Energy and Power Equivalences
The different forms of energy included in energy transfer systems (like kinetic energy, potential energy, internal energy, P-V energy, work & heat) might be measured in several basic units. In common, three kinds of units are used to measure energy: (A) mechanical units, like the foot-pound-force (ft-lbf); (B) thermal units, like the British thermal unit (Btu); and (C) electrical units, like the watt-second (W-sec). In the cgs and mks systems, the mechanical units of energy are joule (j) and the erg, the thermal units are kilocalorie (kcal) and calorie (cal), and the electrical units are watt-second (W-sec) and the erg. However the units of the various kinds of energy are dissimilar, they are equal.
Some of the most significant experiments in science were those performed by J. P. Joule in 1843, who illustrated quantitatively that there was a direct connection among mechanical and thermal energy. Such experiments illustrated those one kilocalorie equivalents 4,186 joules. These similar experiments, whenever performed by using English system units, illustrate that one British thermal unit (Btu) equivalent 778.3 ft-lbf. Such experiments established the likeness of mechanical and thermal energy. The other experiments recognized the likeness of electrical energy with both mechanical and thermal energy. For engineering applications, such equivalences are stated by the relationships shown below.
1 ft-lbf = 1.286 x 10-3 Btu = 3.766 x 10-7 kW-hr
1 Btu = 778.3 ft-lbf = 2.928 x 10-4 kW-hr
1 kW-hr = 3.413 x 103 Btu = 2.655 x 106 ft-lbf
There is one extra unit of energy met in engineering applications. This is the horsepower-hour (hp-hr). It is a mechanical unit of energy defined by the relationship shown below:
1 hp-hr = 1.980 x 106 ft-lbf
These relations can be used to convert among the different English system units for the different forms of energy.
Most of the calculations including the energy of the working fluid in an energy transfer system are executed in Btu's. The kinds of mechanical energy (like potential energy, kinetic energy, & mechanical work) and other kinds of energy (like P-V energy) are generally given in foot-pounds-force. These are transformed to Btu's by using 1 Btu = 778.3 ft-lbf.
This translation factor is frequently used. In fact, a constant termed as the mechanical equivalent of heat, generally symbolized by the symbol J and at times termed to as Joule's constant, is stated as:
The power is defined as the time rate of doing work. It is equal to the rate of the energy transfer. The power has units of energy per unit time. Since with energy, power might be measured in many basic units, though the units are equal. In the English system, mechanical units of power are foot-pounds-force per second and per hour (ft-lbf/sec or ft-lbf/hr) and horsepower (hp). Thermal units of power are British thermal units per hour (Btu/hr), and the electrical units of power are watts (W) or kilowatts (kW). For engineering applications, the likeness of such units is stated by the relationships shown below.
1 ft-lbf/sec = 4.6263 Btu/hr = 1.356 x 10-3 kW
1 Btu/hr = 0.2162 ft-lbf/sec = 2.931 x 10-4 kW
1 kW = 3.413 x 103
Btu/hr = 737.6 ft-lbf/sec
Horsepower is associated to foot-pounds-force per second (ft-lbf/sec) by the relationship shown below:
1 hp = 550.0 ft-lbf/sec
Such relationships can be used to translate the English system units for power.