The International System of Units, or SI, is largely derived from the metric system. Consequently, SI prefixes are based on powers of 10. Positive powers, such as 10^2 = 100, lead to larger quantities, while negative powers, such as 10^-2 = 0.01, lead to smaller quantities. For example, the prefix "milli-" means 0.001, and so 1 milliliter is equal to 0.001 liters and 1 liter is equal to 1,000 milliliters. The SI system consists of seven base units and the dozens of SI-derived units.
Length and Volume
The SI base unit of length is the meter (m). An SI-derived quantity, or one that can be derived solely from basic SI units, is volume. The standard SI unit of volume is the liter (L) for liquids and cubic meter (m^3) for solids. For quantifying small volumes, milliliters (mL) are often the unit of choice. In addition, 1 mL is equivalent to 1 cubic centimeter (1 cm^3).
Time and Temperature
The SI base unit of time is the second (s). For quantifying very short durations, milliseconds (ms) are often the unit of choice. Units of time are not typically used with the SI prefixes. Rather, 1 second is equivalent to 1/60 minutes or 1/3,600 hours. A related SI-derived quantity is velocity. The standard SI unit of velocity is m/s. The basic SI unit of temperature is the Kelvin (K). Like seconds, Kelvins are not typically used with the SI prefixes.
Substance Amount and Mass
The basic SI unit of substance amount is the mole (mol). One mole is defined as 6.02 x 10^23 units (atoms, molecules, etc.) of a substance. Another way to relate the amount of a substance is to describe its mass. The standard SI unit of mass is the kilogram (kg). For quantifying small volumes, grams (g) or milligrams (mg) are often the units of choice.
Electricity and Luminosity
The SI base unit of electrical current is the ampere (A). Related SI-derived quantities are voltage and resistance. The standard SI unit of voltage is the volt (V) and the standard SI unit of resistance is the Ohm (symbolized by the Greek letter omega). Luminosity of light is measured in candelas (cd).
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