earth? • Distance from earth to sun: 149,600,000 km • Speed of light: 299 792 458 m/s

  18. Uncertainties in measurement Because of the limitations of measuring instruments, measurements always have uncertainties.

  19. But quantum physics (physics of “really small” objects) dictates an ultimate, unavoidable limitation to device sensitivity and, hence, to the precision of a measurement.

  20. Measurements and social policies • Speed limit SLEX: 100 km/hr • Depth of Marikina river to signal evacuation: 17 m above sea level • Frequency of alternating current: 60 Hz • Hydrocarbon content of car emission (>2003): below 600 parts per million • Advertisements’ claim: Safeguard kills 99.9% germs

  21. THREE POINTS: • Measurement is at the heart of sciences because it is the ultimate test of any physical theory. • A measurement is composed of a magnitude, a unit and an uncertainty value. • Practice: writing numbers in scientific notation, reporting measurements with the correct significant figures, and dimensional analysis.

  22. Notes: • Seating arrangement, seat plan for recitation • Encourage recitation: deciding factor especially if grade is at the border • Assignment: Research proposal for Gen. Physics, index cards • Complete class list • Consultation hours: TTH, 430 – 6pm • Discovery channel video: Mentalism • Scope of Class rep’s job • Short quiz: 1/4 sheet • Textbook: Giancoli and Shipman • Test questions will mostly be lifted from the textbook • Ask if significant figures will be taught in lab

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