NEB Class 12 • Physics • Thermodynamics • Practice Set

Second Law of Thermodynamics Practice Set: NEB 12 Physics

Master Second Law of Thermodynamics through physical meaning, diagrams, a reliable answer method and exam-focused practice.

ConceptsWorked reasoningNEB practice
Second Law of Thermodynamics concept diagramA wave, measurement markers and an energy arrow illustrate the central relationships.

Worked practice

Build the result from meaning, not memory

The second law fixes the natural direction of thermal processes: heat flows spontaneously from hotter to colder bodies, and no cyclic engine can convert all absorbed heat into work.

Worked example

An engine absorbs 1200 J and rejects 720 J. It produces 480 J, so η=0.40 or 40%. The rejected heat is essential, not wasted arithmetic.

The governing relationship is For an engine, W=Qh−Qc and efficiency η=W/Qh=1−Qc/Qh. A reversible Carnot limit is ηmax=1−Tc/Th with absolute temperatures. Check the convention, geometry or boundary condition before calculation.

Independent practice ladder

  1. Define the key physical quantities and give their SI units.
  2. Draw and label the relevant wave, ray, mode or energy diagram.
  3. Recalculate the worked example after changing one input by a factor of two.
  4. Explain the result without equations.
  5. Create a Nepal-relevant situation such as a classroom speaker, musical pipe, optical slit or thermal machine.
  6. Compare two cases and predict the larger output before calculating.
  7. Find and repair one deliberately wrong sign, node or unit.
  8. Write a three-mark answer and a five-mark answer.

Answer protocol: diagram → known data → principle → symbolic equation → SI substitution → result → physical check. Using Celsius in a Carnot ratio, claiming 100% efficiency, or confusing energy conservation with direction.

NEB answer method

Diagram → principle → equation → check

First identify the physical system and what the question asks. Label the diagram with SI quantities, then write the principle in words before selecting a formula. Solve symbolically where possible and check dimensions, limiting cases and direction.

StageQuestion to askEvidence in answer
ModelWhat is idealised?Boundary or assumption stated
RepresentationWhich diagram fits?Labels, nodes, rays or energy arrows
CalculationWhich relation follows?Symbols then SI values
EvaluationDoes it make sense?Unit, sign and trend check

Make the idea concrete in Nepal

Connect the model to a school laboratory, musical instrument, auditorium, mobile communication, optical instrument or energy device. The example should clarify the physics rather than merely mention a place. For a visual experiment, use PhET Wave Interference and record which variable you changed and what observation followed.

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Frequently asked questions

Questions students ask about Second Law of Thermodynamics

The second law fixes the natural direction of thermal processes: heat flows spontaneously from hotter to colder bodies, and no cyclic engine can convert all absorbed heat into work.

For an engine, W=Qh−Qc and efficiency η=W/Qh=1−Qc/Qh. A reversible Carnot limit is ηmax=1−Tc/Th with absolute temperatures.

Draw the situation, list known quantities in SI units, choose the governing relation symbolically, substitute once and finish with a unit and reasonableness check.

Using Celsius in a Carnot ratio, claiming 100% efficiency, or confusing energy conservation with direction.

Alternate one concept explanation, one diagram and two numericals. Revisit errors after a day instead of only rereading the solution.

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References and next steps

Scope checked against CDC Nepal; concept support from OpenStax and PhET. Continue with the Second Law of Thermodynamics cluster.

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