Physics · Class 11

Audit the direction of heat flow

Conservation of energy alone permits either direction of heat transfer. The second law selects the spontaneous direction by requiring nondecreasing total entropy. Heat flows from hot to cold without external work.

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Audit the direction of heat flow

வெப்பம் இயல்பாக சூடான இடத்திலிருந்து குளிர்ந்த இடத்திற்குச் செல்கிறது. தனிமைப்படுத்தப்பட்ட அமைப்பின் மொத்த என்ட்ரோபி குறையாது.

Key notes, relationships and model limits

Key relationship

ΔStotal=−Q/Th+Q/Tc; spontaneous isolated heat transfer requires ΔStotal≥0.

Remember

Common trap

The first law alone does not determine whether a process can occur spontaneously.

Model limits

Two reservoirs at constant temperatures, exchanging only the selected heat. Negative Q means cold-to-hot transfer without external work; no refrigerator is included.

What you will learn

Use entropy accounting to assess a proposed heat-transfer direction.

Try the experiment

Try both scenarios, change one control, and predict the result before checking the graph and readouts.

Think of it this way

Change one input while keeping the others fixed, as you would in a controlled experiment.

Worked example

Reversing the same isolated transfer gives −0.5 J/K, which is forbidden. A refrigerator can transfer heat that way only with work and other energy exchanges.

Why this matters

The model connects audit the direction of heat flow with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

Transfer 300 J from 600 K to 300 K. What is total entropy change?

−0.5 J/K
+0.5 J/K
+1.5 J/K

Read the explanation

−300/600+300/300=−0.5+1=+0.5 J/K.

Course connection: Class XI Chapter 11. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.