UNITS

Units: It is the measurement of Physical quantity.It is one of the important factor in Engineering

                     field.It is mainly classified into two units.

                               1)Fundamental units

                               2)Derived units

Fundamental units:

               It is the base unit for all physical quantity.There are seven base units are kilogram,candela,metre,second,ampere,kelvin and mole.Most probably fundamental units are denoted as Length(L),Mass(M) and Time(t).

Derived units:

                               The units which is derived from the Fundamental units are called Derived units 

e.g., Volume, area,velocity,pressure.

System of units:

                               There are four types of system of units,which are commonly used and universally

recognised.

                               1)C.G.S units

                               2)F.P.S  units

                               3)M.K.S units

                               4)S.I. units

C.G.S units :

                               In this system,fundamental units of Length,mass,time are centimetre,gram and time.It is also known as absolute units(not depend on arbitrary units)

F.P.S units :

                           In this system,fundamental units of length,mass and time are foot,pound and second respectively.

M.K.S units:

                                 In this system,fundamental units of length,mass and time are mass,kilogram and second respectively.It is also known as engineer's units.

International System of Units(S.I. units):

                                 When Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass, length and time. Giorgi later identified the need for an electrical base unit. Theoretically any one of electric current,potential difference, electrical resistance, electrical charge or a number of other quantities could have provided the base unit, with the remaining units then being defined by the laws of physics. In the event, the unit of electric current was chosen for SI. Another three base units (for temperature, substance and luminous intensity) were added later.It is the most widely used system of measurement.The seven base units are metre,kilogram,second,ampere,kelvin,mole and candela.

Important units:

SI Units and Symbols used in the Guide Subject Physical Quantity Symbol Name Unit Mechanics Mass m, M kilogram kg Linear position Length, Distance Radius x, r  l, d R meter m Time t,  second s Linear angle, Angular position ,  radian rad Spherical angle steradian sr Area A - m2 Volume V - m3 Moment of inertia I - kg*m2 Density - kg/m3 Linear velocity v, u, c - m/s Angular velocity ,  - rad/s Linear momentum p - kg*m/s Angular momentum L - kg*m2/s Linear acceleration a - m/s2 Angular acceleration - rad/s2 Force F newton N=kg*m/s2 Torque - N*m Impulse I - N*s Work Energy W E joule J=N*m Power P watt W=J/s Dynamic viscosity - Pa*s Electricity and Magnetism Current I ampere A Charge Q, q, e coulomb C=A*s Current density j - A/m2 Volume charge density - C/m3 Surface charge density - C/m2 Linear charge density - C/m Electric potential Voltage emf V volt V=J/C Electric field E - N/C, V/m Electric flux - V*m Electric moment pe - C*m Resistance R, r ohm =V/A Specific resistance - *m Capacitance C farad F=C/V Specific conductivity - (*m)-1 Magnetic field B tesla T=N/(A*m) Magnetic flux weber Wb=T*m2=V*s Inductance Mutual-inductance L M henri H=Wb/A Magnetic moment pm - A*m2 Polarization P - C/m2 Magnetization I - A/m Thermodynamics Temperature T kelvin K Substance quantity M mole mol Pressure P - Pa Heat Q - J Heat capacity Entropy C S - J/K Specific heat c - J/(kg*K) Molar heat cm - J/(mol*K) energy flux j - W/m2 Surface tension - N/m Stress Elasticity modulus E pascal Pa=N/m2 Oscillations and Waves Wavelength - m Wave number k - m-1 Frequency f hertz Hz Energy density - J/m3 Energy flux J - J/m2 Intensity I - J/(m2*s) Reactance Impedance X Z ohm =V/A Optics Focal length f - m Luminous intensity I candela cd Luminous flux lumen lm=cd*m2 Illuminance E lux lk=lm/m2 Brightness L - cd/m2 Linear absorption coefficient - m-1 Quantum Physics Mass absorption coefficient - m2/kg Radioactive activity A becquerel Bq=s-1 Absorbed dose D gray Gy=J/kg

                                     

                                   

(Most Important) Unit Conversions

• length
• area
• volume
• mass
• pressure
• energy
• power
• temperature
• radioactivity
• scientific notation

Length

• 1 (statute) mile (mi) = 1.6093 kilometer (km)
• 1 (nautical) mile (mi) = 1.8520 kilometer (km)
• 1 foot (ft) = 0.3048 meter (m)
• 1 yard = 0.9144 meter (m)
• 1 inch (in) = 2.54 centimeter (cm)
• 1 angstrom (A) = 10^-8 centimeter (cm) = 10^-10 meter (m)

Area

• 1 square kilometer (km²) = 10⁶ square meters (m²) = 100 hectares (ha)
• 1 hectare (ha) = 10,000 square meters (m²)
• 1 acre (ac) = 4047 square meters (m²) = 0.4047 hectare (ha)

Volume

• 1 milliliter (ml) = 1 cubic centimeter (cm³)
• 1 cubic meter (m³) = 1000 liters (L)
• 1 (US) quart (qt) = 0.9461 liter (L)
• 1 (US) gallon (gal) = 3.7854 liter (L)
• 1 (US) pint = 0.4723 liter (L)
• 1 (US) fluid ounce = 29.6 milliliter (ml)

Mass

• 1 metric ton (m.t.) = 1000 kilograms (kg)
• 1  pound (lb) = 0.4535924 kilogram (kg)
• 1 ounce (oz) = 28.3495 grams (g)

Pressure

• 1 pascal (Pa) = 1 newton/square meter (N/m²) = 1 Kg m^-1 s^-2
• 1 bar = 0.98692 atmosphere (atm) = 10⁵ pascals (Pa)
• 1 pound per square inch (psi) = 68.97 millibars (mb) = 6897 pascals (Pa)

Energy

• 1 joule (J) = 1 newton meter (Nm)
• 1 calorie (cal) = 4.184 joule (J)
• 1 kilowatt hour (kWh) = 3.6 x 10⁶ joules (J) = 8.60 x 10⁵ calories

Power (energy per unit time)

• 1 watt (W) = 1 joule per second (J/s) = 14.34 calories per minute (cal/min)

Temperature

• from Fahrenheit to Celsius: C = (F - 32) x 5/9
• from Celsius to Fahrenheit: F = (C x 9/5) + 32
• from Celsius to Kelvin: K = C + 273.15

Radioactivity

• 1 Curie (Ci) = 3.7x10¹⁰ Becquerel (Bq)
• 1 Gray (Gy) = 1 J/kg tissue = 100 rad

Commonly used prefixes...

Scientific notation

Prefix...........	Exponent	Examples of exponential usage:
peta (P)	15	amount of CO2 in atmosphere (as C)- 750 Pg (or Gt)
tera (T)	12
giga (G)	9	pool sizes: amount of water in ocean - 1.37 x 109 km3
mega (M)	6	rates: flow of Anatarctic Circumpolar Current - 200 x 106 m s-1
kilo (k)	3	distance New York - Albany ~ 350km
hecto (h)	2	atmospheric pressure: ~1012.5 hPa (=1012.5 mbar)
base unit	0
milli (m)	-3	typical amount of medicine ~1ml
micro (m)	-6
nano (n)	-9	wavelength of violet light ~400nm
pico (p)	-12	size of atom ~10pm
femto (f)	-15	detection limit of gas chromatographs for SF6 : ~1 fmole/L