PHY035 · Thermodynamics · Classes 11–12
Charles law
Objective: To verify Charles's law: at constant pressure the volume of a gas is proportional to its absolute temperature.
🧰 Apparatus
- Capillary tube with an air column trapped by a liquid index
- Water bath
- Thermometer
- Heater
- Scale
📐 Formulas and SI units
V₁/T₁ = V₂/T₂ (T in K)
T(K) = θ(°C) + 273.15
💡 Theory
Heating a gas makes its particles move faster; at constant pressure it must expand. V/T = constant when T is in kelvin. Extending the V–T graph backwards reaches zero volume at −273 °C, absolute zero.
🪜 Procedure
- Fix the capillary tube upright in the water bath with the open end up.
- Record the air-column length and the water temperature.
- Heat the bath in 10 °C steps, stirring well.
- Record the length at each temperature.
- Plot length against temperature (°C) and extend the line to find where the volume would be zero.
📋 Observations Simulated values
✅ Result and conclusion
1 L of gas at 300 K heated to 600 K at constant pressure doubles to 2 L.
🔬 In a real lab
The liquid index sticks a little and the bath is not uniform, so the extrapolated absolute zero is usually within ±10 °C.
⚠️ Safety
Hot water bath: use heat-proof gloves and keep the heater cable dry.
❓ Quick check
1. Charles's law needs temperature in…
2. Doubling the kelvin temperature makes the volume…
3. A balloon put in a freezer…
🏋️ Practice
Easy2 L at 300 K heated to 450 K. V?
Answer: 3 L
Medium27 °C in kelvin?
Answer: 300 K
ChallengeAbsolute zero in °C?
Answer: −273.15 °C