A glass of water falls and breaks. Play the video backward and the pieces leap together and land back in your hand. It makes you laugh, because nature never does that. Thermodynamics takes exactly this “can go back, cannot go back” and splits it into two kinds of process.
1. A process that can go back
A system passes from one state to another by one of two kinds of process: reversible or irreversible. A process that can run backward, with the heat and work at every stage of the forward and backward runs equal and opposite, is a reversible process. Bangla books also call it a pratyagami process.
Heat ice at 0°C and it turns to water; take the same heat out and it turns back to ice. That is reversible. But a little heat is always lost to nature, so the book says a perfectly reversible process does not exist in nature.
At normal pressure and 273 K, ice melting absorbs exactly the heat that the same amount of water gives out when it freezes. That heat is 80 cal, or 336 J, per gram. So melting 5 g of ice takes 5 × 336 = 1680 J, and 5 g of water freezing gives out the same 1680 J.
2. What going back requires
In a reversible process the change happens very slowly, in extremely small steps. So at every step the system keeps thermodynamic equilibrium, which is why the process is called quasi-static. It also has none of the dissipative effects such as inelasticity, viscosity, friction, electrical resistance or magnetic hysteresis, so it is non-dissipative.
At the end, the system and its surroundings can both go back to their initial states with no net change in either.
So there are two conditions for a reversible process: (a) it must take place very slowly, and (b) no dissipated energy may be produced. A process that works against friction, viscosity or resistance is therefore not reversible.
Drop a ball on a steel plate, then run the film backward. What happens with no friction, and what happens with it?
rewindplay, then rewind
Can it get home?
The heights and the share lost as heat are model numbers.
The book calls these apparently reversible: a very slow isothermal or adiabatic change; ice and water at 0°C; an elastic ball dropped on an elastic steel plate, which rises back to its first height; and a spring stretched very slowly within its elastic limit.
3. What cannot go back
A process that, having gone forward, cannot run backward is an irreversible process. It is also called an irretraceable process.
A broken glass does not join up again. Here is the book's other example: join a full vessel and an empty one by a tube and the gas flows into the empty one. It does no external work, and it never comes back by itself. Open the tap and see.
follow the routeopen the tap
It cannot go back
One vessel is full of gas, the other is empty, and the tap between them is shut. What do you do first?
The tap is open. What does the gas do now?
The gas has spread through both vessels, and the external work is 0. Now ask: does it return by itself?
No, it does not return by itself. So what would returning take?
Returning needs outside work, and the process itself runs one way. So it is irreversible.
Not yet. Follow the order of events: tap, spreading, no return, work.
An irreversible process happens suddenly and spontaneously. Every natural process is spontaneous, so every natural process is irreversible. The system shows no tendency to return to its initial state. It is a fast process, and it does not keep thermodynamic equilibrium.
The book's examples: heat produced by a current through a resistance, heat produced by friction, heat flowing from a warm body to a cool one, and the explosion of gunpowder in a gun.
4. Sort them, then compare them
Matching examples comes up in almost every paper, and many students guess. For each case ask two questions: is it very slow, and is there waste such as friction or resistance?
sort themThink what friction does
What can be undone
Pick up a case, then drop it in a box. Ask: is it slow and free of waste, or fast and one-way?
Right. Next one.
Not yet. Is there friction, resistance or a sudden spreading? Then it cannot go back. Only a very slow, non-dissipative change can.
All in place. The reversible ones are slow and non-dissipative; the irreversible ones are spontaneous and one-way. An isothermal expansion is reversible only when it is very slow.
The book compares the two on four points: speed, spontaneity, returning to the initial state, and equilibrium. For each row say the answer yourself first, then drag the handle to check.
side by sideFind the match in each row
Two kinds of process
reversible
irreversible
5. Check yourself
The first question is in the exam's style.
check yourselfpick one answer
Question 1
An irreversible process:
i. is a fast process ii. is a spontaneous process iii. does not keep the system in thermodynamic equilibrium
i and ii are true, but the third is true as well, so iii cannot be left out.
i and iii are true, but spontaneous (ii) is true as well.
ii and iii are true, but fast (i) is true as well.
Right. An irreversible process is fast, spontaneous and does not keep equilibrium: all three are true.
Now the second, keeping the conditions and examples in mind.
check yourselfpick one answer
Question 2
Which condition is necessary for a process to be reversible?
A fast process is a feature of the irreversible kind; a reversible one must be very slow.
Right. These are the two conditions of a reversible process.
A spontaneous process is irreversible, so it cannot be a condition for reversibility.
A process against friction is not reversible.
The third is the last test.
check yourselfpick one answer
Question 3
About a reversible process:
i. It is a very slow process ii. The system keeps thermodynamic equilibrium iii. It is a spontaneous, one-way process
Right. The process is very slow (i) and equilibrium is kept (ii); iii is false.
iii is false: a spontaneous, one-way process is irreversible.
i is true so it cannot be left out; and iii is false.
Since iii is false, all three cannot be true.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionWhat is a reversible process?
AnswerOne that can run backward, with heat and work equal and opposite at every stage.
QuestionDoes a perfectly reversible process exist in nature, and why?
AnswerNo; a little heat is always lost.
QuestionWhat are the two conditions for a reversible process?
AnswerIt must be very slow and produce no dissipated energy (friction, viscosity, resistance).
QuestionWhat is an irreversible process?
AnswerOne that, having gone forward, cannot run backward.
QuestionWhy is every natural process irreversible?
AnswerBecause all natural processes occur spontaneously.
QuestionCompare reversible and irreversible processes on four points.
AnswerSlow/fast; not spontaneous/spontaneous; returns to initial state/does not return; equilibrium kept/not kept.
QuestionHow much heat melts 5 g of ice at 0°C? (336 J per gram)
Answer5 × 336 = 1680 J; freezing gives out the same heat.