The definition of energy. An easy to understand energy definition. Energy is the ability to make something happen. Science and technology education from FT Exploring.
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What is the Definition of Energy?
We use energy to run, to make things happen.
I know, all the school books say energy is the ability to do work. But what does that mean? Well it sort of means the ability to make something happen (the nearly-well-known Dave Watson definition). Everytime a force is exerted on something through a distance (which is the definition of work) something had to move, which means something happened. But is that the definition of Energy? My thermo books say work is a process of energy transfer. Not a single one of my numerous thermodynamics text books says energy is "the ability to do work" (see fun definitions below).

And what about heat flow? Energy can be transferred through heat flow, like when you put a pot of water on the stove and the water gets hotter. Something happened for sure. Something changed. The water got hot and eventually, if left on the hot stove long enough, will start to boil. What forces are involved in this case? There doesn't appear to be anything being pushed through a distance. Does there?

Some of the more thoughtful text books are careful to explain that the ability to do work applies to mechanical energy, not heat (or thermal energy - see the page on types of energy). But we're not done yet. If you get to the really thoughtful text books (or web sites) they will explain that what we call heat or heat flow is really the 35,000 foot view (macro level) of the result of trillions and trillions and trillions of interactions between atoms and molecules. At the atomic, or micro level, you can make the case that it looks a lot like work, as individual atomic particles are exchanging energy by doing work on each other (again see the page on types of energy).


Relevant Digression
It's okay to admit we don't have everything perfectly sorted out.
If we had all the answers we wouldn't need science.

When it comes to the definition of energy, I want you to realize that it is important, but also to relax a little.

The fact is, no one knows what Energy is!
Really! No one knows what ENERGY is!
(See text book quotes below).

We only know how to describe the characteristics of its various manifestations mathmatically. The same is true of other physical phenomena, such as gravity.

The "ability to do work" is more of a characteristic of energy than a definition. Don't you think? (Or what if we call it a "defining characteristic"?)

A catapult is a device to convert stored energy into kinetic energy.
A device for converting work from muscles into stored elastic energy and then converting that into kinetic energy. Things happen when energy changes form.
Back to the Defining Characteristics
For the purpose of explaining energy to beginners, I think the best description is the following nearly-well-known Dave Watson definition:
Energy is a property or characteristic (or trait or aspect?) of matter that makes things happen, or, in the case of stored or potential energy, has the "potential" to make things happen.

By "happen", we mean to make things move or change condition. Examples of changes in condition are changes in shape, volume, and chemical composition (results of a chemical reaction). There are also changes in pressure, temperature, and density which we call a "change of state" in thermodynamics. Phase changes, such as changing from solid to liquid, or liquid to vapor, or back the other way, are also good examples of condition changes. Something happened!

Without energy, nothing would ever change, nothing would ever happen. You might say energy is the ultimate agent of change, the mother of all change agents.

Whenever anything happens or changes there is an energy change. Either energy changes form, as when a generator changes mechanical energy into electrical energy; or energy changes location, as when heat flowing too fast out of your body makes you cold, or heat flowing into a pot of water makes the water turn into steam. You get the idea.

Okay, I suppose "making things happen" is just as much a defining characteristic as the "ability to do work". But it's something beginners can comprehend. You don't have to understand the concept of work to understand this. Also, this more general way of describing energy includes heat flow. When an ice cube melts (a phase change), or a cup of hot coffee cools down, something happened, energy flowed, but no work was done (at least on the macro level). The "ability to do work" only applies to mechanical energy.

Extra Credit Reading - What the Books Don't Say
I looked for the definition of energy in 9 college thermodynamic text books, 3 college text books on heat transfer, and 3 college text books on physics. Only a few of the thermo books even discussed a definition for energy. Most just jumped right into thermo without so much as a nod in the direction of an energy definition. Work was discussed but not as a definition for energy, just as the well-known process of energy conversion.  

If you aren't already having too much fun, I've gathered some of the more interesting comments, descriptions, and "definitions" I found on energy, and listed them below. Who says nerds aren't fun?

I suggest bringing these to a party. Be sure and remind everyone that even while they are hearing these being read, their brains are converting food energy into thought. No one knows quite how the hec that happens, either. Be prepared for enthusiastic applause when you finish. (Okay, I did get booed off the stage at the last party, but they were a bunch of business majors.)

Source: Richard Feynman (very famous & smart physicist), "Lectures on Physics"

Quote: "It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount."
Source: Quoted in a book called Energies, by Vaclav Smil, attributed to David Rose:

Quote: Energy, "is an abstract concept invented by physical scientists in the nineteenth century to describe quantitatively a wide variety of natural phenomena."
Source: Thermodynamics, by Virgil Moring Faires, and Clifford Max Simmang, MacMillan Publishing Co., Inc. (a college text book)

Quote: "Energy is inherent in all matter. Energy is something that appears in many different forms which are related to each other by the fact that conversion can be made from one form of energy to another (comment from DW - see my pages on Energy Changes and the First Law of Thermodynamics). Although no simple definition can be given to the general term energy, E, except that it is the capacity to produce an effect (comment from DW - "Hey that's like my definition!"), the various forms in which it appears can be defined with precision."
Source: (A good ol' Schaum's Outline for gosh sakes!) Theory and Problems of Thermodynamics, by M.M. Abbott, H.C. Van Ness, Schaum's Outline Series in Engineering, McGraw-Hill Book Company

Quote: "Energy is a mathematical abstraction that has no existence apart from its functional relationship to other variables or coordinates that do have a physical interpretation and which can be measured. For example, the kinetic energy of a given mass of material is a function of its velocity, and it has no other reality."
(comment from DW - Phew! I guess that's kind of a mystic thing going there. And that's the first paragraph! How many people would keep reading after that? Well, me for one. I couldn't resist, here's the next paragraph:)
More Quote (2nd paragraph in book): "The first law of thermodynamics is merely a formal statement asserting that energy is conserved. Thus it represents a primitive statement about a primitive concept. Moreover, energy and the first law are coupled: The first law depends on the concept of energy, but it is equally true that energy is an essential thermodynamic function precisely because it allows formulation of the first law."
(comment from DW - Get out of town! My appologies to the folks at Schaum's for poking fun, but come on, that's just plain ridiculous, not to mention unbearably pompous.)
Source: Fundamentals of Classical Thermodynamics, 3rd Edition, by Gordon J. Van Wylen, Richard E. Sonntag, John Wiley & Sons,
(comment from DW - Years ago I taught a semester of Engineering Thermodynamics at Penn State Behrend while also working full time as an Engineer at GE - what an ordeal! This was the book we used.)

Quote: "One very excellent definition of thermodynamics is that it is the science of energy and entropy. However, since we have not yet defined those terms, an alternate definition in terms with which we are already familiar is: thermodynamics is the science that deals with heat and work and those properties of substances that bear a relation to heat and work."
(comment from DW - Okay, the authors say they haven't yet defined energy, implying they are going to define it. Well I looked all through the book finding no such definition of energy. It's not even mentioned in the index. Sneaky, huh?)
Source: Theory and Practice of Heat Engines, by Virgil Moring Faires, The MacMillan Company.
(comment by DW - this is an old one, copyright 1948. Apparently the laws of energy haven't changed since then.)

Quote: "While it is difficult to define energy in a general sense, it is simple to explain particular manifestations of energy. The forms of energy to be considered here are: mechanical potential energy, mechanical kinetic energy, internal energy, flow work, shaft work, transferred heat, and, occasionally, chemical and electrical energy. Other manifestations of energy, such as atomic energy, subatomic energy, will not be discussed."
(comment by DW - It was not my experience that the average first year thermo student felt it was "simple to explain particular manifestations of energy". Also, note that neither work nor heat are described as a form of energy, but he does have that process thing in there "transferred heat". )

Okay, that's enough fun for one page. Those few fellow nerds that are still with me, must be real die-hards. So, to repeat myself again, most of the books I looked at did not even mention a definition for energy, they just started talking about it. From now on, that's what I'll do.
Energy Intro Page     The First Law of Thermo     Types of Energy
The Second Law of Thermo

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