The energy is defined as the capacity of doing work. The energy possessed by a system may be of two kinds.
1. Stored energy: such as potential energy, internal energy, kinetic energy etc.
2. Transit energy: such as heat, work, flow energy etc. The stored energy is that which is contained within the system boundaries, but the transit energy crosses the system boundary. The store energy is a thermodynamic property whereas the transit energy is not a thermodynamic property as it depends upon the path. For example, the kinetic energy of steam issuing out from a steam nozzle and impinging upon the steam turbine blade is an example of stored energy. Similarly, the heat energy produced in combustion chamber of a gas turbine is transferred beyond the chamber by conduction/ convection and/or radiation, is an example of transit energy.
Form of Energy
1. Potential energy (PE)
The energy possessed by a body or system by virtue of its position above the datum (ground) level. The work done is due to its falling on earth’s surface.
Potential energy,PE = Wh = mgh N.m
Where, W = weight of body, N ; m = mass of body, kg
h = distance of fall of body, m
g = acceleration due to gravity, = 9.81 m/s2
2. Kinetic Energy (KE)
The energy possessed by a system by virtue of its motion is called kinetic energy. It means that a system of mass m kg while moving with a velocity V1 m/s, does 1/2mV12 joules of work before coming to rest. So in this state of motion, the system is said to have a kinetic energy given as;
K.E. = 1/2mv12 N.m
However, when the mass undergoes a change in its velocity from velocity V1 to V2, the change in kinetic energy of the system is expressed as;
K.E. = 1/2mv22 – 1/2mv12
3. Internal Energy (U) It is the energy possessed by a system on account of its configurations, and motion of atoms and molecules. Unlike the potential energy and kinetic energy of a system, which are visible and can be felt, the internal energy is invisible form of energy and can only be sensed. In thermodynamics, main interest of study lies in knowing the change in internal energy than to know its absolute value. The internal energy of a system is the sum of energies contributed by various configurations and inherent molecular motions. These contributing energies are
(1) Spin energy: due to clockwise or anticlockwise spin of electrons about their own axes.
(2) Potential energy: due to intermolecular forces (Coulomb and gravitational forces), which keep the molecules together.
(3) Transitional energy: due to movement of molecules in all directions with all probable velocities within the system, resulting in kinetic energy acquired by the translatory motion.
(4) Rotational energy: due to rotation of molecules about the centre of mass of the system, resulting in kinetic energy acquired by rotational motion. Such form of energy invariably exists in diatomic and polyatomic gases.
(5) Vibrational energy: due to vibration of molecules at high temperatures.
(6) Binding energy: due to force of attraction between various sub–atomic particles and nucleus.
(7) Other forms of energies such as: Electric dipole energy and magnetic dipole energy when the system is subjected to electric and/or magnetic fields. High velocity energy when rest mass of the system mo changes to variable mass m in accordance with Eisenstein’s theory of relativity). The internal energy of a system can increase or decrease during thermodynamic operations. The internal energy will increase if energy is absorbed and will decrease when energy is evolved.
4. Total Energy
Total energy possessed by a system is the sum of all types of stored energy. Hence it will be given by
Etotal = PE + KE + U = mgh + 1/2mv2 + U
It is expressed in the unit of joule (1 J = 1 N m)
