Physics · Units And Measurements · NEET
There are exactly seven SI base units. They are: metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for thermodynamic temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity. For NEET, memorise both the quantity and its unit symbol, because questions often give you the unit and ask for the quantity, or the other way around.
Yes. Many students remember only metre, kilogram, second, ampere and kelvin, then forget the mole and the candela. Both are official SI base units. Mole (mol) measures the amount of substance (number of particles), and candela (cd) measures luminous intensity (brightness of a light source in a given direction). NEET has asked which unit belongs to which quantity, so leaving these two out is a common mistake.
By international agreement, the SI base unit of mass is the kilogram (kg), not the gram. This is the one base unit whose name already contains a prefix ('kilo'). The gram is treated as a smaller multiple of the base unit (1 g = 10^-3 kg). For NEET you should write mass in kilograms when you plug values into formulas so that answers come out in correct SI derived units like newton and joule.
A base unit is the unit of a base (fundamental) quantity that is defined on its own and does not depend on any other unit. There are seven of them. A derived unit is built by combining base units using multiplication or division. For example, speed = distance/time gives m/s, and force = mass x acceleration gives kg m s^-2, which we call the newton (N). So base units are the building blocks and derived units are the structures made from them.
No. The radian (for plane angle) and steradian (for solid angle) are supplementary units, not base units. They are dimensionless. So the count of base units stays at seven; the radian and steradian are a separate small group. NEET has asked about plane angle and solid angle having 'units but no dimensions', so keep these two apart from the seven base units.
The speed of light in vacuum is taken as unity. If light takes 6 min 40 s to reach the Earth from the Sun, the distance between the Sun and the Earth in the new unit is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Metre (m) - length, kilogram (kg) - mass, second (s) - time, ampere (A) - electric current, kelvin (K) - temperature, mole (mol) - amount of substance, candela (cd) - luminous intensity.
The ampere (symbol A) is the SI base unit of electric current. It is the only electrical quantity among the seven base units; charge (coulomb) is a derived unit equal to ampere x second (A s).
Yes. Thermodynamic temperature is a base quantity and its SI base unit is the kelvin (K). Note that we write 'kelvin', not 'degree kelvin', and the symbol has no degree sign.
Almost every dimensional-analysis, unit-conversion and derived-unit question in the Units and Measurements chapter starts from the seven base units. Knowing them lets you build derived units, check equations for correctness, and avoid trap options in the exam.
The mole (symbol mol). It measures how many elementary particles (atoms, molecules, ions) are present, based on the Avogadro number. It is one of the two base units students most often forget.
