Not given in the exam, recall it.
What it is for
Charles'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 |
|---|---|---|
| V₁ | initial volume | m³ |
| T₁ | initial absolute temperature | K |
| V₂ | final volume | m³ |
| T₂ | final absolute temperature | K |
Rearrangements
V₂ = V₁T₂ / T₁T₂ = V₂T₁ / V₁
Worked example
Common trap
Understanding Charles's law
Charles's law V₁ / T₁ = V₂ / T₂ applies to a fixed mass of gas at constant pressure. V is volume, which may be in m³, litres or cm³, and T is the absolute temperature in kelvin (K). The subscripts 1 and 2 mark the initial and final states.
The law says volume is directly proportional to absolute temperature: heat a gas and it expands in step with its kelvin temperature. This is because hotter particles move faster and push the walls out to keep pressure steady.
To find an unknown, rearrange to V₂ = V₁T₂ / T₁ or T₂ = V₂T₁ / V₁. Temperature must be in kelvin, so convert any Celsius value first.
A worked example, step by step
A gas of volume V₁ = 400 cm³ is at θ₁ = 27 °C. It is heated at constant pressure to θ₂ = 177 °C. Find the new volume V₂.
Convert to kelvin first: T₁ = 27 + 273 = 300 K, T₂ = 177 + 273 = 450 K.
Write the rearranged formula: V₂ = V₁T₂ / T₁. Substitute with units: V₂ = (400 cm³ × 450 K) / 300 K.
Top line: 400 × 450 = 180000. Divide by 300: 180000 / 300 = 600.
So V₂ = 600 cm³. The kelvin units cancel, leaving cm³.
The gas expands to 600 cm³, quoted to three significant figures.
Common mistakes and how this is tested
The biggest trap is leaving temperature in Celsius. If you used 27 and 177 directly the ratio would be wrong; you must convert to 300 K and 450 K first.
The law also holds only at constant pressure for a fixed mass of gas.
Other errors: assuming volume doubles when Celsius temperature doubles (it does not); mixing volume units across the equation; and forgetting that a gas contracts when cooled, so V₂ can be smaller than V₁.
Under calculate, show the kelvin conversion and the substitution with units. Explain may ask why a balloon expands in the sun, using particle motion.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)