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:
- The selection of unit and
- 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.
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
Characteristics of a standard unit:
The unit chosen for measuring any physical quantity should fulfill the following requirements:
- It should be well defined.
- It should be of suitable size i.e., neither too large nor too small in comparison to the physical quantity to be measured.
- It should be easily accessible.
- It should be reproducible.
- It should not change with time and
- It should not change with the changing physical conditions like temperature, pressure, etc.