Measurement plays an important role since the start of civilization. There was a time when the length, mass and time were measured by various methods. As the metric system is introduced in this era, the measurement method has become standard for everyone. Measurement of physical quantity that involves comparisons with internationally accepted reference standards are known as unit. For example, length is a physical quantity and meter is a unit. Whereas measurement determines how large or small is the physical quantity. There is a common global standard like International Systems of Units used by everyone. This is a very important chapter in physics for 11th standard students of CBSE Board. Solutions to all the questions in this chapter can be easily found in any good solution key that provides NCERT Solutions for Class 11 Physics. Science, medicine, engineering use larger and smaller units of measurement to aid their research in problem solving.
Units
In early times different countries use different systems of units and measurement. The three such units are:
- CGS system: centimeter,gram and second
- FPS system: foot, pound and second
- MKS system: meter, kilogram and second
Now the international system of units are used globally abbreviated as SI for international usage in scientific, technical and industrial work.
There are seven basic units of measurement mentioned below:
| Base Quantity | Name | Symbol |
| Length | Meter | m |
| Mass | Kilogram | Kg |
| Time | Second | s |
| Electric current | ampere | A |
| Thermodynamic temperature | Kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Measurement
It refers to the assigning numbers to characteristics of an object or event that can be compared with other objects or events. It is very important in the field of science, technology and quantitative research. Measurements generally use the International System of Units for comparison.
Measurement of Length
There are many devices used for measuring length of an object or event. For example, vernier calliper used to measure the length to an accuracy of 10-4 m. A screw gauge and spherometer is used to measure the length as less than 10-5 m. If we want to measure the length of a large distance for example, distance between the planets and earth. This can be done by parallax method.
Measurement of Mass
It is a basic property of matter. The SI Unit of mass is kilogram. But while dealing with atoms and molecules, the kilogram unit will not be convenient. In this case, the unified atomic mass unit (u) has been established for expressing the mass of atoms. The mass of the objects ranges from planets, stars to atoms and molecules.
Measurement of Time
Time is usually measured by clock but now the atomic standard of time is used. This is based on the periodic vibrations produced in cesium atoms. It is sometimes called an atomic clock.
Measurement is important in all experiments in science, medicines, research etc. There can also be some uncertainty in measuring instruments or methods. These uncertainties are called errors. But it is also important to understand the accuracy and precision in measurement. Accuracy means the how close is the measure value to the true value. Precision means to what limit the quantity is measured. There are two types of errors: systematic and random errors.
Systematic errors: tend to be in one direction either positive or negative. These are categorised as instrumental errors, imperfection in experimental procedure and personal errors.
Random errors: tend to occur irregularly with respect to size and sign.
Significant figures
Sometimes all measurement involves errors. The result should be reported in such a way that it indicates the precision of measurement. The number should include all the digits in the number that are known reliably and the first digit that is uncertain. The reliable digits and the first uncertain digit is known as significant figures.
Summary
Units and Measurements is a very important chapter in Physics for 11th standard students of CBSE Board in India. This article gives an overview of different types of units and measurement for length, time and mass.