Physical quantity
Physical Quantities & Units - OCR A-Level Physics
Key Definition
Physical quantity
A property of a material or system that can be measured. It consists of a numerical magnitude and a unit.
A property of a material or system that can be measured. It consists of a numerical magnitude and a unit.
- All physical quantities are either base quantitiesThe seven fundamental physical quantities from which all others are derived: mass, length, time, current, temperature, amount of substance, luminous intensity. or derived quantitiesPhysical quantities defined in terms of the base quantities through equations, e.g. velocity (m/s), force (N = kg m s⁻²)..
- There are seven SI base quantities, but OCR requires you to know six.
- Every measurement in physics can be traced back to combinations of these base units.
Key Definition
SI base units
The fundamental units from which all other units are derived. They are defined by international agreement and are independent of each other.
The fundamental units from which all other units are derived. They are defined by international agreement and are independent of each other.
- Mass: kilogram (kg)
- Length: metre (m)
- Time: second (s)
- Electric currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).: ampereThe SI unit of current. One ampere is a flow of one coulomb of charge per second. (A)
- TemperatureA measure of the average kinetic energy of the particles in a substance. An SI base quantity measured in kelvin (K).: kelvin (K)
- Amount of substanceA measure of the number of entities (atoms, molecules, ions) in a sample. SI unit: mole (mol).: mole (mol)
Examiner Tips and Tricks
- The kilogram is the only SI base unit with a prefix (kilo) built into its name.
- When converting, remember that the base unit is kg, not g.
- This trips students up in $F = ma calculations$.