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**SI Units In this presentation you will:**

explore International System (SI) units Next >

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Introduction When measuring physical quantities in science, it is essential to use standard units. They are important for communicating scientific information. NASA lost a $300 million Mars orbiter because one engineering team used metric units while another used United States customary units for a key spacecraft operation. Next >

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Introduction After a ten month journey, the space craft entered the atmosphere too low and burned up. US customary units, sometimes called the American system or “English units,” is based upon the Imperial system. Next >

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**The International System (SI) Units**

The SI system (Le Système International d’Unités) has been accepted by the scientific community for measuring quantities. Quantity Symbol Unit Unit Symbol Length l meter m Mass kilogram kg Time t second s Temperature T or q kelvin K Amount of substance n mole mol Electric current I ampere A Luminous intensity Iv candela cd There are seven fundamental base units from which all others are derived. Next >

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**Length (l) Standard unit: meter Symbol: m**

The meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. 1 km = 1000 m 1 mm = m 1μm = m 1 nm = m Next >

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**Mass (m) Standard unit: kilogram Symbol: kg**

The kilogram is equal to the mass of an object known as the international prototype of the kilogram. 1 tonne = 1000 kg 1 g = kg 1 mg = kg The kilogram is the only SI unit that is still defined by an object. However, work is underway to define it in terms of a physical constant. Next >

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**Time (t) Standard unit: second Symbol: s**

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium133 atom. 1 min = 60 s 1 hour = 3600 s 1 ms = s 1 μs = s 1 ns = s Next >

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**Temperature (T or q) Standard unit: kelvin Symbol: K**

The kelvin, unit of thermodynamic temperature, is the fraction 1/ of the thermodynamic temperature of the triple point of water. The kelvin is based on absolute zero, the lowest temperature possible. This is 0 K, which is approximately -273 °C, or -460 °F. Next >

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**Amount of substance (n)**

Standard unit: mole Symbol: mol The mole is the amount of substance of a system that contains exactly × 1023 specified elementary entities, which may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. Next >

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**Electric Current (I) Standard unit: ampere Symbol: A**

The ampere is the electric current in the direction of the flow of exactly × 1018 elementary charges per second. 1 mA = A 1μA = A 1 nA = A Next >

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**Light Intensity (Iv) Standard unit: candela Symbol: cd**

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. A normal candle emits roughly 1 cd. A 100 W incandescent light bulb emits about 120 cd. Next >

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**Question 1 Which of the following is the SI unit of length? A) second**

B) meter C) ampere D) candela Next >

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**Question 1 Which of the following is the SI unit of length? A) second**

B) meter C) ampere D) candela Next >

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**Question 2 Which of the following is the SI unit of current? A) second**

B) meter C) ampere D) candela Next >

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**Question 2 Which of the following is the SI unit of current? A) second**

B) meter C) ampere D) candela Next >

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**The SI System – Derived Units**

All other units are called derived units in the SI system, and these are based on the seven fundamental base units. Quantity Symbol Unit Unit Symbol Derivation Force l newton N kg·m/s2 Energy E joule j N·m = m2·kg/s2 Voltage V volt J/C = m2·kg/s3·A Charge Q coulomb C s·A Frequency F hertz Hz 1/s Power P watt W J/s = m2·kg/s3 Radiation dose D gray Gv J/kg = m2/s2 Next >

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**The SI System – Prefixes**

The SI system also uses a set of standard prefixes to represent multiples of units or sub divisions of units. Multiples Name deca kilo mega giga tera peta Symbol da k M G T P Factor 101 103 106 109 1012 1015 Examples: 1 km = 1000 m 1 GW = 1,000,000,000 W Divisions Name deci milli micro nano pico femto Symbol d m μ n p f Factor 10-1 10-3 10-6 10-9 10-12 10-15 Examples: 1 mA = A 1 nm = m Next >

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**Question 3 Which is the largest amount of energy? A) 2300 J B) 2.3 GJ**

C) 230 MJ D) 23 kJ Next >

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**Question 3 Which is the largest amount of energy? A) 2300 J B) 2.3 GJ**

C) 230 MJ D) 23 kJ Next >

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**Question 4 Which is the smallest amount of force? A) 8.4 mN B) 0.84 μN**

C) 8.4 nN D) 8.4 N Next >

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**Question 4 Which is the smallest amount of force? A) 8.4 mN B) 0.84 μN**

C) 8.4 nN D) 8.4 N Next >

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**Summary In this presentation you have seen:**

how the SI system of units is based on 7 fundamental base units, from which all other units can be derived the prefixes used to describe multiples of subdivisions of units End >

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