1. Systéme International d´unités
SI Units
Systéme International d´unités

2. Defining the kilogram

3. The need for SI Units
At the end of the eighteenth century, science and technology were growing by leaps and bounds across the developed world.
New scientific studies needed to be shared between countries and needed to have the same units of measurement in order to be accurately compared
In 1791, the metric system was established in Europe
In 1875, The Metre Convention was established – a group of international scientists that would get together every 4-6 years to discuss units of measurement
The most recent additional was the mole in 1971

4. Units that cannot be derived from other units
Base Units
Units that cannot be derived from other units

5. Mass SI Unit: kilogram (kg)
Original definition (1793) – The grave was defined as the mass of one cubic decimetre of pure water at its densest point (4° C)
Current definition (1889) – The mass of the International Prototype Kilogram or “Big K”
The Indus Valley Civilization were the first to develop a system of weights and measures (4000 BC)

6. Length SI Unit: metre (m)
Original definition (1793): 1/10,000,000 of the distance between the North Pole and the equator, in a line going through Paris
Current definition (1983): The distance traveled by light in a vacuum in 1/299,792,458 seconds
The ancient Egyptians (3000 BC) used the unit cubit to measure length – the length from the elbow to the tip of the middle finger. It is believed that yards, feet, and inches were derived from this.

7. Time SI Unit: second (s) Original Definition (Medieval): 1/86,400 day
Current Definition (1967): the time it takes to transition between two states of caesium 133
Ancient calendars marked the passage of time as early as 6000 years ago
Ancient time keepers include Egyptian sundials, Persian water clocks, and European hourglasses

8. Temperature SI Unit: kelvin (K)
Original definition (1743): established the centigrade scale (°C) by assigning 0°C to the freezing point of water and 100°C to the boiling point of water
Current definition (1967): assigned 0 K to absolute zero – the point at which all atomic motion stops

9. Amount of a substance SI Unit: mole (mol)
Original definition (1900): The molecular weight of a substance in grams
Current definition (1967): The amount of substance that contains as many “parts” as kg of Carbon-12
Avogadro’s number: 6.02 x 1023 molecules per mole

10. Derived units

11. Weight The force on an object due to gravity
NOT the same as mass: Weight = mass x gravity
SI Unit: newton (N)
The ancient Greek had many definitions of weight:
Aristotle – weight was the opposite of levity and the two competed to determine if an object would sink or float. The earth had ultimate weight and fire had ultimate levity.
Plato described weight as an objects desire to seek out its kin
Galileo was the first to determine that weight was related to the mass of an object

12. SI Unit: meter per second (m/s or ms-1)
Speed
SI Unit: meter per second (m/s or ms-1)
Used to describe the time it takes an object to travel a given distance

13. SI Unit: square meters (m2)
Area
SI Unit: square meters (m2)
Used to describe the space occupied by a two dimensional object

14. Volume SI Unit: cubic meter (m3)
Used to describe the space an object occupies

15. Density SI Unit: kilogram per meter cubed (kg/m3 or kgm-3)
Describes how compact a substance is
Density = mass/volume

16. Energy SI Unit: Joule (J) Named after James Prescott Joule
Energy is the capacity to do work or to produce heat
Calorie (cal) is the heat needed to raise 1 gram of water by 1°C
1 cal = 4.18 J

17. Prefixes

18. Larger than the base deca – 101 10 hecto – 102 100 kilo – 103 1000
mega –
giga –
tera –

19. Smaller than base deci – 10-1 0.1 centi – 10-2 0.01 milli – 10-3 0.001
micro –
nano –
pico –

20. How to be accurate, precise, and complete in your answers
Making Measurements
How to be accurate, precise, and complete in your answers

21. Making Measurements
Qualitative – measurements are words, like heavy or hot
Quantitative – measurements involve number (quantities) and depend on:
The reliability of the measuring instrument
The care with which it is read (This depends on YOU!)
Scientific Notation
Coefficient raised to the power of ten (ex. 1.3 x 107 instead of )

22. Accuracy, Precision and Error
Accuracy – how close a measurement is to the true value
Precision – how close the measurements are to each other (reproducibility)
Neither accurate nor precise
Precise, but not accurate
Precise AND accurate

23. Accuracy, Precision, and Error
Accepted value – the correct value based on reliable references
Experimental value – the value measured in the lab by you
Error – accepted value – experimental value
Can be positive or negative
Percent error – the absolute value of the error divided by the accepted value, then multiplied by 100%