Termodynamika a molekulová fyzika
What you must know
This domain covers the ideal gas law (pV = nRT or pV = NkT), the first law of thermodynamics (ΔU = Q − W, with W done by the gas), and heat quantities Q = mcΔT and latent heat Q = mL. Understand internal energy of an ideal gas depends only on temperature, and that isothermal, isobaric, isochoric and adiabatic processes each hold one quantity fixed. Molecular physics adds the kinetic interpretation of temperature and the meaning of absolute zero.
Traps
- Sign of work: if the gas is compressed, work is done on the gas, so W (by the gas) is negative.
- Using Celsius in the gas law; T must be in kelvin.
- Confusing heat capacity c (per kg) with molar heat capacity C.
- Forgetting latent heat during a phase change (temperature stays constant).
- Mixing up isothermal (T constant) with adiabatic (Q = 0).
Worked example
2 mol of ideal gas is heated at constant pressure from 300 K to 400 K. Find the work done by the gas. At constant pressure W = pΔV = nRΔT = 2 × 8.31 × 100 ≈ 1662 J. The heat supplied would be Q = nCₚΔT, larger than W because some energy raises internal energy - a classic follow-up question.
Rule of thumb: identify which of p, V, T, Q, W is held fixed, write the first law, then substitute. Always convert to kelvin and check the sign of W.