Uncertainty in Measurements

Transcription

1 Uncertainty in Measurements Ø A measurement is a number with a unit attached. Ø It is not possible to make exact measurements, and all measurements have uncertainty. Ø We will generally use metric system units. These include: the meter, m, for length measurements the gram, g, for mass measurements the liter, L, for volume measurements The second, s, for time measurements Chapter 1 1

2 Length Measurements Ø Let s measure the length of a candy cane. Chapter 1 2

3 Length Measurements Ø Ruler A has 1 cm divisions, so we can estimate the length to ± 0.1 cm. The length is 4.2 ± 0.1 cm. Ø Ruler B has 0.1 cm divisions, so we can estimate the length to ± 0.05 cm. The length is 4.25 ± 0.05 cm. Chapter 1 3

4 Length Measurements Ø Ruler A: 4.2 ± 0.1 cm; Ruler B: 4.25 ± 0.05 cm. Ø Ruler A has more uncertainty than Ruler B. Ø Ruler B gives a more precise measurement. Chapter 1 4

5 Ø The mass of an object is a measure of the amount of matter it possesses. Ø Mass is measured with a balance and is not affected by gravity. Ø Mass and weight are not interchangeable. Mass Measurements Chapter 1 5

6 Mass versus Weight Ø Mass and weight are not the same. Ø Weight is the force exerted by gravity on an object. Chapter 1 6

7 Ø Volume is the amount of space occupied by a solid, liquid, or gas. Ø There are several instruments for measuring volume, including: Graduated cylinder Syringe Buret Pipet Volumetric flask Volume Measurements Chapter 1 7

8 Significant Digits Ø Each number in a properly recorded measurement is a significant digit (or significant figure). Ø The significant digits express the uncertainty in the measurement. Chapter 1 8

9 Significant Digits Clock reaction = change from colorless to blue after about 35 seconds. Ø Stopwatch A displays 35 s à 2 sig figs Ø Stopwatch B displays 35.1 s à 3 sig figs Ø Stopwatch C displays s à 4 sig figs Chapter 1 9

10 Significant Digits Ø To determine the significant digits in a measurement, read the number from left to right, counting digits from the first digit that is not zero. Ø All non-zero digits are significant. Ø Example: g has 4 significant figures m has 5 significant figures. 358 kg has 3 significant figures. Chapter 1 10

11 Significant Digits Ø The following guidelines can be used while encountering zeros in measurements. Ø Zeros between non-zero digits are always significant. Ø Example: 1005 kg has 4 significant figures cm has 3 significant figures. 205 meters has 3 significant figures. 108 C has 3 significant figures. Chapter 1 11

12 Significant Digits Ø Zeros at the beginning of the number are not significant. They only indicate the position of the decimal point. Ø Examples: 0.02 g has 1 significant figure cm has 2 significant figures L has 3 significant figures. Chapter 1 12

13 Significant Digits Ø Zeros at the end of the number are significant only if the number contains a decimal point. Ø Examples: 3.0 cm has 2 significant figures g has 3 significant figures g has 4 significant figures ml has 4 significant figures cm has 2 significant figures. Chapter 1 13

14 Exact Numbers Ø When we count something, it is an exact number. Ø Significant digit rules do not apply to exact numbers. Ø An example of an exact number: there are 3 coins on this slide. Chapter 1 14

15 Rounding Off Nonsignificant Digits Ø All numbers from a measurement are significant. However, we often generate nonsignificant digits when performing calculations. Ø We get rid of nonsignificant digits by rounding off numbers. Chapter 1 15

16 Rounding Examples Ø A calculator displays and 3 significant digits are justified. Ø The first nonsignificant digit is a 4, so we drop all nonsignificant digits and get 12.8 as the answer. Ø A calculator displays and 3 significant digits are justified. Ø The first nonsignificant digit is a 5, so the last significant digit is increased by one to 9, all the nonsignificant digits are dropped, and we get 12.9 as the answer. Chapter 1 16

17 Rules for Rounding Numbers Ø Ø Ø If the first nonsignificant digit is less than 5, drop all nonsignificant digits. If the first nonsignificant digit is greater than or equal to 5, increase the last significant digit by 1 and drop all nonsignificant digits. If a calculation has two or more operations, retain all nonsignificant digits until the final operation and then round off the answer. Chapter 1 17

18 Rounding Off & Placeholder Zeros Ø Round the measurement 151 ml to two significant digits. If we keep 2 digits, we have 15 ml, which is only about 10% of the original measurement. Therefore, we must use a placeholder zero: 150 ml Recall that zeros are not significant if the number does not have a decimal point. Ø Round the measurement 2788 g to two significant digits. We get 2800 g. Remember, the zeros are not significant if the number does not have decimal places, and 28 grams is significantly less than 2800 grams. Chapter 1 18

19 Adding & Subtracting Measurements Ø When adding or subtracting measurements, the answer is limited by the value with the most uncertainty (the answer has the same # of decimal places as the measurement with the least # of decimal places.) Let s add three mass measurements. The measurement g has the greatest uncertainty (± 0.1 g). The correct answer is g g 0.25 g g g Chapter 1 19

20 Multiplying & Dividing Measurements Ø When multiplying or dividing measurements, the answer is limited by the value with the fewest significant figures. Let s multiply two length measurements. (5.15 cm)(2.3 cm) = cm 2 The measurement 2.3 cm has the fewest significant digits, two. The correct answer is 12 cm 2. Chapter 1 20

21 Ø Exponents are used to indicate that a number has been multiplied by itself. Ø Exponents are written using a superscript; thus, (2)(2)(2) = 2 3. Ø The number 3 is an exponent and indicates that the number 2 is multiplied by itself 3 times. It is read 2 to the third power or 2 cubed. Ø (2)(2)(2) = 2 3 = 8 Exponential Numbers Chapter 1 21

22 Powers of Ten Ø A power of 10 is a number that results when 10 is raised to an exponential power. Ø The power can be positive (number greater than 1) or negative (number less than 1). Chapter 1 22

23 Scientific Notation Ø Numbers in science are often very large or very small. To avoid confusion, we use scientific notation. Ø Scientific notation utilizes the significant digits in a measurement followed by a power of ten. The significant digits are expressed as a number between 1 and 10. Chapter 1 23

24 Scientific Notation Ø There are 26,800,000,000,000,000,000,000 helium atoms in 1.00 L of helium gas. Express the number in scientific notation. Ø Place the decimal after the 2, followed by the other significant digits. Ø Count the number of places the decimal has moved to the left (22). Add the power of 10 to complete the scientific notation atoms Chapter 1 24

25 Applying Scientific Notation Ø To use scientific notation, first place a decimal after the first nonzero digit in the number followed by the remaining significant digits. Ø Indicate how many places the decimal is moved by the power of 10. A positive power of 10 indicates that the decimal moves to the left. A negative power of 10 indicates that the decimal moves to the right. Chapter 1 25

26 Scientific Notation : Example Ø The typical length between two carbon atoms in a molecule of a compound is m. What is the length expressed in scientific notation? Ø Place the decimal after the 1, followed by the other significant digits. Ø Count the number of places the decimal has moved to the right (7). Add the power of 10 to complete the scientific notation m Chapter 1 26

27 Scientific Notation : Example Ø A scientific calculator has an exponent key (often EXP or EE ) for expressing powers of 10. Ø If your calculator reads 7.45 E-17, the proper way to write the answer in scientific notation is Ø To enter the number in your calculator, type 7.45, then press the exponent button ( EXP or EE ), and type in the exponent (17 followed by the +/ key). Chapter 1 27

28 Measurements: Precision and Accuracy 1. Precision - how closely a series of measurements of the same object resemble each other. - the closer the measurements are to each other, the more precise the measurement. 2. Accuracy - a measurement s relationship to the property s true value. The precision of a measurement is not necessarily equal to its accuracy!!!