Measurements Measurements Definitions Measurement comparison between measured quantity

Measurements

Measurements: Definitions • Measurement: – comparison between measured quantity and accepted, defined standards (SI) • Quantity: – property that can be measured and described by a pure number and a unit that names the standard

Measurement • Types: – Qualitative: • describe a substance without using numbers (measurements). – Quantitative: • require measurement to be made and have to be described by a QUANTITY (number and unit)

Measurement Requirements • Know what to measure • Have a definite agreed upon standard • Know how to compare the standard to the measured quantity (tool)

Types of measurement • Quantitative– use numbers + units to describe the measured quantity. Examples: the density of iron is 7. 8 g/cm 3. • Qualitative– use description (language) without numbers to describe the measurement • Quantitative or qualitative? – – 4 feet extra large Hot 100ºF _____________________

Measuring • Numbers without units are meaningless. • The measuring instrument limits how good the measurement is

Scientific Notations • A shortcut method for writing very large and very small numbers using powers of ten 602, 000, 000, 000 • The number is written as M x 10 n – n is + number = large numbers (>0) – n is - number = small numbers (<0)

Accuracy, Precision, and Certainty: How good are the measurements? Accuracy how close the measurement is to the actual value Precision how well can the measurement be repeated. (How well do the measurements agree with each other? )

Let’s use a golf anaolgy

Accurate? No Precise? Yes

Accurate? Yes Precise? Yes

Precise? No Accurate? Maybe?

Accurate? Yes Precise? We cant say!

In terms of measurement • Three students measure the room to be 10. 2 m, 10. 3 m and 10. 4 m across. • Were they precise? • Were they accurate?

Assessing Uncertainty • The person doing the measuring should asses the limits of the possible error in measurement

Significant figures (sig figs) • How many numbers mean anything • When we measure something, we can (and do) always estimate between the smallest marks. 1 2 3 4 5

Significant figures (sig figs) • The better marks the better we can estimate. • Scientist always understand that the last number measured is actually an estimate 1 2 3 4 5

Significant Digits and Measurement • Measurement – Done with tools – The value depends on the smallest subdivision on the measuring tool • Significant Digits (Figures): – consist of all the definitely known digits plus one final digit that is estimated in between the divisions.

Sig Figs • • Only measurements have sig figs. Counted numbers are exact A dozen is exactly 12 A piece of paper is measured 11 inches tall. • Being able to locate, and count significant figures is an important skill.

Significant Figures: Examples Measured Value Uncertainty Ruler Division Known digits Estimated digit 1. 07 cm +/-0. 01 cm 0. 1 cm 1, 0 7 3. 576 cm +/-0. 001 cm 0. 01 cm 3, 5, 7 6 22. 7 cm +/- 0. 1 cm 2, 2 7

Significant Rules examples • What is the smallest mark on the ruler that measures 142. 15 cm? – __________ • 142 cm? – __________ • 140 cm? – __________ • Does the zero count? • We need rules!!!

Rules of Significant Figures Pacific: If there is a decimal point present start counting from the left to right until encountering the first nonzero digit. All digits thereafter are significant. Atlantic: If the decimal point is absent start counting from the right to left until encountering the first nonzero digit. All digits are significant.

Sig figs. How many SF in the following measurements? 1. 2. 3. 4. 5. 6. 458 g 4085 g 4850 g 0. 0485 g 0. 004085 g 40. 004085 g

Sig Figs. 7. 405. 0 g 8. 4050 g 9. 0. 450 g 10. 4050. 05 g 11. 0. 0500060 g

Rounding rules Look at the number next to the one you’re rounding. 0 - 4 : leave it 5 - 9 : round up Round 45. 462 to: a) four sig figs b) three sig figs c) two sig figs d) one sig fig

Calculations with Significant Figures

Multiplication and Division Same number of sig figs in the answer as the least in the question 1) 3. 6 x 653 = 2350. 8 3. 6 has 2 SF 653 has 3 SF • answer can only have 2 SF Answer: 2400

Multiplication and Division • • Same rules for division practice 4. 5 / 6. 245 4. 5 x 6. 245 9. 8764 x. 043 3. 876 / 1983 16547 / 714

Practice • • 4. 8 + 6. 8765 520 + 94. 98 0. 0045 + 2. 113 6. 0 x 102 - 3. 8 x 103 5. 4 - 3. 28 6. 7 -. 542 500 -126 6. 0 x 10 -2 - 3. 8 x 10 -3

The Metric System: SI System An easy way to measure

The Metric System • Easier to use because it is a decimal system • Every conversion is by some power of 10. • A metric unit has two parts – A prefix and a base unit. • prefix tells you how many times to divide or multiply by 10.

SI Prefixes • • Exa peta tera Giga mega kilo Hecta deca Unit Centi milli micro Nano pico femto Atto Check blackboard for details

Fundamental Units SI Unit Name Abbreviation Length Meter M Mass Kilogram Kg Time Second s Temperature Kelvin K Electric current Ampere A Quantity of matter Mole Mol luminosity Candela Cd

Mass • • Quantity of matter The same in the entire universe Based on Pt/Ir alloy standard 1 gram is defined as the mass of 1 cm 3 of water at 4 ºC. • 1000 g = 1000 cm 3 of water at 4 ºC • 1 kg = 1 L of water 4 ºC

Measuring Temperature 0ºC • • • Celsius scale. water freezes at 0ºC water boils at 100ºC body temperature 37ºC room temperature 20 - 25ºC

Measuring Temperature • • Kelvin starts at absolute zero (-273 º C) degrees are the same size C = K -273 K = C + 273 Kelvin is always bigger. Kelvin can never be negative. Absolute zero: temp. at which a system cannot be farther cooled.