What is a physical quantity?
A physical quantity is a property of an object that can be measured, such as length, mass, time or temperature. When we measure a quantity, we give a number and a unit. For example, a pencil may be 15 cm long. Here 15 is the number and cm is the unit. Without a unit, the number has no clear meaning.
Base quantities and SI units
Some quantities are called base quantities because all other quantities are built from them. Scientists everywhere use the same standard units called SI units so that results can be shared and compared clearly.
| Base quantity | SI unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | kelvin | K |
| Electric current | ampere | A |
Key idea
Every measurement = number + unit. Example: length = 15 cm, mass = 2 kg, time = 30 s.
Prefixes for large and small values
We use prefixes to write very large or very small values neatly. For example, 1 kilometre (km) = 1000 m, and 1 millimetre (mm) = 0.001 m. The prefix kilo means one thousand, while milli means one thousandth.
- kilo (k) = 1000 times the unit
- centi (c) = one hundredth of the unit
- milli (m) = one thousandth of the unit
Remember
- A physical quantity can be measured and needs a number and a unit.
- SI units are standard units used by scientists everywhere.
- Prefixes like kilo and milli make values easier to write.