Energy (Physics in Action)
What Is Energy?
EVERYONE HAS HEARD OF ENERGY. WE KNOW IT IS RELATED to the cars we drive, the lights we turn on at night, or the food we cook for dinner. Most of us feel that it is somehow related to power. We also use the word energy to describe a person (“very energetic”) or vague feelings (for instance, someone might say, “I sense an energy here”).
In everyday life, it is perfectly acceptable to use the word energy in these various ways. In physics, though, energy is a precisely defined idea. Unfortunately, it is easy to confuse the everyday meanings of energy with its technical definition. It is also unfortunate that many explanations of energy in physics are either confusing or incorrect. So, it will be helpful, first, to make a list of things that are not energy. Most of these things have something to do with energy, but they are not energy. The distinction will make it much easier to understand what energy is.
- Energy is not electricity.
- Energy is not a force.
- Energy is not sunlight.
- Energy is not oil.
- Energy is not a fluid.
- Energy is not power.
- Energy is not radiation.
- Energy is not infrared light.
In short, energy is not any material object at all. Material objects can have or carry energy but themselves are not energy. In this way, energy is sort of like the idea of color: objects have colors, but color is not an object. If you could imagine a slightly kooky bumper- car amusement park ride where the cars could exchange their colors whenever they collided, you would have a good start toward understanding energy. A car might, for instance, carry “redness” from one place to another, much like light carries energy from one place to another.
Another common definition of energy is “the capacity to do work,” where “work” is defined within physics. This definition is largely correct; the student who thinks of energy this way will have a fair grasp of the concept. But in some contexts, especially in biology, it is a little bit incomplete, and we will point this out.
Regardless of what energy is, we will learn in this book how to calculate the amount of energy there is in many situations—and once you have calculated the amount of energy there is in a situation, then the total amount of energy calculated must always stay the same, even if the situation changes. That is, the situation is only allowed to change to another situation with the same amount of calculated energy. This is what we mean when we say, “Energy is always conserved.” According to the law of conservation, energy cannot be created or destroyed— the calculated amount cannot go up or down.
Energy comes in many forms, and at bottom, all forms of energy share the same principles. Nonetheless, the formula used to calculate the amount of energy differs from one situation to another. In the course of this book, we will see many of these forms: from the fundamental definition of energy to mechanical energy, potential energy, heat energy, electrical energy, chemical energy, and nuclear energy.
1 What Is Energy?
2 Kinetic Energy of a Skater: 5,000 Joules
3 Potential Energy of Liberty’s Torch: 3.4 Million Joules
4 Heat Power of the Sun in 1 m2: 1,500 Watts
5 Electrical Energy of an AA Battery: 8 Watt-Hours
6 Chemical Energy of 1 Kilogram of Sugar: 17 Million Joules
7 Relativistic Energy of 1 Kilogram of Helium Fusion: 270 Trillion Joules
8 Household Energy Use: 43 Kilowatt-Hours
9 Conclusion . Glossary .
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