recall on the relation Power, Energy, Work, Flow

Understand and master relation between Power, Energy, Work, Flow:

memo form the main relation and conversion of physical units:


The Work unit is Joule, Work describes a force applied over a distance. 1 joule work corresponds to a force of 1 newton moving object on a 1 meter. (The force applied parallel to the displacement)
The energy of an object represents the work (joules) can be produced by this object so, energy is measured in the same unit that the work in joules..


The kinetic energy of an object measuring the work that the object can perform by moving.
The potential energy (also called geodesic) of an object, measuring the work that this object can perform by its altitude variation.
Example: A change in altitude of 10 meters of an object of mass 2 kg can provide work 2kg X 9.81 X 10 meters = 196.2 joules. We can say that this object has a potential energy (geodesic) of 196.2 joules
To facilitate some conversions and power calculations, the energy of an object can be expressed following its mass. We then speak of mass energy: Joules per Kg (J/Kg)
Example: back to our object of mass 2kg in the previous example that the potential energy was 196.2 joules. Its mass energy (joule / kg) is therefore 196.2 / 2 = 98.1 This amounts to simply remove the mass factor in the formula for energy
  • Potential energy mass (joules/kg) = g X H(m)
  • Mass kinetic energy(joules/kg) = 0.5 X speed² (m/sec)
From energy to power: The energy thus expresses the work that can make an object. The ability to perform an amount of work in one second is defined as power of an object (in watts). A motor that can apply a force of one newton on 1 meter per second, delivers a power of 1 Watt. Another engine that can perform the same work (1newton 1 meter) in 0.5 seconds is 2 times more powerful!
Power can also be expressed by a force and speed: Power can also be expressed by a Torque N.m (1 newton force applied to a lever for 1 meter= 1 n.m) and a rotational speed (Rad/sec) 1 Rad/sec = 9,5 Rpm: Flow rate to the power:


The flow rate Expresses the speed of (m/sec ) of a quantity of matter.This amount of matter can be described by its volume(m3) or by its mass (kg)
The volume flow Qv, the volume (m3) moved into a second: unit = m3/sec . The volume flow rate can be expressed as a function of the section S (m²) through the fluid flows at a speed V (m / sec) the relation:
The mass flow Qm, the mass(Kg) moved into a second: unit = Kg/sec
One can very simply convert m3/sec kg / sec if we know the density "ρ" (say "rho") of the fluid. Eg water having a density (kg/m3) of 1000 kg / m3, the mass flow Qm(kg/sec)= the volumetric flow Qv(m3) X 1000 (kg/m3)
Relation between flow rate and pressure head:The flow of water through the system will be done by converting the potential energy into kinetic energy according to the relationship described by the formula known as the theorem of Torricelli :
speed = sqrt( 2g.H)
  • g is the gravity (9.81ms²)
  • H is the net head (m)
  • speed in m/sec
This theorem gives us the fluid velocity as a function of height, for example, a tank filled with one meter of water is clogged at its base. Its kinetic energy output is zero because no flow is done .If the fluid stopper is removed will elapse according to the relation speed = sqrt( 2g.H). = sqrt (2 X9.81X1) = 4.429 m/sec