First Law of Thermodynamics

The first law of thermodynamics is just the energy-conservation principle applied to a thermodynamic system: any change in the system’s internal energy must come from heat transferred into it, minus the energy it gives out as work. If heat enters the system, internal energy tends to increase; if the system expands and pushes on its surroundings, it loses energy by doing work. This balance explains why temperature, pressure, and phase can change when a gas is heated or compressed.

Use this formula whenever a problem involves heat transfer, work done by or on a gas, and a change in internal energy. It is especially common in ideal-gas processes, piston problems, and any thermodynamics question asking how much energy a system gains or loses.

• Using the wrong sign for work, especially confusing work done by the system with work done on the system
• Mixing up heat absorbed and heat released without adjusting the sign of Q
• Forgetting that all terms must use the same units before calculating