Units

Definition:The standard measure of each kind of physical quantity is called a UNIT.

Measurement:Measurement implies comparison with a standard value. The magnitude of a physical quantity can be written as:

Numerical ratio x Units

Types of Units

In Science and Engineering there are tow kinds of Units:Fundamental UnitsDerived Units

Units to measure Length, Mass and Time are known as Fundamental Units.

All other units which can be expr3esseed in terms the fundamental units are called Derived Units.

Dimension

Definition: The unique quality of every quantity which

distinguishes it from all other quantities is called dimension.

Dimensional symbols for Length, Mass and Time are: L, M, T

All Derived units would be defined as multiple of fundamental dimensions: Volume….L3

Speed …..LT-1

Acceleration …..LT-2

System of Units

British System of units describes fundamental in centimeter, grams and seconds and therefore called CGS system.

The System that uses Meter , Kilogram and Second is called MKS.

International System of Units…SI mostly uses MKS units but is rationalized in application.

Types of SI

SI units are divided into Three:Base UnitsDerived UnitsSupplementary Units

Base UnitsQuantity; Name: Symbol:

Length metre m

Mass kilogram Kg

Time second s

Electric current ampere A

Thermodynamic temperature kelvin K

Luminous intensity candela cd

Amount of substance mole mol

Derived SI Units… 1

Quantity: Name: Symbol:

Area Square metre m2

Volume Cubic metre m3

Speed Metre per second m/s

Acceleration Metre per second square

m/s2

Density Killogram per cubic metre

kg/m3

Derived SI Units…..2

Quantity: Name: Symbol:

Concentration of mass Mole per cubic metre

mol/m3

Activity 1 per second S-1

Specific volume cubic metre per killogram

m3/kg

luminance candila per square metre

cd/m3

SI derived units with special names

Quantity Name: Symbol:

In form of other

units

In form of SI units

Frequency herts Hz - s-1

Force newton N - m.kg.s-2

Pressure pascal Pa N/m2 m-1.kg.s2

SI derived units with special names

Quantity Name: Sym--bol:

In form

of other units

In form of SI units

Energy joule J N.m m2.kg.s-2

Radiant flux watt W J/s m2.kg.s-3

Electric charge

coulomb C A.s s.A

Electric tension

volt V W/A m2.kg.s-3.A-1

SI derived units with special names

Quantity Name: Sym--bol:

In form

of other units

In form of SI units

capacitance farad F C/V m2.kg-1.s4

Electric resistance

ohm V/A m-2.kg.s-3.A-2

Conductance siemens S A/V m-2.kg-1.s3.A-2

SI derived units with special names

Quantity Name: Symbol:

In form of

other units

In form of SI units

Magnetic flux weber Wb V.S m2.kg.s-2.A-1

Flux density tesla T Wb/m2 Kg.s-2.A-1

Inductance henry H Wb/A m2.kg.s-2.A-2

Luminous flux lumen lm - cd.sr

Definition related to Instruments

True or actual Value: That magnitude to measuring system which can be approached and never evaluated is termed as True Value.

Indicated Value: The value which is indicated by the instrument.

Correction: The revision applied to the critical value so that the final result obtained improves the worth of result is called correction.

Overall error: The difference of sacle reading and true value.

Range: The region between the limits an instrument can operate.

Definition related to Instruments

Sensitivity: The ratio of output response to a specified change in the output is called sensitivity. Minimum measurable change in variable is called

Resolution sensitivity. Lowest level of measured variable that produces

effective response is called Threshold sensitivity.

Scale Sensitivity: The ratio of change in scale to corresponding change in pointer deflection.

Scale Readability: