1. What Is Energy?
We can think of energy as anything that can carry out an action or maintain a process. Without energy, everything comes to a halt. Though energy is not as tangible as mass, distance or force, its effects are just as real. Energy, and energy flow, will play a central role in our modeling:

**Energy is the capacity to do work or to transfer heat**

Work is the product of force and distance. For example, a force of 40 pounds moving 15 feet represents 600 ft-lbs of energy. This mechanical aspect of energy is manifested in many activities. Think of the motion of an automobile, a cheetah, or a rocket.

The “Big Three” of engines - gasoline, diesel, and steam turbine have carried economies from the industrial revolution to the modern era. All three are heat engines - they transform heat, or thermal energy to mechanical energy.

In the other direction, mechanical energy can be dissipated or degraded to heat, as with brakes on an automobile. All forms of energy - even light, sound, and the biochemical energy in food - are ultimately dissipated as heat.

The heat equivalent of non-thermal forms of energy is found by various methods. A laboratory approach calculates the energy in an egg, for example, by putting it in a bomb calorimeter and reducing it to ash. The the difference between the Calories in and the Calories out is contributed by the egg -about 150 Calories. Sterile factory egg or fertilized chicken, each is assigned the same number of Calories

2. Work and Power
Lifting 200 pounds two feet is a challenge for most of us. On the other hand, almost anyone can pull a rope with 40 pounds (#) of resistance for 10 feet. Yet, from the view-point of output, both accomplish equal amounts of gravitational work, 400 foot-pounds: 200 lbs x 2 ft = 400 ft-lbs or 40 lbs x 10 ft = 400 ft-lbs.

3. Thermal Energy
One measure of heat energy is the BTU (British Thermal Unit). A BTU is the energy that goes into heating one pound of water one degree Fahrenheit. Amazingly, this is the equivalent of 778 ft-lbs of work. In other words, the thermal energy that goes into raising the temperature of an 11 ounce mug of coffee one degree (°F) is approximately equivalent to the work of lifting 55 pounds up one flight of stairs. (A pint, or 16 fluid ounces of water weighs about one pound. Rule of thumb -- A pint’s a pound the world around.)
You can see the mechanical equivalent of thermal energy on the figure bellow:


There are a seemingly endless number of energy units and their conversions, with contributions from the English and French systems, as well as heat and mechanics. The BTU and ft-lb, for example, are from the English system. The end of this chapter has a list of energy units, as well as an extensive conversion table.

4. Conservation of Energy
Here is a fundamental principle of physics, known as the First Law of Thermodynamics --
Energy can neither be created nor destroyed, it can only be transformed.

In a closed system or tank, the energy remains constant. If the energy at the start is Q0, then it remains at Q0. In an open system, if the storage does not change, the ingoing and outgoing energy must be equal. If the storage changes, this must be reflected in the energy balance.
Energy conservation law:


The energy input to a system might not balance the energy that goes out, as in Fig. 7-5. In the system on the left, the input is 75 units of energy but only 60 units go out. Since the First Law requires that the energy be conserved, system had to gain 15 units of energy. (The energy units could be Cal, BTU, kw-hr - whatever fits the context.) In the right system the input is short 15 units of energy so we can infer that the system must have lost 15 units. The sinks are depositories of leakage or rejected energy. It is usually low-grade heat, as in respiration, an engine exhaust, or a non-insulated hot water heater. The outputs represent useful work. Examples of this are plant growth , walking or the execution of a work of art.

6. Efficiency
The (thermodynamic) efficiency of a process is inversely related to the entropy. This is the ratio of useful output to input and so is always less than 100%. There is no internal storage, so the sum of the inputs must equal the outgoing energy. The efficiency for the below process is 22.5%.
Calculating energy efficiency:


Industrial economies are built on heat engines, and much effort goes into improving their efficiencies. The Big Three of heat engines are the Diesel (oil), Otto (gasoline), and Rankine (steam) engines. Mileage is a good, practical measure of the efficiency of engines used for transportation. The increase in miles per gallon (mpg) for gasoline engines has been significant over the past ten years. Sometimes it is charged that oil companies repress engine designs that will get 300+ mpg. The charges are based on calculations that convert almost all the energy of gasoline into useful work. Unfortunately, “the iron law of entropy” will exact its tax by sucking much of the energy into the heat sink. Can engineers improve the design of a gasoline engine so that it is almost 100% efficient . . . ? The answer, unfortunately, is No. The thermodynamic efficiency of internal combustion engines is 50-55%, and this is a maximum. Actual efficiency is always less, running about 35% for current automobile engines.