A7LAM CSCA ACADEMYCSCA Academy / Physics / Heat
CSCA Physics · Lesson 11 of 16Gases and kinetic theory
Gaz et théorie cinétique · الغازات والنظرية الحركية للجزيئات
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A gas is a huge number of molecules in random motion. Temperature measures their average kinetic energy, and pressure comes from their impacts on the walls.
Key ideas
- Molecules move randomly (Brownian motion is the evidence) and attract each other weakly.
- Temperature measures the average kinetic energy of the molecules: T(K) = t(°C) + 273.
- Ideal gas: pV = nRT, or p₁V₁/T₁ = p₂V₂/T₂ for a fixed amount of gas.
- Constant T: pV is constant. Constant V: p/T is constant. Constant p: V/T is constant.
Formulas to know
- pV = nRT, R = 8.31 J/(mol·K)
- p₁V₁/T₁ = p₂V₂/T₂
- T = t + 273 K
Worked example
A sealed rigid tank at 300 K has p = 1 × 10⁵ Pa. It is heated to 600 K. New pressure?
- Constant V: p₁/T₁ = p₂/T₂
- Replace: p₂ = 1×10⁵ × 600 / 300
- Compute: p₂ = 2 × 10⁵ Pa
Answer: 2 × 10⁵ Pa
Practice questions
1. 27 °C in kelvin?
246 K · 300 K ✓ · 27 K · 327 K
27 + 273 = 300 K.
2. At constant temperature, a gas is squeezed to half its volume. Its pressure:
halves · doubles ✓ · stays the same · is multiplied by 4
pV is constant.
3. A hotter gas means:
bigger molecules · faster molecules on average ✓ · more molecules · lower pressure
Temperature measures average kinetic energy.