# Units for Measurement

## •    Measurement is the comparison of an unknown quantity with a known quantity which is known as ‘Unit of Measurement’. For the measurement of any physical quantity a constant quantity is chosen as a standard which we called a unit. The standard quantity is matched or compared with the given physical quantity to find the number which expresses how many times a standard quantity is contained in a physical quantity which is termed as measurement. •    In ancient days, the inaccurate methods of measurement were used such as use of palm, foot etc., as standard to measure length. As the parts of human body differ in lengths for different people, this led to a lot of confusion. To solve the problem standard units were introduced.•     A standard unit should be of convenient size, possible to define without any doubt and easily reproduced. A standard unit should not change with respect to space and time and it should not be perishable. •    Standard units are further classified into two categories namely. Fundamental units: These are the units which can be neither derived from one another nor resolved into anything more basic. Length, mass, time, temperature, electric charge and electric current are examples of fundamental units. Derived units: These are the units which can be obtained by the combination of one or more fundamental units. Area, volume, density are some examples of derived units. There are also certain guidelines for writing units. •    CGS, MKs, FPS and SI are different systems of units which cover different specific units under them. CGS stands for centimetre, gram and second whereas MKS stands for metre, kilogram and second. FPS stands for foot, pound and second. The SI system of unit stands for international system of units and uses metre, kilogram and second as standard. •    The standard international system of units provides formal definitions for the meter, the kilogram, and the second, and also specifies and defines other additional units like the Kelvin for temperature, the candela for luminous intensity, the coulomb for electric charge, the ampere for electric current, the mole for material quantity., the radian for angle and steradian for solid angle.

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• Q1

Define one metre.

Marks:1

One metre is defined as 1,650,763.73 times the wavelength of specified orange red spectral line in emission spectrum of krypton-86.

• Q2

Name the three commonly used systems of units.

Marks:1

The three most commonly used systems of units are cgs, mks, and SI.

• Q3

What is mks system of units?

Marks:1

In cgs system of units, m stands for metre, k stands for kilogram and s stands for second.

• Q4

What is CGS system of units?

Marks:1

In cgs system of units, c stands for centimeter, g stands for gram and s stands for second.