Second Law of Thermodynamics: NEB Class 12 Physics Guide

Curriculum scope

Connect the second law to the first

The CDC Grade 12 page and curriculum control scope. The unit covers directional statements, heat engines/refrigerators, efficiency/COP, Carnot ideas, reversibility and entropy at the assigned depth.

Prerequisite: First Law guide. The first law balances energy; it would not forbid a hypothetical cyclic device converting one-reservoir heat wholly to work. The second law supplies the missing feasibility restriction.

Direction and irreversibility

Natural processes have a preferred direction

Heat flows spontaneously from hotter to colder bodies; reverse transfer needs work or another compensating effect. Friction converts organised mechanical energy into dispersed internal energy. Free expansion and mixing occur spontaneously but do not reverse by themselves.

Kelvin–Planck: impossible for a cyclic device to take heat from a single reservoir and convert it entirely into work without another effect. Clausius: impossible for a cyclic device’s sole effect to transfer heat from cold to hot. These are equivalent expressions of the second law, discussed by OpenStax.

Heat engines

An engine must reject some heat

A cyclic engine absorbs Qh from hot reservoir, produces work W and rejects Qc to cold reservoir. First law over cycle gives W=Qh−Qc. Efficiency η=W/Qh=1−Qc/Qh. Since Qc cannot be zero for a cyclic engine between finite reservoirs, η<1.

See OpenStax heat engines.

Refrigerators and heat pumps

Work moves heat against its spontaneous direction

A refrigerator removes Qc from cold region, receives work W and delivers Qh=Qc+W to hot surroundings. Refrigerator COP=Qc/W. A heat pump’s heating COP=Qh/W=COP_R+1. COP may exceed one because it measures moved heat per work input, not energy-conversion efficiency.

Reversible limit

Carnot efficiency depends only on reservoir temperatures

For a reversible engine between absolute temperatures Th and Tc, ηC=1−Tc/Th. Use kelvin. It is an upper bound; real engines are less efficient because of finite temperature differences, friction, turbulence and other irreversibilities.

The Carnot-cycle resource explains reversible comparisons. Reaching 100% would require Tc=0 K in the formula and is not physically attainable.

Entropy

Entropy tracks dispersal and irreversibility

For a reversible transfer, dS=dQrev/T. Entropy is a state function; for an isolated system, total entropy does not decrease. A reversible ideal process has zero total entropy production, while irreversible processes produce positive total entropy.

“Entropy is disorder” can be a rough analogy but is not enough for calculations. Use energy dispersal and state-function reasoning. The OpenStax entropy section shows how an irreversible path can have state-change entropy calculated through an imagined reversible path.

Solution method

Device diagram → energy balance → performance → feasibility

1. Draw hot/cold reservoirs and arrows.
2. Label magnitudes Qh, Qc and W.
3. Apply Qh=W+Qc.
4. Choose efficiency or COP definition.
5. Convert reservoir temperatures to kelvin.
6. Compare with Carnot or second-law direction.

Common mistakes: treating efficiency like COP, using Celsius in ratios, setting rejected heat to zero, reversing Qh/Qc arrows and assuming energy balance proves feasibility. For tuition call 9846662070; the MKS panel supports post-Grade-12 planning.

Checked sources

References and related learning

• CDC Nepal: Physics Grade 12
• CDC Nepal: Secondary Curriculum
• OpenStax: Heat Engines
• OpenStax: Second-Law Statements
• OpenStax: Carnot Cycle
• OpenStax: Entropy

Continue with the Second Law Study Guide. Sources were checked on 2 August 2026; follow current CDC, NEB and school notices if requirements change.