What are the 7 basic SI units?
2 Answers
The International System of Units (SI) defines seven base units, each corresponding to a fundamental physical quantity. Here’s a detailed overview of each unit, including the quantity it measures, its symbol, and its definition:
1. **Meter (m)** - **Length**
- **Definition**: The meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 seconds.
- **Usage**: It is used to measure distances, heights, and dimensions in various fields, including physics, engineering, and everyday measurements.
2. **Kilogram (kg)** - **Mass**
- **Definition**: The kilogram is defined by taking the fixed numerical value of the Planck constant \( h \) to be \( 6.62607015 \times 10^{-34} \) when expressed in the unit \( \text{m}^2 \cdot \text{kg} \cdot \text{s}^{-1} \).
- **Usage**: This unit is widely used to measure mass in scientific research, industry, and commerce.
3. **Second (s)** - **Time**
- **Definition**: The 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.
- **Usage**: This unit is used universally to measure time intervals, durations, and frequencies.
4. **Ampere (A)** - **Electric Current**
- **Definition**: The ampere is defined as the constant current that, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section, and placed 1 meter apart in vacuum, would produce a force equal to \( 2 \times 10^{-7} \) newtons per meter of length between these conductors.
- **Usage**: It measures electric current, a key parameter in electrical and electronic applications.
5. **Kelvin (K)** - **Thermodynamic Temperature**
- **Definition**: The kelvin is defined as the fraction \( \frac{1}{273.16} \) of the thermodynamic temperature of the triple point of water.
- **Usage**: This unit is crucial in fields like thermodynamics and physical chemistry for temperature measurement.
6. **Mole (mol)** - **Amount of Substance**
- **Definition**: The mole is defined as the amount of substance containing as many elementary entities (atoms, molecules, ions, etc.) as there are atoms in 0.012 kilograms of carbon-12. This is approximately \( 6.022 \times 10^{23} \) entities.
- **Usage**: It is fundamental in chemistry for quantifying amounts of substances.
7. **Candela (cd)** - **Luminous Intensity**
- **Definition**: The candela is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency \( 540 \times 10^{12} \) hertz and has a radiant intensity in that direction of \( \frac{1}{683} \) watt per steradian.
- **Usage**: This unit measures the brightness of light sources and is essential in fields like photometry and lighting design.
### Summary Table
| Quantity | Unit | 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 |
These seven base units form the foundation for the SI system, allowing for a coherent and consistent way to express measurements across various scientific disciplines.
1. **Meter (m)** - **Length**
- **Definition**: The meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 seconds.
- **Usage**: It is used to measure distances, heights, and dimensions in various fields, including physics, engineering, and everyday measurements.
2. **Kilogram (kg)** - **Mass**
- **Definition**: The kilogram is defined by taking the fixed numerical value of the Planck constant \( h \) to be \( 6.62607015 \times 10^{-34} \) when expressed in the unit \( \text{m}^2 \cdot \text{kg} \cdot \text{s}^{-1} \).
- **Usage**: This unit is widely used to measure mass in scientific research, industry, and commerce.
3. **Second (s)** - **Time**
- **Definition**: The 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.
- **Usage**: This unit is used universally to measure time intervals, durations, and frequencies.
4. **Ampere (A)** - **Electric Current**
- **Definition**: The ampere is defined as the constant current that, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section, and placed 1 meter apart in vacuum, would produce a force equal to \( 2 \times 10^{-7} \) newtons per meter of length between these conductors.
- **Usage**: It measures electric current, a key parameter in electrical and electronic applications.
5. **Kelvin (K)** - **Thermodynamic Temperature**
- **Definition**: The kelvin is defined as the fraction \( \frac{1}{273.16} \) of the thermodynamic temperature of the triple point of water.
- **Usage**: This unit is crucial in fields like thermodynamics and physical chemistry for temperature measurement.
6. **Mole (mol)** - **Amount of Substance**
- **Definition**: The mole is defined as the amount of substance containing as many elementary entities (atoms, molecules, ions, etc.) as there are atoms in 0.012 kilograms of carbon-12. This is approximately \( 6.022 \times 10^{23} \) entities.
- **Usage**: It is fundamental in chemistry for quantifying amounts of substances.
7. **Candela (cd)** - **Luminous Intensity**
- **Definition**: The candela is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency \( 540 \times 10^{12} \) hertz and has a radiant intensity in that direction of \( \frac{1}{683} \) watt per steradian.
- **Usage**: This unit measures the brightness of light sources and is essential in fields like photometry and lighting design.
### Summary Table
| Quantity | Unit | 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 |
These seven base units form the foundation for the SI system, allowing for a coherent and consistent way to express measurements across various scientific disciplines.
The International System of Units (SI) is a standardized system used for measurement across science and industry. It is based on seven base units, each representing a fundamental dimension of measurement. Here are the seven basic SI units:
1. **Meter (m)**: This is the SI unit for length. One meter is defined as the distance traveled by light in a vacuum in \( \frac{1}{299,792,458} \) seconds. The meter is used to measure dimensions such as height, width, and depth.
2. **Kilogram (kg)**: This is the SI unit for mass. Originally defined as the mass of a liter of water, the kilogram is now defined by the Planck constant, which is set to exactly \( 6.62607015 \times 10^{-34} \) kg·m²/s. It is used to measure the amount of matter in an object.
3. **Second (s)**: This is the SI unit for 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. The second measures time intervals.
4. **Ampere (A)**: This is the SI unit for electric current. It is defined by taking the fixed numerical value of the elementary charge (e) to be \( 1.602176634 \times 10^{-19} \) coulombs. The ampere measures the flow of electric charge.
5. **Kelvin (K)**: This is the SI unit for thermodynamic temperature. It is defined by setting the Boltzmann constant to exactly \( 1.380649 \times 10^{-23} \) J/K. The Kelvin scale starts at absolute zero, the point where no more thermal energy can be extracted from a system.
6. **Mole (mol)**: This is the SI unit for the amount of substance. It is defined as containing exactly \( 6.02214076 \times 10^{23} \) elementary entities (e.g., atoms, molecules). This number is known as Avogadro's number, and the mole measures the quantity of a substance.
7. **Candela (cd)**: This is the SI unit for luminous intensity. It is defined by setting the luminous efficacy of monochromatic radiation of frequency \( 540 \times 10^{12} \) Hz to exactly \( 683 \) lumens per watt. The candela measures the power emitted by a light source in a given direction.
These base units serve as the foundation for all other SI units, which are derived from them. For example, units like the newton (N) for force and the joule (J) for energy are derived from these base units.
1. **Meter (m)**: This is the SI unit for length. One meter is defined as the distance traveled by light in a vacuum in \( \frac{1}{299,792,458} \) seconds. The meter is used to measure dimensions such as height, width, and depth.
2. **Kilogram (kg)**: This is the SI unit for mass. Originally defined as the mass of a liter of water, the kilogram is now defined by the Planck constant, which is set to exactly \( 6.62607015 \times 10^{-34} \) kg·m²/s. It is used to measure the amount of matter in an object.
3. **Second (s)**: This is the SI unit for 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. The second measures time intervals.
4. **Ampere (A)**: This is the SI unit for electric current. It is defined by taking the fixed numerical value of the elementary charge (e) to be \( 1.602176634 \times 10^{-19} \) coulombs. The ampere measures the flow of electric charge.
5. **Kelvin (K)**: This is the SI unit for thermodynamic temperature. It is defined by setting the Boltzmann constant to exactly \( 1.380649 \times 10^{-23} \) J/K. The Kelvin scale starts at absolute zero, the point where no more thermal energy can be extracted from a system.
6. **Mole (mol)**: This is the SI unit for the amount of substance. It is defined as containing exactly \( 6.02214076 \times 10^{23} \) elementary entities (e.g., atoms, molecules). This number is known as Avogadro's number, and the mole measures the quantity of a substance.
7. **Candela (cd)**: This is the SI unit for luminous intensity. It is defined by setting the luminous efficacy of monochromatic radiation of frequency \( 540 \times 10^{12} \) Hz to exactly \( 683 \) lumens per watt. The candela measures the power emitted by a light source in a given direction.
These base units serve as the foundation for all other SI units, which are derived from them. For example, units like the newton (N) for force and the joule (J) for energy are derived from these base units.