SI Units

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# SI Units - PowerPoint PPT Presentation

SI Units. Systéme International d´unités. Defining the kilogram. http:// www.youtube.com / watch?v =ZMByI4s-D-Y. 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.

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Presentation Transcript

### SI Units

Systéme International d´unités

Defining the kilogram

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

### Base Units

Units that cannot be derived from other units

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)

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.

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

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

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 0.012 kg of Carbon-12

Avogadro’s number: 6.02 x 1023 molecules per mole

### Derived units

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
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

Area

SI Unit: square meters (m2)

Used to describe the space occupied by a two dimensional object

Volume

SI Unit: cubic meter (m3)

Used to describe the space an object occupies

Density

SI Unit: kilogram per meter cubed (kg/m3 or kgm-3)

Describes how compact a substance is

Density = mass/volume

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

### Prefixes

Larger than the base

deca – 101 10m

hecto – 102 100m

kilo – 103 1000m

mega – 106 1000000m

giga – 109 1000000000m

tera – 1012 1000000000000m

Smaller than base

deci – 10-1 0.1m

centi – 10-2 0.01m

milli – 10-3 0.001m

micro – 10-6 0.000001m

nano – 10-9 0.000000001m

pico – 10-12 0.000000000001m

### Making Measurements

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 107instead of 13000000)
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

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%