Lessons 24–25 · 2 lessons · P. Habibullayev, A. Boydedayev, A. Bahromov, K. Suyarov, J. Usarov, M. Yuldasheva. Physics Grade 9, revised and expanded 3rd edition. G‘afur G‘ulom Publishing House, Tashkent, 2019
25
Irreversibility of thermal processes. The second law of thermodynamics
Textbook: p. 72
GoalDistinguish reversible and irreversible processes; state the second law of thermodynamics (Clausius).
New words
reversible process · qaytar jarayonirreversible process · qaytmas jarayonsecond law of thermodynamics · termodinamikaning ikkinchi qonunidirection of heat transfer · issiqlik uzatish yo‘nalishi
Explanation
A reversible process is one in which the system goes first one way and then back to its initial state with no change left in the surroundings. The oscillation of a frictionless pendulum or of a load on a spring is close to reversible. All heat processes are irreversible: a hot body gives heat to its surroundings, but heat does not pass spontaneously from a cold body to a hot one; gas in a vessel connected to an empty vessel spreads between them and does not gather back by itself; a bullet that hits a barrier warms up, but the warm bullet and barrier never set the bullet moving again by themselves. The German scientist R. Clausius generalised this in the second law of thermodynamics: heat cannot pass from a colder system to a hotter one without some change in both systems or in surrounding bodies. The first law only says energy is conserved; the second shows the direction of processes.
Worked examples
Classify: (a) a frictionless pendulum – close to reversible; (b) ice melting in warm tea – irreversible; (c) a glass breaking – irreversible; (d) coffee cooling in a cold room – irreversible.
A refrigerator moves heat from its cold interior to the warm room, but not spontaneously: a compressor does work using electricity and the surrounding air is warmed. So it does not contradict Clausius’s statement.
Class activity
Observation: put a drop of food colouring (or tea) into a glass of warm water and watch it spread. Does the colouring gather back in one place by itself? Which law does this illustrate? Use only food colouring or tea; an adult prepares the warm water.
Practice
1
Define a reversible process and give an example.
After the process runs backwards and the system returns to its initial state, no change remains in the surroundings; the oscillation of a frictionless pendulum.
2
Can heat pass spontaneously from a 20 °C body to a 60 °C body?
No, heat passes spontaneously only from hot to cold (second law).
3
Which is irreversible: oscillation of a load on a spring (no friction) or the cooling of hot tea?
The cooling of hot tea.
4
Does a refrigerator violate the second law? Why?
No: heat is transferred not spontaneously but at the expense of the compressor’s work, and the surroundings change.