In physics, there are several fundamental quantities that we measure. For each of these quantities, there is a standard unit used universally.
1. Length (or Distance)
- Definition: The measure of how far apart two points are in space.
- SI Unit: Meter (m)
- Common Prefixes:
- Kilometer (km) = 103 m10^3 \, \text{m}103m
- Centimeter (cm) = 10−2 m10^{-2} \, \text{m}10−2m
- Millimeter (mm) = 10−3 m10^{-3} \, \text{m}10−3m
2. Mass
- Definition: A measure of the amount of matter in an object. It is often confused with weight, but mass does not change with location (unlike weight, which depends on gravity).
- SI Unit: Kilogram (kg)
- Other Units:
- Gram (g) = 10−3 kg10^{-3} \, \text{kg}10−3kg
- Milligram (mg) = 10−6 kg10^{-6} \, \text{kg}10−6kg
3. Time
- Definition: The measure of the duration of an event or the interval between two events.
- SI Unit: Second (s)
- Common Prefixes:
- Minute (min) = 60 seconds
- Hour (h) = 60 minutes = 3600 seconds
4. Temperature
- Definition: A measure of the average kinetic energy of particles in a substance, which determines how hot or cold the substance is.
- SI Unit: Kelvin (K)
- Other Units:
- Celsius (°C): Commonly used in everyday life (related to Kelvin by T(K)=T(°C)+273.15T(K) = T(°C) + 273.15T(K)=T(°C)+273.15).
- Fahrenheit (°F): Mainly used in the United States.
5. Electric Current
- Definition: The flow of electric charge (usually electrons).
- SI Unit: Ampere (A), often shortened to Amp
- Other Units: Coulomb per second (C/s)
6. Amount of Substance (or Quantity of Matter)
- Definition: The measure of the number of entities (atoms, molecules, ions) in a substance.
- SI Unit: Mole (mol)
- Note: One mole of any substance contains Avogadro’s number (6.022×10236.022 \times 10^{23}6.022×1023) of entities.
7. Luminous Intensity
- Definition: The measure of the perceived power of light.
- SI Unit: Candela (cd)
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