Expressing Quantities & SI UnitsEdexcel GCSE Physics: Revision notes
Section 1
What are SI units?
Scientists use a standard set of units, called SI units, so that measurements can be compared consistently around the world. Every physical quantity has a recognised SI unit — for example, distance is measured in metres (m), mass in kilograms (kg), and time in seconds (s).
You must be able to recall and use the correct SI unit for each physical quantity covered in the specification (see Appendix 3), and always give the correct unit alongside a numerical answer.
Exam answers that give a correct number but the wrong (or no) unit will usually lose a mark — always state the unit.
Section 2
What are multiples and sub-multiples of units?
Prefixes are used with SI units to represent very large or very small quantities without writing lots of zeros.
| Prefix | Symbol | Multiplier |
|---|---|---|
| giga | G | ×10⁹ |
| mega | M | ×10⁶ |
| kilo | k | ×10³ |
| centi | c | ×10⁻² |
| milli | m | ×10⁻³ |
| micro | µ | ×10⁻⁶ |
| nano | n | ×10⁻⁹ |
For example, 1 kilometre (km) = 1000 m, and 1 millimetre (mm) = 0.001 m.
Mixing up milli (×10⁻³) and micro (×10⁻⁶) is a common error — always double-check which prefix is being used before converting.
Section 3
How do you convert between units?
To convert between units, multiply or divide by the appropriate conversion factor.
Converting hours to seconds: 1 hour = 60 minutes = 60 × 60 seconds = 3600 seconds
So to convert hours to seconds, multiply by 3600. To convert seconds back to hours, divide by 3600.
The same logic applies to any unit conversion: identify the relationship between the two units, then multiply or divide accordingly.
2.5 hours in seconds: 2.5 × 3600 = 9000 s.
Section 4
How should you express numbers in physics?
Physics answers should be given to an appropriate number of significant figures, usually matching the least precise data given in the question (often 2 or 3 significant figures).
Standard form is used to write very large or very small numbers concisely, as A × 10ⁿ, where 1 ≤ A < 10 and n is a (positive or negative) integer.
For example, the speed of light (300,000,000 m/s) is written in standard form as 3 × 10⁸ m/s.
Must Know
- Every physical quantity has a standard SI unit — always give the unit with a numerical answer
- Know the prefixes: giga (G, ×10⁹), mega (M, ×10⁶), kilo (k, ×10³), centi (c, ×10⁻²), milli (m, ×10⁻³), micro (µ, ×10⁻⁶), nano (n, ×10⁻⁹)
- 1 hour = 3600 seconds
- Use significant figures appropriate to the data given in the question
- Standard form: A × 10ⁿ, where 1 ≤ A < 10
That's the notes covered.
Carry on to the next subtopic.