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Thermal Physics · Physics

Heat and work are energy transfers. Internal energy is stored energy.

Build clean definitions that make the First Law feel inevitable rather than mysterious.

This topic

Heat, Work, and Internal Energy

Identify the system, classify energy transfer correctly, and avoid the most common sign and language errors.

Transfer
Heat as energy transfer
Heat is energy transferred because of a temperature difference—never a “thing contained” in a body.
  • Heat flows from higher to lower temperature
  • Heat is process-dependent, not a state variable
  • Common language traps (“heat in the object”)
Start nano-lesson Heat is a verb, not a noun.
Transfer
Work done by and on a system
Work is energy transfer via forces acting through distances (or more generally, generalized forces/displacements).
  • Mechanical work idea and sign
  • Boundary work (P–V work) preview
  • “By the system” vs “on the system” clarity
Start nano-lesson Always state your sign convention.
State
Internal energy of a system
Internal energy is the energy associated with the microscopic degrees of freedom of a system.
  • State function: depends only on state
  • Microscopic motion + interactions (conceptual)
  • Not the same as “thermal energy” in vague terms
Start nano-lesson ΔU is meaningful; U’s zero is chosen.
Distinctions
Heat vs temperature
Temperature is a state variable. Heat is energy in transit. They answer different questions.
  • Temperature describes state, not transfer
  • Same heat input can yield different ΔT
  • System definition controls interpretation
Start nano-lesson Separate “state” from “process.”
Practice
Practice & Exercises
Practice classification, sign conventions, and short explanations that reveal conceptual clarity.
  • Heat vs work classification drills
  • System boundary and sign-convention prompts
  • Internal energy change reasoning
  • “Spot the misconception” questions
  • Exam-style short-response sets
Open practice Clarity beats memorization.