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### Energy and Different Forms of Energy

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Potential EnergyThe energy possessed by a body by virtue of its position relative to some fixed datum is known as potential energy.This energy is stored in the body, being released if the body is allowed to fall to its original datum.Consider a water tank with water of mass M stored at the height of Z as shown in the graphic.At Z, the Potential Energy of the water (of weight Mg) is given by the work done to lift it through the height Z.Potential Energy of the water (Nm or joules) is given by the product of Force in newton and the distance moved in metre.We prefer to use joules to indicate work or energy as the unit for Torque is also newtons-meter.
Kinetic EnergyThis is the energy possessed by a body by reason of its motion.Consider water flowing out of the tank at a velocity c as shown in the graphic.Kinetic Energy of the water flow is given by half the product of mass of water in kilogram and square of velocity of water in metres per second.For per kilogram of water the specific kinetic energy is half the square of the velocity in metres per second.
Internal EnergyThis is the energy stored in a substance by virtue of the movement of its molecules.It depends only upon temperature and is independent of changes in pressure and volume.Internal Energy is given the symbol U and is measured in joules.As with all the other symbols in the steady flow energy equation, if we wish to refer to the specific internal energy we use the lower case version of this symbol, i.e., u. Specific refers to the amount per one kilogram of the substance.
Flow or Displacement EnergyAny fluid entering or leaving a system must push or displace an equal volume ahead of itself for there to be steady flow.From the basic principle which states that work done is equal to force times distance moved, for water flow as shown in the figure.Flow Energy is given by the product of the pressure and volume and its unit is joules.
EnthalpyInternal Energy, pressure and volume are properties of the fluid. It is therefore convenient to combine them into one single property known as Enthalpy (H).Enthalpy is the combination of internal energy and flow energy.The graphic shows the energy transfer involved when a substance changes phase.Specific enthalpy is given by the sum of internal energy and the product of pressure and volume and its unit is joules.