First Law of Thermodynamics: NEB Class 12 Physics Guide

Curriculum boundary

Start with the current Grade 12 scope

The CDC Grade 12 Physics page and secondary curriculum control the Nepal scope. The unit connects heat, work and internal energy and applies the first law to isochoric, isobaric, isothermal, adiabatic and cyclic processes at the required depth.

Prerequisites are ideal-gas variables, heat capacity and energy conservation. Review Ideal Gas and Quantity of Heat. International sources explain the model but do not replace CDC requirements.

Energy ledger

Define system, transfer and sign before arithmetic

A thermodynamic system is the matter or region chosen for study; surroundings are everything else. A boundary may allow energy transfer as heat or work. Heat is energy transfer caused by temperature difference, not a substance stored in the system. Work is organised energy transfer associated here mainly with moving a piston boundary.

With the stated convention, ΔU=Q−W. Heat added gives Q>0; heat removed Q<0. Expansion work by the gas gives W>0; compression work on the gas gives W<0. Other books may define work on the system as positive and write ΔU=Q+W_on. Both are consistent if not mixed.

Internal energy is a state function: ΔU depends only on initial and final equilibrium states. Q and W are path quantities. The OpenStax first-law treatment illustrates different paths with the same ΔU.

P–V work

Area under the process path represents boundary work

For a quasi-static volume change, differential work by gas is dW=P dV and W=∫P dV. On a P–V graph, work is signed area under the path. Expansion moves right and produces positive W; compression moves left and produces negative W under this convention. A vertical isochoric path has zero volume change and zero boundary work.

For a curved path, use the process equation or graph area; multiplying one pressure by total ΔV may be invalid. See work, heat and internal energy.

Common processes

Use the fixed condition to simplify the ledger

Isothermal does not mean no heat transfer; adiabatic does not mean constant temperature. A rapid insulated expansion may be approximately adiabatic and cool as internal energy supplies work. Explore process variables with PhET Gas Properties while stating idealisation limits.

Worked comparisons

Keep state and path claims separate

Isochoric heating: 500 J enters a rigid container. W=0, ΔU=+500 J. Adiabatic expansion: gas does 300 J work with Q=0, so ΔU=−300 J. Compression: 200 J work is done on gas, hence W_by=−200 J; if 50 J heat leaves, Q=−50 J and ΔU=−50−(−200)=+150 J.

Two paths connect the same states with ΔU=400 J. Path A has W=100 J, so Q=ΔU+W=500 J. Path B has W=350 J, so Q=750 J. Heat differs because work differs, while internal-energy change remains the same.

Reliable method

System → convention → process → ledger → check

1. Draw system boundary and name initial/final states.
2. Write the sign convention explicitly.
3. Identify fixed condition and P–V path.
4. Calculate work with sign.
5. Use ΔU=Q−W and solve symbolically.
6. Check units, signs and physical energy flow.

Common mistakes are calling U heat, treating Q/W as state functions, assuming isothermal means Q=0, ignoring compression sign and using PΔV for a varying-pressure path. For online or physical tuition call 9846662070. The MKS Education panel provides test preparation and pre-counselling after Grade 12.

Checked sources

References and related learning

• CDC Nepal: Physics Grade 12
• CDC Nepal: Secondary Curriculum
• OpenStax: Work, Heat and Internal Energy
• OpenStax: First Law
• OpenStax: Thermodynamic Processes
• PhET: Gas Properties

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