Topic 3 introduces thermal physics: the relationship between temperature, energy, and the behaviour of gases at the molecular level. It connects macroscopic measurements (temperature, pressure) to microscopic behaviour (molecular kinetics).
Temperature measures average kinetic energy of molecules. Celsius and Kelvin scales: T(K) = T(°C) + 273. Thermal equilibrium: two objects in contact reach the same temperature. Internal energy = total random KE + PE of all molecules. Heating changes internal energy; temperature change depends on specific heat capacity.
Q = mcΔT: energy to change temperature. Specific heat capacity c (J·kg⁻¹·K⁻¹): water ≈ 4200, aluminium ≈ 900. During phase changes, temperature remains constant: Q = mL where L is specific latent heat (fusion for melting, vaporisation for boiling). Energy goes into breaking/forming intermolecular bonds, not increasing KE.
Ideal gas law: pV = nRT (R = 8.31 J·mol⁻¹·K⁻¹) or pV = NkT (k = 1.38 × 10⁻²³ J·K⁻¹). Assumptions: molecules are point particles, no intermolecular forces except during collisions, collisions are elastic, random motion. Real gases deviate at high pressure (volume matters) and low temperature (intermolecular forces matter).
Pressure arises from molecular collisions with container walls: p = ⅓ρ<c²>. Average KE per molecule: ½m<c²> = 3/2 kT. Root-mean-square speed: c_rms = √(3kT/m). Temperature is proportional to average molecular kinetic energy. This explains gas laws macroscopically through molecular behaviour.
Temperature measures the average kinetic energy of molecules — it tells you how hot something is. Heat (thermal energy transfer) is the energy that flows between objects due to a temperature difference. A large lake at 20°C contains far more total thermal energy than a hot spark at 1000°C, even though the spark has higher temperature. Heat always flows from higher to lower temperature until thermal equilibrium is reached.
Book a Trial + Diagnostic session. Get a personalized Learning Path with clear milestones, tutor match, and a plan recommendation — all within 24 hours.
Book Trial + Diagnostic →