Total energy of a liquid: Pressure, Kineti, Potential energy

Total energy of a liquid

A liquid in motion possesses pressure energy, kinetic energy and potential energy.

(i) Pressure energy

It is the energy possessed by a liquid by virtue of its pressure.

Consider a liquid of density ρ contained in a wide tank T having a side tube near the bottom of the tank as shown in Fig. . A frictionless piston of cross sectional area 'a' is fitted to the side tube. Pressure exerted by the liquid on the piston is P = h ρ g where h is the height of liquid column above the axis of the side tube.

If x is the distance through which the piston is pushed inwards, then

Volume of liquid pushed into the tank = ax

∴ Mass of the liquid pushed into the tank = ax ρ

As the tank is wide enough and a very small amount of liquid is pushed inside the tank, the height h and hence the pressure P may be considered as constant.

Work done in pushing the piston through the distance x = Force on the piston × distance moved

(i.e) W = Pax

This work done is the pressure energy of the liquid of mass axρ.

∴ Pressure energy per unit mass of the liquid = Pax /axp = P/p

(ii) Kinetic energy

It is the energy possessed by a liquid by virtue of its motion.

If m is the mass of the liquid moving with a velocity v, the kinetic energy of the liquid = ½ mv²

Kinetic energy per unit mass = 1/2mv²   /  m   = v² / 2

(iii) Potential energy

It is the energy possessed by a liquid by virtue of its height above the ground level.

If m is the mass of the liquid at a height h from the ground level, the potential energy of the liquid = mgh

Potential energy per unit mass = mgh/ m = gh

Total energy of the liquid in motion = pressure energy + kinetic energy + potential energy.

Total energy per unit mass of the flowing liquid = P/p + v²/2 + gh