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Bell-Ringer

Bell-Ringer. Why is accuracy so important in science? What is the difference between accuracy and precision? What is meant by the term “significance?”. Section 2-3 Using Scientific Measurements. Coach Kelsoe Chemistry Pages 44-57. Accuracy vs. Precision.

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Bell-Ringer

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  1. Bell-Ringer • Why is accuracy so important in science? • What is the difference between accuracy and precision? • What is meant by the term “significance?”

  2. Section 2-3Using Scientific Measurements Coach Kelsoe Chemistry Pages 44-57

  3. Accuracy vs. Precision • Accuracy refers to the closeness of measurements to the correct or accepted value of the quantity measured. • Precision refers to the closeness of a set of measurements of the same quantity made in the same way

  4. High Accuracy, High Precision • The arrows on the drawing have high accuracy, because they are on the bullseye, and are close to each other.

  5. Low Accuracy, Low Precision • The arrows on the drawing have low accuracy, because they are not near the bullseye, and are not precise because they are not near each other.

  6. Low Accuracy, High Precision • The arrows on the drawing have low accuracy, because they are not near the bullseye, but are close to each other.

  7. High Accuracy, Low Precision • The arrows on the drawing have high accuracy (on average), because they are near the bullseye, but are not close to each other.

  8. Percent Error • Percent error is calculated by subtracting the experimental value from the accepted value, dividing the difference by the accepted value, and then multiplying by 100. Percent Error = Valueaccepted-Valueexperimental x 100 ______________________ Valueaccepted

  9. Percent Error % • Percent error has a negative value if it is greater than the experimental value, and a positive value if the accepted value is less than the experimental value.

  10. Significant Figures • Significant figures in measurement consist of all the digits known with certainty plus one final digit, which is somewhat uncertain or is estimated. • For example, if I asked you to measure the length of a paper clip to one hundredths of a centimeter, and you measured 5.78 cm, the 8 would be uncertain. • “Significant” does not necessarily mean “certain.”

  11. Rules for Significant FiguresKNOW THESE!!! • 37 has two • 92116 has five • 80.4 has three • 50019 has five • 0.01265 has four • 0.0000038 has 2 • All nonzero numbers ARE significant. • Zeros appearing between nonzero digits ARE significant. • Zeros appearing in front of all nonzero digits ARE NOT significant.

  12. Rules for Significant FiguresKNOW THESE!!! • 85.0000 has six • 72.000000 has 8 • 46.0 has three • 2000 has one, but.. • 2000. has four • For our purposes, if it has a decimal, the zeros count! • Zeros at the end of a number and to the right of a decimal point ARE significant. • Zeros at the end of a number but to the left of a decimal point may or may not be significant.

  13. Bellringer • Which number is the only number that we question its “significance?” • What is the difference between accuracy and precision? • How do WE calculate percent error?

  14. Rounding Significant Figures • The only time you round up is when: • Greater than 5 • 42.68 g  42.7 g • 5, followed by nonzero digit(s) • 2.7851 cm  2.79 cm • 5, not followed by nonzero digit(s), and preceded by an odd digit • 4.635 kg  4.64 kg (because 3 is odd)

  15. Rounding Significant Figures • The last number will stay the same when the digit following the last digit to be retained is: • Less than 5 • 17.32 m  17.3 m • 5, not followed by nonzero digit(s), and the preceding significant digit is even • 78.65 mL  78.6 mL (because 6 is even)

  16. Using Significant Figures Rounding Addition or Subtraction – the answer should have the same number of decimals places as the measurement used with the fewest decimal places Multiplication and Division – the answer should have the same number of significant figures as the measurement used with the fewest significant figures. Conversion factors are NOT used to determine significant figures!

  17. Scientific Notation • Scientific Notation - method of expressing numbers using exponential notation; a measurement is expressed as a number between 1 and 10 multiplied by a whole-number power of ten • M x 10n 1  M < 10 • n is an integer (whole number)

  18. Relationships • Two quantities (x and y) are directly related if dividing one by the other gives a constant value • x / y = k or x = yk • Graph would be a straight line • Two quantities (x and y) are inversely related to each other if their product is constant • y = k / x or yx = k • Graph would be a hyperbola

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