Not given in the exam, recall it.
What it is for
Pressure 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 |
| T₁ | initial absolute temperature | K |
| p₂ | final pressure | Pa |
| T₂ | final absolute temperature | K |
Rearrangements
p₂ = p₁T₂ / T₁T₂ = p₂T₁ / p₁
Worked example
Common trap
Understanding the pressure law
The pressure law links the pressure and absolute temperature of a fixed mass of gas held at constant volume. p₁ is the initial pressure and p₂ the final pressure, both in pascals (Pa). T₁ and T₂ are the initial and final absolute temperatures in kelvin (K).
When a gas is heated at constant volume its molecules move faster and strike the walls harder and more often, so the pressure rises in direct proportion to the absolute temperature. This is why p/T stays constant, giving p₁/T₁ = p₂/T₂.
To make one quantity the subject, cross-multiply first. For the new pressure use p₂ = p₁T₂ / T₁; for the new temperature use T₂ = p₂T₁ / p₁.
Always convert Celsius to kelvin before substituting.
A worked example, step by step
A sealed steel can holds gas at p₁ = 120 kPa and T₁ = 27 °C. It is cooled to T₂ = −23 °C with the volume unchanged.
Find the new pressure.
First convert to kelvin: T₁ = 27 + 273 = 300 K and T₂ = −23 + 273 = 250 K.
Write the formula, then substitute with units:
p₂ = p₁T₂ / T₁ = (120 kPa × 250 K) / 300 K = 100 kPa.
The kelvin units cancel, leaving the answer in kilopascals: p₂ = 100 kPa (3 significant figures). Cooling lowers the pressure, which matches our expectation because pressure falls with absolute temperature at constant volume.
Common mistakes and how this is tested
The most common error is using Celsius instead of kelvin. The law only works with absolute temperature, so convert every temperature with K = °C + 273 before you divide.
Other frequent slips: forgetting the volume must stay constant (the law fails if the gas expands or is compressed); mixing up which pressure pairs with which temperature (p₁ goes with T₁); and rounding the kelvin conversion too early.
Under calculate you must show the formula, the kelvin substitution and the final unit. Under explain you may be asked why pressure rises with temperature at constant volume, so link it to faster molecules and harder, more frequent collisions.
Under define, state the proportional relationship in words.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)