First law of thermodynamics

First Law of Thermodynamics

The First Law of Thermodynamics is a core concept in physics and chemistry that states energy cannot be created or destroyed, only transformed or transferred. It declares that energy cannot be created or destroyed; it can only change from one form to another.

Definition

The First Law of Thermodynamics states that the total energy of an isolated system remains constant. In other words, the increase in the internal energy of a system is equal to the heat added to the system minus the work done by the system on its surroundings.

Mathematical Expression

The first law is commonly expressed in the following form:

ΔU = Q - W

• ΔU = The variation in the system’s internal energy
• Q = Heat added to the system
• W = Work performed by the system on its surroundings

Derivation of the Mathematical Form

Consider a system undergoing a process in which heat energy Q is added and the system does work W on the surroundings. According to the conservation of energy:

Energy added to the system = Change in internal energy + Work done by the system

Mathematically,

Q = ΔU + W

Rearranging the equation gives the standard form of the first law:

ΔU = Q - W

Sign Conventions

• Heat absorbed by the system (Q) is positive.
• Heat released by the system (Q) is negative.
• Work done by the system on surroundings (W) is positive.
• When work is performed on the system by the surroundings, (W) is negative.

Applications

• Describes how energy varies during chemical reactions.
• Helps analyze engines, refrigerators, and heat pumps.
• Serves as the foundation for understanding thermodynamic cycles and processes.

The First Law of Thermodynamics emphasizes the conservation of energy in all physical and chemical processes. It helps predict how energy flows within a system and its surroundings, forming a foundation for further studies in thermodynamics.