First Law of Thermodynamics Study Pack

Kibin's free study pack on First Law of Thermodynamics includes a 3-section study guide, 8 quiz questions, 10 flashcards, and 1 open-ended Explain review question. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.

Last updated May 21, 2026

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First Law of Thermodynamics Study Guide

Master the First Law of Thermodynamics by working through its core equation, ΔU = Q − W, and understanding how internal energy, heat, and work interact. This pack covers thermodynamic sign conventions, special processes like isothermal and adiabatic, and why perpetual motion machines violate energy conservation — giving you the conceptual and problem-solving foundation college physics demands.

Key Takeaways

  • •The First Law of Thermodynamics states that the change in a system's internal energy equals the heat added to the system minus the work done by the system: ΔU = Q − W.
  • •Internal energy is the total kinetic and potential energy of all particles within a system; it is a state function, meaning its change depends only on initial and final states, not the path taken.
  • •Heat and work are distinct energy transfer mechanisms — heat flows due to a temperature difference, while work involves a force acting through a displacement (such as gas expanding against a piston).
  • •In thermodynamic sign conventions, Q is positive when heat flows into the system and W is positive when the system does work on its surroundings.
  • •Special processes — isothermal, adiabatic, isochoric, and isobaric — each constrain one variable, simplifying the First Law equation and revealing how energy is redistributed in each case.
  • •The First Law is a statement of energy conservation: energy cannot be created or destroyed, only transferred between a system and its surroundings as heat or work.
  • •A perpetual motion machine of the first kind — a device that produces work with no energy input — is impossible because it would violate the First Law.

The Core Statement: Energy Conservation in Thermodynamic Systems

The First Law of Thermodynamics is the principle of energy conservation applied specifically to heat and work, establishing that any change in a system's stored energy must be fully accounted for by energy entering or leaving as heat or work.

Defining a Thermodynamic System

  • •A system is any clearly defined region of matter under study — for example, a gas confined in a cylinder — while everything outside it is called the surroundings.
  • •The boundary between system and surroundings can be real (like cylinder walls) or imaginary, but it determines what counts as an energy transfer.
  • •An isolated system exchanges neither heat nor work with its surroundings, a closed system exchanges energy but not matter, and an open system exchanges both.

The Mathematical Statement of the First Law

  • •The First Law is written as ΔU = Q − W, where ΔU is the change in internal energy, Q is the net heat added to the system, and W is the net work done by the system on its surroundings.
  • •Because ΔU depends only on the initial and final states of the system, it is called a state function — unlike Q and W individually, which depend on the process path.
  • •The equation enforces a strict energy bookkeeping: if a system loses 500 J as work and gains 200 J as heat, its internal energy must decrease by exactly 300 J.

Internal Energy, Heat, and Work as Distinct Physical Quantities

Understanding the First Law requires distinguishing carefully between three quantities that are easy to conflate: internal energy stored within a system, heat transferred across a boundary due to temperature differences, and work done through mechanical interaction with the surroundings.

Internal Energy

  • •Internal energy (U) is the sum of all microscopic kinetic energies (from particle motion) and potential energies (from inter-particle interactions) within the system.
  • •For an ideal monatomic gas, internal energy depends only on temperature: U = (3/2)nRT, where n is the number of moles and R is the universal gas constant.
  • •Internal energy is a property of the system's state — it does not describe a process or a transfer.

Heat as Energy Transfer

  • •Heat (Q) is energy that crosses the system boundary specifically because of a temperature difference between the system and surroundings.
  • •Heat is not stored in a system; it exists only during the transfer process. Saying a system 'contains heat' is a common misconception.
  • •Q is positive when energy flows into the system (system absorbs heat) and negative when energy flows out (system releases heat).

Work Done by a Gas

  • •The most common form of work in thermodynamics is pressure-volume work: W = PΔV for a process at constant pressure, where P is pressure and ΔV is the change in volume.
  • •When a gas expands (ΔV > 0), it pushes outward on its surroundings and does positive work, reducing its own internal energy if no heat is added.
  • •When a gas is compressed (ΔV < 0), the surroundings do work on the gas (W is negative from the system's perspective), which tends to increase internal energy.
  • •Graphically, the work done by a gas during any process equals the area under the curve on a pressure-volume (P-V) diagram.

About this Study Pack

Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.

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