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Текст 1. What is the Definition of Energy?






What is the Definition of Energy?

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. Every time 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".

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). 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.

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! We only know how to describe the characteristics of its various manifestations mathematically. 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?

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.

 

Ответьте на вопросы к тексту:

1. What is energy?

2. How can heat flow be explained?

3. What’s the definition of energy according to Dave Watson?

4. What are the examples of energy change?


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