The Energy Principle

Last updated: March 16, 2023

The first law of thermodynamics states that for any given system, the change in energy is equal to the difference between the heat transferred to the system and the work done by the system on its surroundings during a given time interval.

The energy referred to in this principle represents the total energy of the system minus the sum of the potential, kinetic, and internal (molecular) forms of energy, such as electrical and chemical energy. The internal energy changes are commonly disregarded in water distribution analysis because of their relatively small magnitude.

In hydraulic applications, energy is often represented as energy per unit weight, resulting in units of length. Using these length equivalents gives engineers a better feel for the resulting behavior of the system. When using these length equivalents, the state of the system is expressed in terms of head. The energy at any point within a hydraulic system is often represented in three parts:

Pressure Head: P/ γ
Elevation Head: Z
Velocity Head: V²/2g

Where:

P = Pressure (N/m²)
γ = Specific weight (N/m³)
Z = Elevation (m)
V = Velocity (m/s)
g = Gravitational acceleration constant (m/s²)

These quantities can be used to express the headloss or head gain between two locations using the energy equation.