Engineering 10

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# Engineering 10 - PowerPoint PPT Presentation

Engineering 10. Units & Conversions. Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu. Units Introduction. People measure quantities through comparisons with standards. Every measured quantity has an associated “unit” Which is the name of the Standard.

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## Engineering 10

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Engineering 10

Units &Conversions

Bruce Mayer, PE

Units Introduction
• People measure quantities through comparisons with standards.
• Every measured quantity has an associated “unit” Which is the name of the Standard.
• Need to define sensible and practical "units" and "standards" that scientists & engineers everywhere can agree upon
• Even though there exist an almost infinite number of different physical quantities, we need no more than a handful of “base” standards.
SI System of Units
• Système International d'Unités (International System of Units)
• A CompletelyConsistentSet of BasicUnits
• Requires NO Conversion factors
• e.g., 5280 ft = 1 mile
• Defined by UNCHANGING Physical Phenomena
• Except for one...

http://www.bipm.org/en/si/

SI System History
• In 1960 The 11th General Conference on Weights and Measures (GCWM) adopted the name SI System, for the recommended practical system of units of measurement.
• The 1960 GCWM Specified Seven well-defined “Base” units which, by convention, are regarded as DIMENSIONALLY INDEPENDENT

http://www.bipm.org/en/si/

¿¿ Class Question ??
• Which of you Have Heard of the unit:
• METER (m)
• KILOGRAM (kg)
• SECOND (s)
• AMPERE or AMP (A)
• KELVIN (K)
• MOLE (mol)
• CANDELA (cd)
SI Base Units
• From this List Observe
• Very common Units
• Mass (kg)
• Length (m)
• Time (s)
• Some Not so Common Units
• Current (A)
• Temperature (K)
• Some Uncommon Units
• Substance amt (mol)
• Luminous Int (cd)
• All but the kg are defined by Physical Phenomena
• Examine the Defs
Length or Distance (meter)

1 meter

Laser

1/299792458 s

photon

Meter Defined
• “The path traveled by light in vacuum during a time interval of 1/299792458 of a second.”
Mass (kilogram)kilogram Defined
• a cylinder of PLATINUM-IRIDIUM alloy maintained under vacuum conditions by the International Bureau of Weights and Measures in Paris

If The ProtoType Were Cubic, its Edge Length would be About 36.2 mm (1.42”); quite small

Time (Second)Second Defined
• 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 cesium 133 atom
• This is the Definition of an “Atomic” Clock
• more than 200 atomic clocks are located in metrology institutes and observatories in more than 30 countries around the world
Electric Current (Ampere)Amp Defined
• That constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in a vacuum, would produce between these conductors a force equal to 2 x 10−7Newton per metre of length.
• What’s a Newton?→ 1kg-m/(s2)
Thermo-dynamic temperature (Kelvin)Kelvin (Temperature) Defined
• The unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
• 273.16K = 0.0098 °C
• Room Temperature (72 °F) is about 295.5 Kelvins
• NO “Degree” Sign Used with the Kelvin Unit
Amount of Substance (mole)mole (amt of Substance) Defined
• The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon 12.
• 1 mole = 6.023x1023entities
• entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
Light Brightness (candela)Luminous Intensity Defined
• The luminous intensity, in a given direction, of a source that emits monochromatic radiation (one-color light) of frequency 540 x 1012 Hertz (555 nm) and that has a radiant intensity in that direction of 1/683 watt per steradian

555nmcolor

• The are 4 (12.57) Steradians in a Sphere
• 1 Str = 7.96% of the Sphere Surface
Units Have Evolved
• Candela Predecessor based on a Flame
• Hence the Name
• Temperature Based on Freezing points
• Water
• Platinum
• Second Based on the Sidereal (standard) day
Units Have Evolved
• History of the Meter (or Metre)
• One ten millionth of the distance from the North pole to the equator.
• The distance between two fine lines engraved near the ends of a platinum-iridium bar
• 1 650 763.73 wavelengths of a particular orange-red light emitted by atoms of krypton-86 (86Kr).
• The length of the path traveled by light in a vacuum during a time interval of 1/299 792 458 of a second.
SI Derived Units
• The Seven Base Units May be Algebraically Combined to Produce “Derived Units”
• e.g.:
• Several DerivedUnits have SpecialUsefulness and arethus Given their OWN Names
Old (and Tired) Unit Sets
• MKS
• Stands for Meter-Kilogram-Second as the Most Common Units
• Predecessor to The SI System
• CGS
• Means Centimeter-Gram-Second
• Still Widely Used
• IPS, FPM, FPH
• Inch-Pound-Sec, Foot-Lb-Min, Ft-Lb-Hour

Fundamental Dimension

Base Unit

length

mass

force

time

electric charge [Q]

absolute temperature

luminous intensity

amount of substance

foot (ft)

pound (lbm)

pound (lbf)

second (sec)

coulomb (C)

degree Rankine (oR)

candela (cd)

mole (mol)

Some Are the SAME SI

Conservation of Units
• Principle of conservation of units:
• Units on the LEFT side of an equation MUST be the SAME as those on the RIGHT side of an Equation
• Then Have Dimensional Homogeneity
• Needed to Prevent “Apples & Oranges” Confusion
• e.g., I Buy 100 ft of Wire at One Store and 50 m at another; how much total Wire do I have? (It’s NOT “150”)
• To Determine the Amount of Wire I have I Need to Convert to Consistent (Homogeneous) Units
• Start by Thinking About the Definition of “1”
• AnyThing divided by ITSELF = “1”
• Now Consider a “minute”
• Read as “60 Seconds per minute”
• Units can also be Multiplied and Divided in a manner similar to Numbers
• This how we get, say, “Square Feet”
• e.g.; Consider an 8ft x 10ft Engineer’s Cubicle in Dilbert-Land. How Much WorkSpace Does the Engineer Have?
• Now Back to the Wire
• Want to Know how many FEET of Wire I have in Total
• Check in Table 16.8 and Find “3.2808 ft = 1meter (“3.2808 ft per meter”)
• Multiply the 50m by this special Value of 1
• Can “Cancel” The Units by Division
• So then the Total Wire = 264 ft
• A World-Class Sprinter can Run 100m in 10s.
• How Fast is this in MPH?
• Gasoline In Seoul Costs 1840 Korean-Won (W) for one Liter of Regular Unleaded
• How Much is this in \$ per Gallon
• Find Currency Exchange Rate → \$1 = 1150 W
Several Forms of “1”
• Unit Conversion Factors
• ANYTHING Divided by ItSelf = 1 
Units – Exponent Properties

This summary assumes that no denominators are 0 and that 00 is not considered. For any integers m and n

Raising units to POWERS
• Start again with 1
• Can do the SAME Thing with Units.
• And 123 = 1728 so
• Thus have 1728 “cubic inches” per “Cubic Foot”
• What’s a “Cubic Yard” in “Cubic Feet”?
• So have 27 cubic-ft per cubic-yd
• NOT “9”
7 inches, Water Column
• Ms. Ezersky noted that Natural Gas is delivered by PG&E to homes at a pressure of 4-7 “inches of Water Column”
• This is a unit of pressure, Just Like Pascals or psig
• A U-Tube Manometer can Measure Pressure Differences in Inches of Water Column
7 inches, Water Column
• To Calc the “in-WC” pressure we need to know some Engineering Physics
• From ENGR36
• Where
• ≡ Liquid SPECIFIC WEIGHT
• h ≡ Liquid Column Height
• For Liquid Water at Room Temperature and Pressure
• Now find 7 in-WC in psig

Natural Gas @ 9.5 inWC

7 inches, Water Column
• Convert out the N & m
White Board Examples cont.
• The USA FDA recommends that Adults consume 2200 Calories per Day
• What then is the “Power Rating” of a Grown Human Being?
• Note that there are TWO types of “Calories”
• The Amount of Heat Required to Raise the Temperature of 1 GRAM of water by 1 °C (or 1 Kelvin)
• Often Called the Gram-CAL; This is what is in the Text
• The Amount of Heat Required to Raise the Temperature of 1 KILOgram of water by 1 °C
• Often Called the kgCAL or kiloCal; This is what you read on the side of Food Packaging
Tire Pressure
• Many AutoMobile Tires have a Maximum Pressure Rating of About 44 psig.
• Convert 44 psi to kiloPascals (kPa)
Ton of Refrigeration
• During his Presentation Mr. Ian McClaren of SouthLandIndustries described the “Ice Storage” Cooling System Behind Bldg-1800.
• He Noted that the Cooling Power of this system was Rated in “Tons”
• What is a “Ton” of Cooling Power
Ton of Refrigeration
• A TON of the refrigeration is defined, roughly, as the COOLING effect of melting 2000 lbs of water ICE over a 24 HOUR Period
• From PHYS4C (or ASHRAE HandBook) find that the “Latent Heat of Fusion” for ice is 333.55 kJ/kg
• On WhtBoard Convert a “Ton of Refrigeration” to
• kW and Btu/hr
White Board Examples
• A 2003 Chevy z06 corvette
• Has a 5.7 Liter V8 Engine
• What is the Engine Displacement in cubic-inches?
• Develops 410 HP
• What is the Power in Watts?
• A the Maximum recommended pressure for many 65R15 tires is 44 psi (lbs per sq-inch; NOT lbs)
• What is this Max Pressure in kPa?
All Done for Today

How toSpend theCalories

¿¿ Class Question ??
• Let’s Make List of “METRIC” Type Units