Significant Figures and Significant Figures and MeasurementMeasurement

Geneva High SchoolGeneva High School

Exact & Measured Numbers• There are 2 different types of numbers

– Exact– Measured

• Measured number = they are measured with a measuring device so these numbers have ERROR.

• Also, we all read measuring instruments slightly differently.

How many donuts do you see in this picture?

Hopefully you counted four donuts.This is an example of an exact number.An exact number is obtained when you count objects or use a defined relationship.

Counted objects are always exact, for example:2 soccer balls4 pizzas

Exact relationships with predefined values are also exact numbers. For example:

1 foot = 12 inches1 meter = 100 cm

Check Your Learning

A. Exact numbers are obtained by 1. using a measuring tool

2. counting3. definition

B. Measured numbers are obtained by 1. using a measuring tool

2. counting3. definition

Check Your Learning

A. Exact numbers are obtained by 2. counting

3. definition

B. Measured numbers are obtained by 1. using a measuring tool

Check Your Learning

Classify each of the following as an exact or a measured number.

1 yard = 3 feet

The diameter of a red blood cell is 6 x 10-4 cm.

There are 6 hats on the shelf.

Gold melts at 1064°C.

Check Your Learning

Classify each of the following as an exact (1) or a measured(2) number.

This is a defined relationship. A measuring tool is used to determine

length. The number of hats is obtained by

counting. A measuring tool is required.

Measurements

• All scientific measurements have a number and a unit

• Ex: 11.2 km 65.304 mg

Accuracy

• Accuracy is the closeness of a measurement to the actual value of what is being measured.

• How close a measured value is to the real value/target.

Precision

• Precision is a gauge of how exact a measurement is.

• How repeatable is a measurement?

Accuracy and Precision

Significant Figures

• Significant digits are all the digits that are known in a measurement, plus the last digit that is estimated.

• The more precise a measurement is, the more significant digits the measurement contains.

• You cannot record a piece of data that is more precise than the measuring instrument allows.

Tomorrow’s Quiz

• Scientific Notation• Metric Conversions• Identifying the number of significant

figures (yes, you may use your rules sheet I gave you_

For example…• Suppose you measure the mass of a piece of iron to be 34.73

grams on a digital scale.• You then measure the volume to be 4.42 cubic centimeters.• The calculated density of that iron would be:

Density = 34.73 g = 7.857466g/cm3

4.42 cm3

This calculated answer can’t be recorded like this, because it contains seven significant figures, while the volume of the sample only had three significant figures in the measurement.

You cannot report a calculation or measurement that is more precise than the data used in the calculation.

2.4 Measurement and 2.4 Measurement and Significant FiguresSignificant Figures

• Every experimental measurement has a degree of uncertainty.

• The volume, V, at right is certain in the 10’s place, 10mL<V<20mL

• The 1’s digit is also certain, 17mL<V<18mL

• A best guess is needed for the tenths place.

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17

What is the Length?

1 2 3 4 cm

• We can see the markings between 1.6-1.7cm• We can’t see the markings between the .6-.7• We must guess between .6 & .7• We record 1.67 cm as our measurement• The last digit an 7 was our guess...stop there

Learning CheckLearning Check

What is the length of the wooden stick?1) 4.5 cm 2) 4.54 cm 3) 4.547 cm

19

8.00 cm or 3 (2.2/8)?

Measured NumbersMeasured Numbers

• Do you see why Measured Numbers have error…you have to make that Guess!

• All but one of the significant figures are known with certainty. The last significant figure is only the best possible estimate.

• To indicate the precision of a measurement, the value recorded should use all the digits known with certainty.

20

21

Below are two measurements of the mass of the same object. The same quantity is being described at two different levels of precision or certainty.

Significant Figure Rule 1

• All non-zero digits are significant• Zeros between non-zeros are significant

34.591.0910.3001

All have 4 sig figs

Rule 2

• If number ≥ 1 with a decimal point, then…all digits are significant

7.10010078.00

Rule 3

• If digit ≥ 1 and no decimal point, then all zeros to right of non-zeros are not significant

32,000890400290

Note the 4 rulesNote the 4 rulesWhen reading a measured value, all nonzero digits should

be counted as significant. There is a set of rules for determining if a zero in a measurement is significant or not.

• RULE 1. Zeros in the middle of a number are like any other digit; they are always significant. Thus, 94.072 g has five significant figures.

• RULE 2. Zeros at the beginning of a number are not significant; they act only to locate the decimal point. Thus, 0.0834 cm has three significant figures, and 0.029 07 mL has four.

25Chapter Two

• RULE 3. Zeros at the end of a number and after the decimal point are significant. It is assumed that these zeros would not be shown unless they were significant. 138.200 m has six significant figures. If the value were known to only four significant figures, we would write 138.2 m.

• RULE 4. Zeros at the end of a number and before an implied decimal point may or may not be significant. We cannot tell whether they are part of the measurement or whether they act only to locate the unwritten but implied decimal point.

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Practice Rule #1 Zeros

45.8736.000239 .00023900 48000. 48000 3.982106 1.00040

6355246

•All digits count

•Leading 0’s don’t

•Trailing 0’s do

•0’s count in decimal form

•0’s don’t count w/o decimal

•All digits count

•0’s between digits count as well as trailing in decimal form

2.5 Scientific Notation2.5 Scientific Notation

• Scientific notation is a convenient way to write a very small or a very large number.

• Numbers are written as a product of a number between 1 and 10, times the number 10 raised to power.

• 215 is written in scientific notation as: 215 = 2.15 x 100 = 2.15 x (10 x 10) = 2.15 x

102

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Chapter Two 29

Two examples of converting standard notation to scientific notation are shown below.

Chapter Two 30

Two examples of converting scientific notation back to Two examples of converting scientific notation back to standard notation are shown below. standard notation are shown below.

• Scientific notation is helpful for indicating how many significant figures are present in a number that has zeros at the end but to the left of a decimal point.

• The distance from the Earth to the Sun is 150,000,000 km. Written in standard notation this number could have anywhere from 2 to 9 significant figures.

• Scientific notation can indicate how many digits are significant. Writing 150,000,000 as 1.5 x 108 indicates 2 and writing it as 1.500 x 108 indicates 4.

• Scientific notation can make doing arithmetic easier. Rules for doing arithmetic with numbers written in scientific notation are reviewed in Appendix A.

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2.6 Rounding Off Numbers2.6 Rounding Off Numbers

• Often when doing arithmetic on a pocket calculator, the answer is displayed with more significant figures than are really justified.

• How do you decide how many digits to keep?

• Simple rules exist to tell you how.

Chapter Two 32

• Once you decide how many digits to retain, the rules for rounding off numbers are straightforward:

• RULE 1. If the first digit you remove is 4 or less, drop it and all following digits. 2.4271 becomes 2.4 when rounded off to two significant figures because the first dropped digit (a 2) is 4 or less.

• RULE 2. If the first digit removed is 5 or greater, round up by adding 1 to the last digit kept. 4.5832 is 4.6 when rounded off to 2 significant figures since the first dropped digit (an 8) is 5 or greater.

• If a calculation has several steps, it is best to round off at the end.

Chapter Two 33

Practice Rule #2 RoundingPractice Rule #2 RoundingMake the following into a 3 Sig Fig numberMake the following into a 3 Sig Fig number

1.5587

.0037421

1367

128,522

1.6683 106

1.56

.00374

1370

129,000

1.67 106

Your Final number must be of the same value as the number you started with,129,000 and not 129

Examples of RoundingExamples of RoundingFor example you want a 4 Sig Fig number

4965.03

 

780,582

 

1999.5

0 is dropped, it is <5

8 is dropped, it is >5; Note you must include the 0’s

5 is dropped it is = 5; note you need a 4 Sig Fig

4965

780,600

2000.

RULE 1. RULE 1. In carrying out a multiplication or division, the answer cannot have more significant figures than either of the original numbers.

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•RULE 2. In carrying out an addition or subtraction, the answer cannot have more digits after the decimal point than either of the original numbers.

Chapter Two 37

Multiplication and divisionMultiplication and division

32.27 1.54 = 49.6958

3.68 .07925 = 46.4353312

1.750 .0342000 = 0.05985

3.2650106 4.858 = 1.586137 107

6.0221023 1.66110-24 = 1.000000

49.7

46.4

.05985

1.586 107

1.000

Addition/SubtractionAddition/Subtraction

25.5 32.72 320 +34.270 ‑ 0.0049 + 12.5 59.770 32.7151 332.5

59.8 32.72 330

__ ___ __

Addition and SubtractionAddition and Subtraction

.56 + .153 = .713

82000 + 5.32 = 82005.32

10.0 - 9.8742 = .12580

10 – 9.8742 = .12580

.71

82000

.1

0

Look for the last important digit

Mixed Order of OperationMixed Order of Operation

8.52 + 4.1586 18.73 + 153.2 =

(8.52 + 4.1586) (18.73 + 153.2) =

239.6

2180.

= 8.52 + 77.89 + 153.2 = 239.61 =

= 12.68 171.9 = 2179.692 =

Rule 4

• If number < 1, all zeros to left of non-zeros are not significant

0.0980.00030.90020

Rule 5

• Any number that represents a numerical count or exact definition has an infinite # of sig figs

32 students in the class

Sig Figs in Addition/Subtraction

• Answer is rounded to same # of decimal places as the measurement with the least # of decimal places.

• Ex: 13.1 + 4.25

63.408 This answer would be 80.758 rounded to 80.8

Sig Figs in Multiplication/Division

• Round your answer to the measurement with the least # of sig figs.

Ex: 4.3 x 2 = 8.6 = 9

18.75 ÷ 3.5 = 5.357 = 5.4

Error

Error = experimental value – accepted value

Percent Error

Percent Error = error / accepted value x 100