What is meant by the unit of a physical quantity? Define the basic units in SI system.
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Answer:
All physical quantities in the International System of Units (SI) are expressed in terms of combinations of seven fundamental physical units, which are units for: length, mass, time, electric current, temperature, amount of a substance, and luminous intensity.
Unit of a Physical Quantity
All quantities in terms of which law of Physics are expressed and which can be measured directly or indirectly are called physical quantities.
For the measurement of a physical quantity, we consider a constant quantity as a standard quantity and then find the number which express how many times the standard quantity is contained in the physical quantity. This standard quantity is called the unit. Measurement is thus the comparison of unknown quantity with a known fixed unit quantity.
For example, if the length of a rod is 3 metres, it means that the unit of length is the metre and this unit is contained three times in the length of that rod. Similarly, if the mass of a ball is 2 kilograms, it means that unit of mass is kilogram and this unit is contained two times in the mass of the ball. Hence, to express the magnitude of a physical quantity we should know two things are the unit in which the quantity is measured and the numerical value which express how many times the above mentioned unit is contained in the given physical quantity.
.°. Hence, Magnitude of a physical quantity = numerical value of physical quantity × size of its unit
If "n" is the numerical value of physical quantity and "u" is the size of the unit, then, magnitude Q of the physical quantity is:
[ Q = nu ]
Thus when say that the length of the rod is 3 m i.e. Q = 3 m, it means n = 3 and u = 1 m.
The size of unit is different in different systems of units. Suppose the magnitude of a physical quantity is Q. If n1 is the numerical value of the physical quantity for a unit u1 and n2 is the numerical value of the physical quantities for a unit u2, then,
Q = n1u1 (For first system of units)
Q = n2u2 (For second system of units)
Hence, n1u1 = n2u2
Two things are obvious from equation 1,
1. The magnitude of a given physical quantity is always same irrespective of the system of units adopted that is n u = constant.
2. If u1 > u2, then n1 < n2. Therefore, the larger the magnitude of the unit selected the smaller is the numerical value of the physical quantity.
Basic Units in S.I. System
There are seven main and two supplementary fundamental quantities.
I mentioned three quantity which are as follows:
1. Length: Unit meter, Symbol "m"
=> It is the SI unit of length and is equal to the length of the path travelled by light in vacuum during the time interval of 1 / 299,792,458 of a second.
2. Mass: Unit kilogram Symbol "kg"
=> It is the SI unit of mass and is equal to the mass of a Platinum iridium cylinder of diameter equal to its height which is present in a vault at international bureau of weights and measures at severes near Paris France.
3. Time: Unit second Symbol "s"
=> It is the SI unit of time. The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom.
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