Not given in the exam, recall it.
What it is for
Boyle's law is used in Heat (SPM Physics Form 4). Keep the SI units in every line of working. Heat
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| p₁ | initial pressure | Pa |
| V₁ | initial volume | m³ |
| p₂ | final pressure | Pa |
| V₂ | final volume | m³ |
Rearrangements
p₂ = p₁V₁ / V₂V₂ = p₁V₁ / p₂
Worked example
Common trap
Understanding Boyle's law
Boyle's law p₁V₁ = p₂V₂ applies to a fixed mass of gas at constant temperature. p is pressure and V is volume, with subscripts 1 and 2 for the initial and final states. Pressure may be in pascals (Pa) or kilopascals (kPa), and volume in m³, litres or cm³, as long as both sides match.
The law says pressure and volume are inversely proportional: squeeze a gas into half the volume and its pressure doubles. This happens because the particles hit the walls more often in a smaller space.
To find an unknown, rearrange to p₂ = p₁V₁ / V₂ or V₂ = p₁V₁ / p₂. Temperature is not in the formula because it is held constant.
A worked example, step by step
A gas at p₁ = 200 kPa occupies V₁ = 250 cm³. It expands to V₂ = 500 cm³ at constant temperature.
Find the new pressure p₂.
Write the rearranged formula: p₂ = p₁V₁ / V₂. Substitute with units: p₂ = (200 kPa × 250 cm³) / 500 cm³.
Top line: 200 × 250 = 50000. Divide by 500: 50000 / 500 = 100.
So p₂ = 100 kPa. The cm³ units cancel, leaving kPa.
Doubling the volume has halved the pressure, exactly as the inverse relationship predicts. The answer is 100 kPa, or 1.0 × 10² kPa to two significant figures.
Common mistakes and how this is tested
The key condition is that temperature and the mass of gas stay constant. If the temperature changes, use the general gas law instead.
Because it is a ratio, volume units need not be m³, but they must match on both sides.
Other errors: expecting pressure to rise when volume rises (it falls); mixing kPa with Pa across the equation; and swapping the initial and final labels so the ratio inverts.
Under calculate, show the substitution with matching units. Explain may ask why the pressure changes in terms of particle collisions, and a graph question may ask you to recognise a p against 1/V straight line.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)