Measurements in Science is the process of obtaining the magnitude of a physical quantity and expressing it relative to a certain basic, arbitrarily chosen, internationally accepted reference standard called UNIT.

We need only a limited number of units to express all the physical quantities because these quantities are dependent on each other. We assign a set for fundamental or base quantities only, the units for these are called the Fundamental/Base units.

All remaining quantities can be derived from the base quantities. The units for these derived quantities are called derived units.

A system of units = Fundamental Units + Derived Units

There are different systems of units e.g. CGS(centimeter, gram and second), MKS(meter, gram and second) etc. However, the most commonly used standard is the SI system.

SI System

7 Base quantities

Base Quantity Name Symbol
Length metre m
Mass kilogram kg
Time second s
Electric Current ampere A
Thermodynamics Temperature Kelvin K
Amount of substance mole mol
Luminous Intensity candela cd

2 additional quantities

Base Quantity Name Symbol
Plane Angle radian rad
Solid Angle steradian sr

Errors

Degree of uncertainty associated with a measurement.

Result = measured value ± absolute error

Accuracy is a measure of how close the measured value is to the true value of a quantity.
Precision tells us to what resolution or limit the quantity is measured.

Units and Measurement
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