Name: Units of Measurement
Uploaded: 2019-07-19
Duration: 2 min 13 s
Description: We mention many physical quantities in routine conversations. These physical quantities have units with numerical values. Let's recall some units of measurem...

Question

Define basic and supplementary units of SI system. What are advantage of this system?

Accepted Answer

Basic units are used to measure the fundamental physical quantities. There are seven Base SI units defined as below:

(i) Meter: It is the SI unit of length. The distance travelled by light in vacuum in $\frac{1}{299, 792, 458} second$ is called $1 m$ .

(ii) Kilogram: The mass of cylinder made of platinum-iridium alloy kept at international Bureau of Weights and measures is defined as $1 kg$ .

(iii) second: Cesium- $133$ atom emits electromagnetic radiations of several wavelengths. A particular radiation is selected which corresponds to the transition between the two hyperfine levels of the ground state of $C s - 133$ . Each radiation has a time period of repetition of certain characteristics. The time duration in $9, 192, 631, 770$ time periods is defined as $1 s$ .

(iv) ampere: If the two parallel current carrying wires, separated at a distance of $1 m$ in vacuum experiences a force of $2 \times 10^{- 7} {Nm}^{- 1}$ between them, the equal amount of current which flows in each of them is $1 Ampere$ .

(v) kelvin: The fraction $\frac{1}{273.16}$ of the thermodynamic temperature of triple point of water is called $1 K$ .

(vi) Mole: The amount of substance that contains as many elementary entities as there are number of atoms in $0.012 kg$ of $carbon - 12$ is called a mole.

(vii) Candela: It is the luminous intensity of a black-body of surface area $\frac{1}{600, 000} m^{2}$ placed at the temperature of freezing platinum and at the pressure of $101, 325 N / m^{2}$ .

Two supplementary units are:

(i) Radian: It is used to measure plane angle. One radian is the angle subtended by arc of unit length and unit radius.

(ii) Steradian: It is used to measure solid angle. One steradian is the angle subtended by unit surface area at a distance equal to unity.

Advantage of SI system:

1) It is a coherent system. In this system all the derived units can be obtained by multiplying or dividing the fundamental units.

2) SI is an absolute system. There is no gravitational unit in the system.

3) SI is a rational system of units. i.e This system uses only one unit for one physical quantity which is not so, with the other system of units.

4. It is a metric system, All multiples are expresses in power of ten.

Define basic and supplementary units of SI system. What are advantage of this system?

Important Questions on Units : Fundamental and Derived Units

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