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Thermal Physics · Physics

Energy conservation for thermal systems.

The First Law is not a new law of nature—it’s bookkeeping. Learn to track heat, work, and internal energy cleanly.

This topic

First Law of Thermodynamics

Choose the system, state sign conventions, and apply ΔU = Q − W (or equivalent) without confusion.

Statement
What the First Law says
The change in internal energy equals energy added as heat minus energy leaving as work (with a stated convention).
  • Internal energy as a state function
  • Heat and work as process quantities
  • Why the same ΔU can occur via different paths
Start nano-lesson Write the convention explicitly.
Application
Energy conservation in thermal systems
The First Law is energy conservation applied to a defined system boundary.
  • Closed vs open systems (intro)
  • System boundary controls what counts as work/heat
  • Interpreting “no heat exchange” and “no work”
Start nano-lesson Boundary first, equations second.
Use cases
Applications to gases and cycles
Combine P–V work ideas with heat transfer to analyze expansions, compressions, and closed cycles.
  • Connecting ΔU, Q, and W for a process
  • Cycle idea: net ΔU = 0 over a cycle
  • Net work as area enclosed on a P–V diagram (preview)
Start nano-lesson In a cycle, ΔU totals to zero.
Conventions
Sign conventions
Many errors come from mixing conventions. Decide whether W is “by the system” or “on the system” and stick to it.
  • Two common conventions and how to translate
  • Expansion/compression and the sign of work
  • Quick consistency checks with physical intuition
Start nano-lesson Consistency beats memorization.
Practice
Practice & Exercises
Practice setting up First Law balances, interpreting process statements, and doing sign-consistent computations.
  • Identify Q, W, and ΔU from descriptions
  • Compute unknowns in simple processes
  • Cycle reasoning: net heat vs net work
  • Sign-convention translation problems
  • Exam-style mixed First Law sets
Open practice Start with the system boundary.