Scientific Work

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Scientific Work

  1. 1. Unit 1THE SCIENTIFIC WORK
  2. 2. Physics and Chemistry What do they have in common?  Physicists and Chemists study the same: matter.  Physicists, Chemists and other scientists work in the same way:  SCIENTIFIC METHOD
  3. 3. Physics and Chemistry What makes them different?  Physics studies phenomena that dont change the composition of matter.  Chemistry studies phenomena that change the composition of matter.
  4. 4. SCIENTIFIC METHOD
  5. 5. SCIENTIFIC METHOD  The observation of a phenomenon and curiosity make scientists ask questions.  Before doing anything else, its necessary to look for the previous knowledge about the phenomenon.
  6. 6. SCIENTIFIC METHOD  Hypotheses are possible answers to the questions we asked.  They are only testable predictions about the phenomenon.
  7. 7. SCIENTIFIC METHOD  We use experiments for checking hypotheses.  We reproduce a phenomenon in controlled conditions.  We need measure and collecting data in tables or graphics
  8. 8. SCIENTIFIC METHOD  We study the relationships between different variables.  In an experiment there are three kinds of variables − Independent variables: they can be changed. − Dependent variables: they are measured. − Controlled variables: they dont change.
  9. 9. SCIENTIFIC METHOD  After the experiment, we analyse its results and draw a conclusion.  If the hypothesis is true, we have learnt something new and it becomes in a law  If the hypothesis is false. We must look for a new hypothesis and continue the research.
  10. 10. Magnitudes,measurements and units Physical Magnitude: It refers to every property of matter that can be measured. − Length, mass, surface, volume, density, velocity, force, temperature,... Measure: It compares a quantity of a magnitude with other that we use as a reference (unit). Unit: It is a quantity of a magnitude used to measure other quantities of the same magnitude. Its only useful if every people uses the same unit.
  11. 11. Magnitudes,measurements and units Length of the classroom = 10 m meansThe length of the classroom is 10 times the length of 1 metre.
  12. 12. The International System of Units The SI has: − a small group of magnitudes whose units are fixed directly: the fundamental magnitudes.  E.g.: Length → meter (m); Time → second (s) − The units for the other magnitudes are defined in relationship with the fundamental units: the derivative magnitudes.  E. g.: speed → meter/second (m/s)
  13. 13. The International System of UnitsThe fundamental magnitudes and their unitsLength meter m Mass kilogram kg Time second s Amount of substance mole mol Temperature Kelvin K Electric current amperes A Luminous intensity candela cd
  14. 14. The International System of UnitsSome examples of how to build the units of derivative magnitudes: − Area = Length · width → m·m = m2 − Volume = Length · width · height → m·m·m = m3 − Speed = distance / time → m/s − Acceleration = change of speed / time → (m/s)/s = m/s2
  15. 15. The International System of UnitsSome examples of how to build the units of derivative magnitudes: − Area = Length · width → m·m = m2 − Volume = Length · width · height → m·m·m = m3 − Speed = distance / time → m/s − Acceleration = change of speed / time → (m/s)/s = m/s2
  16. 16. The International System of UnitsMore derivative units. Quantity Name Symbol Area square meter m2 Volume cubic meter m3 Force Newton N Pressure Pascal Pa Energy Joule J Power Watt W Voltage volt V Frequency Hertz Hz Electric charge Coulomb C
  17. 17. The International System of UnitsPrefixes: we used them when we need express quantities much bigger or smaller than basic unit. Power of 10 for Prefix Symbol Meaning Scientific Notation_______________________________________________________________________ mega- M 1,000,000 106 kilo- k 1,000 103 deci- d 0.1 10-1 centi- c 0.01 10-2 milli- m 0.001 10-3 micro- µ 0.000001 10-6 nano- n 0.000000001 10-9
  18. 18. The International System of UnitsPrefixes: the whole list Factor Name Symbol Factor Name Symbol 10-1 decimeter dm 101 decameter dam 10-2 centimeter cm 102 hectometer hm 10-3 millimeter mm 103 kilometer km 10-6 micrometer µm 106 megameter Mm 10-9 nanometer nm 109 gigameter Gm 10-12 picometer pm 1012 terameter Tm 10-15 femtometer fm 1015 petameter Pm 10-18 attometer am 1018 exameter Em 10-21 zeptometer zm 1021 zettameter Zm 10-24 yoctometer ym 1024 yottameter Ym
  19. 19. Changing units We can change a quantity into another unit. Conversion factors help us to do it. A conversion factor is a fraction with the same quantity in its denominator and in its numerator but expressed in different units. 1h 1 km =1 =1 60 min 1000 m 60 min 1000 m =1 =1 1h 1 km
  20. 20. Changing units Lets see a few examples of how to use them 1 km 2570 km ·1 2570 m · = =2,570 km 1000 m 1000 1 h 1 min 3500 h 3500 s · · = =0,972 h 60 min 60 s 3600 2 500 cm² · 1m 100 cm  =500 cm² ·  1m² 10000 cm² =  500 m² 10000 =0,05 m² m m 1 km 3600 s 30 · 3600 km km 30 =30 · · = =108 s s 1000 m 1 h 1000 h h
  21. 21. Significant figures• They indicate precision of a measurement.• Sig Figs in a measurement are the really known digits. 2.3 cm
  22. 22. Significant figures Counting Sig Figs: − Which are sig figs?  All nonzero digits.  Zeros between nonzero digits − Which arent sig figs?  Leading zeros – 0,0025  Final zeros without a decimal point – 250 Examples: − 0,00120 → 3 sig figs; 15000 → 2 sig figs − 15000, → 5 sig figs; 13,04 → 4 sig
  23. 23. Significant figures Calculating with sig figs − Multiplicate or divide: the factor with the fewer number of sig figs determines the number of sig figs of the result:  2,345 m · 4,55 m = 10,66975 m 2 = 10,7 m2  (4 sig figs) (3 sig figs) → (3 sig figs) − Add or substract: the number with the fewer number of decimal places determines the number of decimal places of the result:  3,456 m + 2,35 m = 5,806 m = 5,81 m  (3 decimal places) (2 decimal places) → (2 decimal places)
  24. 24. Significant figures Calculating with sig figs − Exact number have no limit of sig fig:  Example: Area = ½ · Base · height.  ½ isnt taken into account to round the result. − Rounding the result:  If the first figure is 5, 6, 7, 8 or 9, the last figure taken into account is increased in 1  If not, it doesnt change.
  25. 25. Scientific notation Is used to write very large or very small quantities: − 385 000 000 Km = 3.85·108 Km − 0,000 000 000 157 m = 1,57·10-10 m Changing a number to scientific notation: − We move the decimal point until there is an only number in its left side. − The exponent of 10 is the number of places we moved the decimal point:  The exponent is positive if we move it to the left side  Its negative if we move it to the right side.
  26. 26. Measurement errors Its impossible to measure a quantity with total precision. When we measure, well never know the real value of the quantity. Every measurement has an error because: − The measurement instrument can only see a few sig figs. − It may not be well built or calibrated. − We are using it in the wrong way.
  27. 27. Measurement errors There are two ways for expressing the error of a measurement: − Absolute error: it is the difference between the value of the measurement and the value accepted as exact. − Relative error: it is the absolute error in relationship with the quantity.
  28. 28. Measurement errors How to calculate the error. EXAMPLE 1: − We have measured several times the mass of a ball:  20,17 g, 20,21 g, 20,25 g, 20,15 g, 20,28 g − Its supposed that the real value of the ball of the mass is the average value of all the measurements:  Vr = (20,17 g + 20,21 g + 20,25 g + 20,15 g + 20,27 g )/5 = 20,21 g − The absolute error of the first measurement is:  Er = |20,17 g – 20,21 g| = 0,04 g − The relative error is calculate dividing the absolute error by the value of quantity.
  29. 29. Measurement error How to calculate the error. EXAMPLE 2: − We have measured once the length of a piece of paper using a ruler that is graduated in millimetres: 29,7 cm − We suppose that the real value is the measured value. − The absolute error is the precision of the rule:  Ea = 0,1 cm − Relative error:  Er = 0,1 cm / 29,7 cm = 0,0034 = 0,34 %

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