# SI Units

Get Started. It's Free SI Units ## 3. Length

### 3.1. Metre (m)

3.1.1. History of the metre

3.1.1.1. Due to the revolution in france in 1789 all units of measure used by the old regime where replaced by new units.

3.1.1.2. It was officially adopted as the international unit for length in 1875 by the Metre Convention

3.1.1.3. The meter was intended to equal 10-7 or one ten-millionth of the length of the meridian through Paris from pole to the equator

### 3.2. Definition

3.2.1. The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

### 3.3. FunFact !

3.3.1. A light year is 5,865,696,000,000 miles or about 9,460,800,000,000 kilometers.

## 4. Electric Current

### 4.2. Definition

4.2.1. Constant flow of electrons. In a vacuum, 2 parallel lines of conductors, 1m apart from each other would produce a force of 2 x 10^7 newton per meter of length.

### 4.3. Ampere (A)

4.3.1. History

4.3.1.1. The ampere was introduced by the International Electric Congress in Chicago in 1893, It was added along with the unit for electrical resistance, Ohms.

### 4.4. Derived Units

4.4.1. Electric potential difference, electromotive force, Volt (V)

4.4.2. Electric charge, quantity of electricity, Coulomb (C)

4.4.3. Electric resistance, Ohm (Ω)

4.4.5. Electric conductance, Siemens (S)

4.4.6. Magnetic flux, Weber (Wb)

4.4.7. Inductance, Henry (H)

4.4.8. Magnetic flux density, Tesla (T)

## 5. Amount of Substance

### 5.1. Mole

5.1.1. History

5.1.1.1. The mole's history is connected to that of Avogadro's number and molecular mass. In 1805, John Dalton published the first table of atomic weights, which was based on the stoichiometry of chemical reactions/compounds. (The system said that hydrogen was defined as 1) Berzelius and Gerhardt also contributed the the expanding of the atomic weights system, solving the problem of compounds with unknown stoichiometry. (Stoichiometry is chemistry focusing on the relative quantities of reactants in chemical reactions and products made from them).

5.1.1.2. Fact: The name 'mole' came from the German unit 'Mol', translated in 1897.

5.1.2. Derived Units

5.1.2.1. Mole per cubic meter ( mol/m^3 )

5.1.2.2. Catalytic Activity - Katal ( kat )

5.1.2.3. Joule per mole ( J/mol )

5.1.2.4. Joule per mole kelvin (J/molK )

5.1.2.5. Katal per cubic meter ( kat/m^3 )

5.1.2.6. Chemical Concentration - Molal ( mol/kg )

5.1.3. Definition

5.1.3.1. 1 Mole refers to the number of basic articles (atoms, molecules, ions, electrons etc.) in a substance that is equivalent to the number of atoms in 12 gms of Carbon-12. The symbol is - mol

### 5.2. Definition

5.2.1. The number of atoms in 12 gms of Carbon-12 is 6.02 x 10^23 (Avogadro's Constant).

5.2.2. The number of atoms in 12 gms of Carbon-12 is 6.02 x 10^23 (Avogadro's Constant).

5.2.2.1. The number of atoms in 12 gms of Carbon-12 is 6.02 x 10^23 (Avogadro's Constant).

5.2.3. 1 Mole refers to the number of basic articles (atoms, molecules, ions, electrons etc.) in a substance that is equivalent to the number of atoms in 12 gms of Carbon-12. The symbol is - mol

## 6. Temperature

### 6.1. Definition

6.1.1. Temperature is the degree of intensity of heat present in a substance or an object. It can be measured by using a thermometer.

### 6.2. Derived Units

6.2.1. Degrees Celsius (C)

6.2.2. Heat Capacity (J/K)

6.2.3. Specific Heat Capacity (J/kg.K)

6.2.4. Thermal Conductivity (W/m.K)

### 6.3. History of temperature

6.3.1. In 1701, Ole Christensen Romer created one of the first practical thermometers.

6.3.2. Rømer's thermometer used red wine as the temperature indicator.

6.3.3. Rømer created a temperature scale for his thermometer with 0 representing the temperature of a salt and ice mixture

6.3.4. In 1714, through the devotion his life, Daniel Gabriel Fahrenheit invented more precise measurement for the temperature, inventing the term Fahrenheit

6.3.5. In 1742, Andres Celsius invented the term celsius which measured 0 celsius when water froze and 100 celsius when the water boiled

### 6.4. Fact

6.4.1. Absolute zero is a theoretical temperature, Aboslute zero is when all substances have no heat energy

## 7. Time

### 7.1. Definition

7.1.1. time= scalar quantity that measures the flow of events happening in the past, present and future

### 7.2. derived units

7.2.1. velocity (v)

7.2.2. coulomb (C)

7.2.3. hertz (Hz)

### 7.3. Fun Fact

7.3.1. Daylight savings time started out as a joke by Benjamin Franklin to try to get people to work earlier during summer mornings.

## 8. Luminous Intensity

### 8.1. Unit

8.1.1. Candela - cd

8.1.2. Derived units

8.1.2.1. Lux (measure of illumination of a surface)

8.1.2.1.1. Light meters often measure lux values (or footcandles, but these are directly related: one footcandle is 10.764 lx). Formally, lux is a derived unit from lumen, which is a derived unit from candela. Yet, the concept of lux is more easily compared to candela than to lumen.

8.1.2.2. Lumen (SI unit for luminous flux)

8.1.2.2.1. lumen = candela x angular span in steradian

8.1.2.3. candela per square metre (SI unit for luminance)

8.1.2.4. lumen second (SI unit for luminous energy)

8.1.2.5. lux second (SI unit for luminous exposure)

8.1.2.6. lumen per watt (SI unit for luminous eficacy)

8.1.3. A typical candle produces about one candela of luminous intensity

### 8.2. Definition

8.2.1. The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

8.2.2. Simplified: The candela measures the amount of light emitted in the range of a (three-dimensional) angular span.

8.2.3. There is a exponential law for the rate at which luminosity decays. The formulae is L = L0 exp (−t/τ)

### 8.3. History

8.3.1. Before 1946 the standardised ‘candela’, various units for luminosity were used, often based on the brightness of a flame from a “standard candle” that had specified compositions and designs. (e.g., the English ‘candlepower’ was the luminosity produced by a spermaceti candle weighing one sixth of a pound, burning at a rate of 120 grains per hour.)

8.3.2. In 1946, according to international agreement, the unit of luminous intensity was based on the luminance of a Planckian radiator (a blackbody) at the temperature of freezing platinum (1769.5°C)

8.3.3. In 1948 it gained its current name ‘candela’ along with its symbol, cd.

8.3.4. 1979 People realized it was too difficult experimentally to use the Placnk radiator method, especially at such high temperatures, therefore adopted the new definition of the candela (stated above)

### 8.4. Equation

8.4.1. Iv(λ)=683.002y(λ)I(λ)

8.4.2. here, Iv(λ) = luminous intensity in candelas, I(λ) = radiant intensity in W/sr and y(λ) = standard luminosity function