What are the 7 main SI units?
2 Answers
The seven main SI (International System of Units) base units are:
1. **Meter (m)** - Unit of length.
2. **Kilogram (kg)** - Unit of mass.
3. **Second (s)** - Unit of time.
4. **Ampere (A)** - Unit of electric current.
5. **Kelvin (K)** - Unit of thermodynamic temperature.
6. **Mole (mol)** - Unit of amount of substance.
7. **Candela (cd)** - Unit of luminous intensity.
These units serve as the foundation for all other derived units in the SI system.
1. **Meter (m)** - Unit of length.
2. **Kilogram (kg)** - Unit of mass.
3. **Second (s)** - Unit of time.
4. **Ampere (A)** - Unit of electric current.
5. **Kelvin (K)** - Unit of thermodynamic temperature.
6. **Mole (mol)** - Unit of amount of substance.
7. **Candela (cd)** - Unit of luminous intensity.
These units serve as the foundation for all other derived units in the SI system.
The International System of Units (SI) is the modern form of the metric system and is the most widely used system of measurement. It is built around seven base units, each of which represents a fundamental physical quantity. These seven main SI units are:
### 1. **Meter (m) - Length**
- **Definition:** The meter is the SI unit of length. It is defined by the distance light travels in a vacuum in 1/299,792,458 of a second.
- **Usage:** It's used to measure distance or length, such as the height of a building, the length of a room, or the distance between two cities.
### 2. **Kilogram (kg) - Mass**
- **Definition:** The kilogram is the SI unit of mass. It was originally defined as the mass of a specific platinum-iridium alloy cylinder kept in France. However, since 2019, it has been defined based on the Planck constant, a fundamental constant of nature.
- **Usage:** It's used to measure mass, such as the weight of a person, the mass of an object, or the amount of a substance.
### 3. **Second (s) - Time**
- **Definition:** The second is the SI unit of time. It is defined by the duration of 9,192,631,770 cycles of the radiation corresponding to the transition between two energy levels of the cesium-133 atom.
- **Usage:** It measures time intervals, such as the duration of an event, the time of day, or the period of a cycle.
### 4. **Ampere (A) - Electric Current**
- **Definition:** The ampere is the SI unit of electric current. It is defined by the amount of charge passing through a conductor in one second when one coulomb of charge passes through it.
- **Usage:** It measures the flow of electric current, such as the current flowing in a wire or an electrical circuit.
### 5. **Kelvin (K) - Thermodynamic Temperature**
- **Definition:** The kelvin is the SI unit of thermodynamic temperature. It is defined by the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (the point where water coexists in solid, liquid, and gaseous states).
- **Usage:** It measures temperature, especially in scientific contexts, such as the temperature of gases, the freezing and boiling points of substances, or the temperature in outer space.
### 6. **Mole (mol) - Amount of Substance**
- **Definition:** The mole is the SI unit for the amount of substance. It is defined by the number of atoms in 12 grams of carbon-12, which is approximately 6.022 x 10²³ entities (Avogadro's number).
- **Usage:** It measures the quantity of a substance, particularly in chemistry, such as the number of molecules in a given volume of gas or the amount of a chemical compound in a reaction.
### 7. **Candela (cd) - Luminous Intensity**
- **Definition:** The candela is the SI unit of luminous intensity. It is defined by the light emitted by a source in a particular direction, specifically 1/683 of a watt per steradian at a wavelength of 555 nanometers.
- **Usage:** It measures the intensity of light, such as the brightness of a light bulb or the intensity of light sources in photography or lighting design.
These seven base units form the foundation of the SI system, from which all other units of measurement (called derived units) are constructed. Understanding these units is essential for scientific measurements, engineering, and many everyday applications.
### 1. **Meter (m) - Length**
- **Definition:** The meter is the SI unit of length. It is defined by the distance light travels in a vacuum in 1/299,792,458 of a second.
- **Usage:** It's used to measure distance or length, such as the height of a building, the length of a room, or the distance between two cities.
### 2. **Kilogram (kg) - Mass**
- **Definition:** The kilogram is the SI unit of mass. It was originally defined as the mass of a specific platinum-iridium alloy cylinder kept in France. However, since 2019, it has been defined based on the Planck constant, a fundamental constant of nature.
- **Usage:** It's used to measure mass, such as the weight of a person, the mass of an object, or the amount of a substance.
### 3. **Second (s) - Time**
- **Definition:** The second is the SI unit of time. It is defined by the duration of 9,192,631,770 cycles of the radiation corresponding to the transition between two energy levels of the cesium-133 atom.
- **Usage:** It measures time intervals, such as the duration of an event, the time of day, or the period of a cycle.
### 4. **Ampere (A) - Electric Current**
- **Definition:** The ampere is the SI unit of electric current. It is defined by the amount of charge passing through a conductor in one second when one coulomb of charge passes through it.
- **Usage:** It measures the flow of electric current, such as the current flowing in a wire or an electrical circuit.
### 5. **Kelvin (K) - Thermodynamic Temperature**
- **Definition:** The kelvin is the SI unit of thermodynamic temperature. It is defined by the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (the point where water coexists in solid, liquid, and gaseous states).
- **Usage:** It measures temperature, especially in scientific contexts, such as the temperature of gases, the freezing and boiling points of substances, or the temperature in outer space.
### 6. **Mole (mol) - Amount of Substance**
- **Definition:** The mole is the SI unit for the amount of substance. It is defined by the number of atoms in 12 grams of carbon-12, which is approximately 6.022 x 10²³ entities (Avogadro's number).
- **Usage:** It measures the quantity of a substance, particularly in chemistry, such as the number of molecules in a given volume of gas or the amount of a chemical compound in a reaction.
### 7. **Candela (cd) - Luminous Intensity**
- **Definition:** The candela is the SI unit of luminous intensity. It is defined by the light emitted by a source in a particular direction, specifically 1/683 of a watt per steradian at a wavelength of 555 nanometers.
- **Usage:** It measures the intensity of light, such as the brightness of a light bulb or the intensity of light sources in photography or lighting design.
These seven base units form the foundation of the SI system, from which all other units of measurement (called derived units) are constructed. Understanding these units is essential for scientific measurements, engineering, and many everyday applications.