5 · IGCSE Physics
Gas pressure and temperature
Heat or cool a fixed amount of gas in a sealed, rigid container and watch the molecules speed up or slow down. Record pressures and extend the straight line back to find absolute zero.
T (K) = θ (°C) + 273
p ∝ T, so p₁ / T₁ = p₂ / T₂
v_rms = √(3kT / m)
p ∝ T, so p₁ / T₁ = p₂ / T₂
v_rms = √(3kT / m)
- Temperature:
- rms molecular speed:
- Wall hits on screen: measuring…
- Pressure p:
- p / T:
Model: nitrogen (m = 4.65 × 10⁻²⁶ kg), volume fixed at 1.00 litre, p = 101 kPa at 273 K.
Try this
- Record the pressure at five temperatures between −200 °C and 300 °C. Where does the dashed line through your points reach p = 0?
- Switch the graph to kelvin. Go from −123 °C (150 K) to 27 °C (300 K): does the pressure double? By what factor does v_rms change?
- Watch the wall-hit count as you heat the gas. It rises, but more slowly than the pressure — why? (Think about the momentum change in each hit.)
Common error: putting °C values into p₁ / T₁ = p₂ / T₂ — always add 273 to convert to kelvin first, or the answer will be wrong.