1st Law of Thermodynamics piston illustration

The first law of thermodynamics is an expression of the law of conservation of energy adapted for thermodynamic systems.

What does that mean for a physical system within its surroundings that the energy of the universe is constant? It means that as the system exchanges energy with its environment through heat flow or thermodynamic work (pressure-volume work), there must be a corresponding loss or gain to the internal energy of the system. Think about it. This is just common sense. If heat is flowing into a system at constant volume (no work), the internal energy of the system must be increasing. If heat is flowing out, the internal energy of the system must be decreasing. If the surroundings perform work on the system and heat flow is prevented, the change in internal energy must equal the work that is being done on the system. Usually there is some combination of heat flow and work occuring in a thermodynamic transformation. Sum it up (keep the plus and minus signs straight) and the internal energy change will equal the total. The first law of thermodynamics is the easy part of thermodynamics.

Whether an MCAT passage deals with a steam engine or a phase diagram, if an MCAT passage has a theme directly concerned with thermodynamics, there will be a question or two requiring you to reason from the 1st Law. That is important enough, but it is trivial compared to the significance of understanding thermodynamics for understanding science. Stick with it here. Really concentrate. When we move through the chemistry and into biochemistry later in this course, there will be a big payoff in your conceptual understanding if you really push at every stage in the thermodynamics now to understand in a concrete, intuitive way.

WikiPremed Resources

1st Law of Thermodynamics Cards
Chapter from the Wisebridge Learning System for Physics

Thermodynamics Practice Items
Problem set for Thermodynamics in PDF format

Answer Key
Answers and explanations

The First Law of Thermodynamics Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for 1st Law of Thermodynamics
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Understand the meaning of heat flow and thermodynamic work.

Explain the First Law of Thermodynamics in clear, simple terms.

Narrate an adiabatic compression or expansion, an isovolumetric transformation, and an isothermal compression or expansion in terms of the First Law of Thermodynamics.

Be able to interpret heat flow, work, and internal energy change for the model thermodynamic transformations of an ideal gas on a pressure-volume diagram.

Suggested Assignments

Review the basic terms for the 1st law of thermodynamics using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Master the concept and question cards for the 1st law of thermodynamics.

Perform the practice items for thermodynamics. Here is the answer key for the practice items.

In ExamKrackers Chemistry. Read pp. 119-129. Perform practice items 73-80 on pg. 130. (Reading and practice items for heat & temperature, ideal gas & kinetic theory, and 1st law of thermodynamics).

Review the 1st law of thermodynamics web resources.

Conceptual Vocabulary for 1st Law of Thermodynamics

Thermodynamics is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles.
First law of thermodynamics
The first law of thermodynamics states that the increase in the internal energy of a thermodynamic system is equal to the amount of heat energy added to the system minus the work done by the system on the surroundings.
Conservation of energy
The conservation of energy states that the total amount of energy in any system remains constant, although it may change forms.
Mechanical work
Mechanical work is the amount of energy transferred by a force.
Isothermal process
An isothermal process is a thermodynamic process in which the temperature of the system stays constant.
Heat transfer
Heat transfer is the passage of thermal energy from a hot to a cold body.
Internal energy
The internal energy of a thermodynamic system is the total of the kinetic energy due to the motion of molecules and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals.
Isolated system
An isolated system, as contrasted with a open system, is a physical system that does not interact with its surroundings.
Isobaric process
An isobaric process is a thermodynamic process in which the pressure stays constant.
Isochoric process
An isochoric process, also called an isometric process or an isovolumetric process, is a thermodynamic process that occurs without a change in volume.
Adiabatic process
An adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid.
Thermodynamic system
A thermodynamic system, originally called a working substance, is defined as that part of the universe that is under consideration, separated by a real or imaginary boundary from the environment or surroundings
Thermodynamic state
A thermodynamic state is the macroscopic condition of a thermodynamic system as described by its particular thermodynamic parameters.
Thermodynamic process
A thermodynamic process may be defined as the evolution of a thermodynamic system proceeding from an initial state to a final state.
Mechanical equivalent of heat
The mechanical equivalent of heat was an expression of 19th century science stating that mechanical work may be transformed into heat, and conversely heat into work, with the magnitude of one always proportional to the other.
In thermodynamics, work is the quantity of energy transferred from one system to another without an accompanying transfer of entropy.
State function
A state function is a property of a system that depends only on the current state of the system, not on the way in which the system got to that state.
Open system
A open system describes a system in continuous interaction with its environment.
Closed system
A closed system can exchange heat and work with its surroundings but not matter. This is in contrast to an isolated system which can exchange neither heat nor matter with the surroundings.
Phenomenological thermodynamics
Phenomenological thermodynamics is a branch of thermodynamics concerned with the study and analysis of actual phenomena without evaluation of statistical energy-level atomic and molecular details.
Polytropic process
A polytropic process is a thermodynamic process in which the logarithm of the pressure versus the logarithm of the volume is a straight line.