Question

What are the 7 main SI units?

Answer 1

The International System of Units (SI) is based on seven fundamental units from which all other units are derived. These seven units are known as the **base units** and each corresponds to a specific physical quantity. Here are the seven main SI units:

1. **Meter (m)** — Unit of **Length**  
   The meter is defined as the distance light travels in a vacuum in 1/299,792,458 of a second.

2. **Kilogram (kg)** — Unit of **Mass**  
   The kilogram is the only SI base unit with a prefix ("kilo-"). It is defined by fixing the Planck constant (h) to exactly 6.62607015 × 10⁻³⁴ joule-seconds (J·s).

3. **Second (s)** — Unit of **Time**  
   The second is defined by the frequency of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. One second is exactly 9,192,631,770 cycles of this radiation.

4. **Ampere (A)** — Unit of **Electric Current**  
   The ampere is defined by taking the elementary charge (e) as 1.602176634 × 10⁻¹⁹ coulombs, meaning an ampere corresponds to the flow of one coulomb of charge per second.

5. **Kelvin (K)** — Unit of **Temperature**  
   The kelvin is defined by fixing the Boltzmann constant (k) to exactly 1.380649 × 10⁻²³ joules per kelvin (J/K). It relates temperature to the kinetic energy of particles.

6. **Mole (mol)** — Unit of **Amount of Substance**  
   The mole is defined as exactly 6.02214076 × 10²³ elementary entities (Avogadro's number). This is the number of atoms in 12 grams of carbon-12.

7. **Candela (cd)** — Unit of **Luminous Intensity**  
   The candela is defined by fixing the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz to 683 lumens per watt.

These seven SI units form the foundation for all other physical measurements in science and engineering.

Answer 2

The International System of Units (SI) is built on seven base units, each of which defines a fundamental physical quantity. Here are the seven main SI units:

1. **Meter (m)** - The unit of length. One meter is defined as the distance traveled by light in a vacuum in 1/299,792,458 seconds.

2. **Kilogram (kg)** - The unit of mass. One kilogram is defined as the mass of the international prototype of the kilogram, a platinum-iridium cylinder kept at the International Bureau of Weights and Measures (BIPM).

3. **Second (s)** - The unit of time. One second is defined as 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 cesium-133 atom.

4. **Ampere (A)** - The unit of electric current. One ampere is defined as the constant current which, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section, and placed one meter apart in a vacuum, would produce a force between them equal to 2 × 10^−7 newton per meter of length.

5. **Kelvin (K)** - The unit of thermodynamic temperature. One kelvin is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

6. **Mole (mol)** - The unit of amount of substance. One mole is defined as the amount of substance that contains as many elementary entities (such as atoms, molecules, ions) as there are atoms in 0.012 kilograms of carbon-12.

7. **Candela (cd)** - The unit of luminous intensity. One candela is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10^12 hertz and has a radiant intensity in that direction of 1/683 watt per steradian.

These base units provide a standardized system for measuring and expressing physical quantities across various fields of science and engineering.