What are the 7 basic SI units?
2 Answers
The International System of Units (SI) is a comprehensive system used globally for scientific and everyday measurements. It is based on seven fundamental physical quantities, each with its own base unit. Here are the seven basic SI units:
1. **Meter (m)**: This is the SI unit of length. The meter is defined as the distance light travels in a vacuum in 1/299,792,458 seconds. It is used to measure distance or length in various contexts.
2. **Kilogram (kg)**: The kilogram is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant, h, to be 6.62607015 × 10^-34 joule seconds, when expressed in kilograms, meters, and seconds. Previously, it was defined by the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder.
3. **Second (s)**: The second is the SI unit of time. It 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. This definition ensures high precision in timekeeping.
4. **Ampere (A)**: The ampere is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge, e, to be 1.602176634 × 10^-19 coulombs, where the unit of current is derived from the flow of this charge.
5. **Kelvin (K)**: Kelvin is the SI unit of temperature. It is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. This scale is absolute, starting from absolute zero, where all molecular motion ceases.
6. **Mole (mol)**: The mole is the SI unit of amount of substance. It is defined by taking the fixed numerical value of the Avogadro constant, N_A, to be 6.02214076 × 10^23 entities (such as atoms or molecules) per mole. This unit allows for the measurement of quantities in chemical reactions.
7. **Candela (cd)**: The candela is the SI unit of luminous intensity. It is defined as the intensity of a source emitting monochromatic radiation of frequency 540 × 10^12 hertz and having a radiant intensity of 1/683 watt per steradian. This unit measures the perceived brightness of light.
These units form the foundation of the SI system, ensuring consistency and clarity in measurements across various scientific disciplines and everyday life.
1. **Meter (m)**: This is the SI unit of length. The meter is defined as the distance light travels in a vacuum in 1/299,792,458 seconds. It is used to measure distance or length in various contexts.
2. **Kilogram (kg)**: The kilogram is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant, h, to be 6.62607015 × 10^-34 joule seconds, when expressed in kilograms, meters, and seconds. Previously, it was defined by the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder.
3. **Second (s)**: The second is the SI unit of time. It 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. This definition ensures high precision in timekeeping.
4. **Ampere (A)**: The ampere is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge, e, to be 1.602176634 × 10^-19 coulombs, where the unit of current is derived from the flow of this charge.
5. **Kelvin (K)**: Kelvin is the SI unit of temperature. It is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. This scale is absolute, starting from absolute zero, where all molecular motion ceases.
6. **Mole (mol)**: The mole is the SI unit of amount of substance. It is defined by taking the fixed numerical value of the Avogadro constant, N_A, to be 6.02214076 × 10^23 entities (such as atoms or molecules) per mole. This unit allows for the measurement of quantities in chemical reactions.
7. **Candela (cd)**: The candela is the SI unit of luminous intensity. It is defined as the intensity of a source emitting monochromatic radiation of frequency 540 × 10^12 hertz and having a radiant intensity of 1/683 watt per steradian. This unit measures the perceived brightness of light.
These units form the foundation of the SI system, ensuring consistency and clarity in measurements across various scientific disciplines and everyday life.
The International System of Units (SI) is based on seven fundamental units, which are considered the building blocks for all other measurements in science and engineering. Each of these units corresponds to a fundamental physical quantity. Here's an overview of the 7 basic SI units:
### 1. **Meter (m) – Unit of Length**
- **Symbol**: \( \text{m} \)
- **Quantity**: Length
- **Definition**: The meter is the distance light travels in a vacuum in \( 1/299,792,458 \) of a second. It defines spatial dimensions like length, width, and height.
### 2. **Kilogram (kg) – Unit of Mass**
- **Symbol**: \( \text{kg} \)
- **Quantity**: Mass
- **Definition**: The kilogram is defined by the Planck constant \( h \), where \( h = 6.62607015 \times 10^{-34} \) joule-seconds. The mass of a kilogram is linked to this constant, which is fundamental to quantum mechanics.
### 3. **Second (s) – Unit of Time**
- **Symbol**: \( \text{s} \)
- **Quantity**: Time
- **Definition**: The second is defined by 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) – Unit of Electric Current**
- **Symbol**: \( \text{A} \)
- **Quantity**: Electric Current
- **Definition**: The ampere is defined by the elementary charge \( e \), where \( e = 1.602176634 \times 10^{-19} \) coulombs. One ampere represents the current produced when one coulomb of charge passes through a point in a circuit per second.
### 5. **Kelvin (K) – Unit of Temperature**
- **Symbol**: \( \text{K} \)
- **Quantity**: Thermodynamic Temperature
- **Definition**: The kelvin is defined by fixing the Boltzmann constant \( k = 1.380649 \times 10^{-23} \) joules per kelvin. This unit represents absolute temperature, where 0 K (absolute zero) is the lowest limit of temperature.
### 6. **Mole (mol) – Unit of Amount of Substance**
- **Symbol**: \( \text{mol} \)
- **Quantity**: Amount of Substance
- **Definition**: The mole is defined by the Avogadro constant, \( N_A = 6.02214076 \times 10^{23} \) entities per mole. A mole corresponds to this many atoms, molecules, or other particles.
### 7. **Candela (cd) – Unit of Luminous Intensity**
- **Symbol**: \( \text{cd} \)
- **Quantity**: Luminous Intensity
- **Definition**: The candela is defined by the luminous efficacy of monochromatic radiation at a frequency of \( 540 \times 10^{12} \) Hz. One candela represents the intensity of light in a given direction emitted by a source.
### Summary Table:
| **Physical Quantity** | **Unit** | **Symbol** |
|----------------------------|---------------|------------|
| Length | Meter | \( m \) |
| Mass | Kilogram | \( kg \) |
| Time | Second | \( s \) |
| Electric Current | Ampere | \( A \) |
| Temperature | Kelvin | \( K \) |
| Amount of Substance | Mole | \( mol \) |
| Luminous Intensity | Candela | \( cd \) |
These seven base units form the foundation for all other derived units used in science and engineering. Each is carefully defined in terms of physical constants or fundamental properties of nature to ensure precision and consistency worldwide.
### 1. **Meter (m) – Unit of Length**
- **Symbol**: \( \text{m} \)
- **Quantity**: Length
- **Definition**: The meter is the distance light travels in a vacuum in \( 1/299,792,458 \) of a second. It defines spatial dimensions like length, width, and height.
### 2. **Kilogram (kg) – Unit of Mass**
- **Symbol**: \( \text{kg} \)
- **Quantity**: Mass
- **Definition**: The kilogram is defined by the Planck constant \( h \), where \( h = 6.62607015 \times 10^{-34} \) joule-seconds. The mass of a kilogram is linked to this constant, which is fundamental to quantum mechanics.
### 3. **Second (s) – Unit of Time**
- **Symbol**: \( \text{s} \)
- **Quantity**: Time
- **Definition**: The second is defined by 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) – Unit of Electric Current**
- **Symbol**: \( \text{A} \)
- **Quantity**: Electric Current
- **Definition**: The ampere is defined by the elementary charge \( e \), where \( e = 1.602176634 \times 10^{-19} \) coulombs. One ampere represents the current produced when one coulomb of charge passes through a point in a circuit per second.
### 5. **Kelvin (K) – Unit of Temperature**
- **Symbol**: \( \text{K} \)
- **Quantity**: Thermodynamic Temperature
- **Definition**: The kelvin is defined by fixing the Boltzmann constant \( k = 1.380649 \times 10^{-23} \) joules per kelvin. This unit represents absolute temperature, where 0 K (absolute zero) is the lowest limit of temperature.
### 6. **Mole (mol) – Unit of Amount of Substance**
- **Symbol**: \( \text{mol} \)
- **Quantity**: Amount of Substance
- **Definition**: The mole is defined by the Avogadro constant, \( N_A = 6.02214076 \times 10^{23} \) entities per mole. A mole corresponds to this many atoms, molecules, or other particles.
### 7. **Candela (cd) – Unit of Luminous Intensity**
- **Symbol**: \( \text{cd} \)
- **Quantity**: Luminous Intensity
- **Definition**: The candela is defined by the luminous efficacy of monochromatic radiation at a frequency of \( 540 \times 10^{12} \) Hz. One candela represents the intensity of light in a given direction emitted by a source.
### Summary Table:
| **Physical Quantity** | **Unit** | **Symbol** |
|----------------------------|---------------|------------|
| Length | Meter | \( m \) |
| Mass | Kilogram | \( kg \) |
| Time | Second | \( s \) |
| Electric Current | Ampere | \( A \) |
| Temperature | Kelvin | \( K \) |
| Amount of Substance | Mole | \( mol \) |
| Luminous Intensity | Candela | \( cd \) |
These seven base units form the foundation for all other derived units used in science and engineering. Each is carefully defined in terms of physical constants or fundamental properties of nature to ensure precision and consistency worldwide.