Every physical quantity is a number multiplied by a unit. The SI system (Système International) is built on a small set of base quantities, each with its own base unit. All other units are made from these.The six base quantities and units you need for AQA are:• mass: kilogram (kg)
• length: metre (m)
• time: second (s)
• amount of substance: mole (mol)
• temperature: kelvin (K)
• electric current: ampere (A)(There is a seventh SI base unit, the candela for luminous intensity, but it is not used in this course.)Notice that the kilogram is the only base unit that already contains a prefix (kilo). Also, current, not charge, is the base quantity: charge is derived from current and time.Any quantity that is not in this list is a derived quantity. For example, speed is not a base quantity, because its unit m s−1 is made by combining the base units of length and time.Temperature in calculations must be in the base unit, kelvin. To convert from degrees Celsius:T (K) = θ (°C) + 273 Worked example: a gas is at 27 °C. In kelvin, T = 27 + 273 = 300 K.Using base units consistently means equations work without extra conversion factors, and lets you check an equation by comparing the base units on each side.
• length: metre (m)
• time: second (s)
• amount of substance: mole (mol)
• temperature: kelvin (K)
• electric current: ampere (A)(There is a seventh SI base unit, the candela for luminous intensity, but it is not used in this course.)Notice that the kilogram is the only base unit that already contains a prefix (kilo). Also, current, not charge, is the base quantity: charge is derived from current and time.Any quantity that is not in this list is a derived quantity. For example, speed is not a base quantity, because its unit m s−1 is made by combining the base units of length and time.Temperature in calculations must be in the base unit, kelvin. To convert from degrees Celsius: