Units for measurements Physics Notes

Units for measurements
Measuring any physical quantity means comparing it with a standard to determine its relationship with the standard of the same kind.

The chosen standard of the same kind taken as the reference in order to measure a physical quantity is called the unit of that quantity.

The process of measurement of a physical quantity involves:

  1. The selection of unit and
  2. To find out the number of times that unit is contained in the physical quantity.

For example, if we are asked to measure the length of a rope and select meter as the unit of measurement. We place the metering rod successively along PQ and find out that it is contained four times in PQ (figure). Thus, 4 is the numerical value of the length PQ.
Units for measurements Physics Notes 1
So, we write PQ = 4 meters.

In general,
The magnitude of a physical quantity Q = numerical value × size of its unit
If unit V of the quantity is contained n times in the quantity, we write
Q = nu

We know that if the size of the chosen unit is small, then the numerical value of the quantity will be large and vice-versa. It is obvious that the measure of the physical quantity is always the same
nu = constant

If n1 is the numerical value of the physical quantity for unit u1 and n2 for unit u2, then
n1u1 = n2u2

NCERT Solutions Guru Units for measurements Physics Notes

Characteristics of a standard unit:
The unit chosen for measuring any physical quantity should fulfill the following requirements:

  1. It should be well defined.
  2. It should be of suitable size i.e., neither too large nor too small in comparison to the physical quantity to be measured.
  3. It should be easily accessible.
  4. It should be reproducible.
  5. It should not change with time and
  6. It should not change with the changing physical conditions like temperature, pressure, etc.

Physics Notes