What is Energy?

Teaching the Concept of Energy

The concept of energy is both subtle and complex.

Energy is probably the first abstract quantity that students will encounter in their educational careers.

What differentiates energy from other abstract qualities, such as friendship or beauty, is that energy is quantifiable; that is, a specific numerical value can be assigned to the energy properties of an object.

What makes teaching of the concept of energy (and power) difficult is the fact that both energy and power are terms that are used (unscientifically) in everyday language. This may lead one to feel that they have an intuitive notion about energy, a notion which may be inaccurate or perhaps even completely incorrect.

Step one in understanding the concept of energy, and achieving scientific literacy, is to dispel the everyday colloquial notions about energy and replace them with accurate scientific definitions.

Once students have mastered the scientific definitions which apply to work, energy, and power, most of the difficulties in understanding these concepts disappear.

ENERGY: A DEFINITION

One of the most misunderstood, and subtle, concepts in physics is that of ENERGY. The confusion about energy arises from several basic misconceptions. To fully understand energy it helps to begin with some key facts that are known about energy and then to look at some of the consequences of these important facts.

ENERGY is the property of a physically identifiable object (such as a brick) or a field (such as a magnetic field) which allows it to do measurable work on another object or field.


FACTS ABOUT ENERGY

  1. TRUE: Energy is conserved; it can neither be created nor destroyed.
    This simple fact is one of the most important laws of physics. It is known as the Law of Conservation of Energy.

  2. TRUE: The total amount of energy in the Universe is constant ... this total amount NEVER changes (see above).

  3. TRUE: Energy is the currency of nature. No process or event can occur without energy being transferred.


MYTHS ABOUT ENERGY

  1. FALSE: Energy can be created.

  2. FALSE: Energy can be destroyed.


A USEFUL ANALOGY

Wealth is to the economic world - as energy is to the natural world.

For example, wealth can take many forms. It may be in the form of cars, jewels, real estate, money in the bank, or potential earnings. Nothing in the economic world can happen unless money is transferred (wealth is transferred). The currency of the economic world is the dollar.
Similarly, energy can take many forms. It may be in the form of kinetic energy (heat and motion), gravitational or chemical potential energy, or even in the form of mass. Nothing in the natural universe can change unless energy is transferred. The currency of the natural world is the joule.

Another subtle, but very important similarity between wealth and energy can be noticed whenever an event, such as the purchase of an automobile, occurs. We never actually see the dollars transferred, we simply assume the transaction has taken place based on the effects we observe, namely the new automobile owner driving the car off the car dealer's lot.
Similarly, when we observe a bowl of hot soup cooling down we don't actually "see" joules of energy being transferred to the surrounding air, but we assume that this is happening based on the effects that we observe (soup getting cooler and the surrounding air getting warmer).

Hucksters, Quacks, and Charlatans

It is the public's misunderstanding of the basic concepts of energy (and the Law of Conservation of Energy) that allows unscrupulous entrepreneurs to swindle the public out of millions of their hard earned dollars every year.

The range of fraudulent products and outrageous claims runs from perpetual motion machines to miracle medical cures.


SOME AMAZING DISCOVERIES ABOUT ENERGY

  1. In 1905 Albert Einstein made a remarkable discovery. As result of a theoretical study he was able to show that mass was in fact just a very compact (although very different) form of energy. Einstein predicted that if matter were converted to energy the amount of energy released would be tremendous.

    In fact he showed that the relationship between matter and energy could be described by the equation

    E = m c2

    where E is energy released (in joules) when m kilograms of matter is converted into energy. c is the velocity of light in a vacuum, namely, 3 x 108 m/s therefore, c2 is 9 x 1016, that is, 9 followed by 16 zeros or 90 000 000 000 000 000.

  2. Here is a truly amazing discovery! Most of the energy in the Universe is in a few forms. The first source of energy is from ordinary matter and radiation, that is stars and galaxies (galaxies are clusters of billions of stars, as well as clouds of dust and gas) The visible (known) mass-energy part of the Univese, all the matter and energy with which we are familiar, makes up only about 4-5% of the total mass-energy of the Universe!

    The second major component of the Universe is in the form of "cold dark matter". This is an (as yet undiscovered) exotic form of matter that makes up about 23% of the Universe. It's existence is known from its gravitational effect on the motions of clusters of galaxies and its effect on the dynamics of stars within large galaxies.

    Finally, there has recently been discovered an exotic form of energy which appears to be causing the expansion of the Universe to undergo acceleration. This "dark energy" appears to dominate the Universe, and makes up over 70% of the Universe. It is extraordinarily weak and diffuse and uniformly distributed throught the Universe. It cannot be detected in the laboratory. It cannot be detected in galaxies or even clusters of galaxies. It can only be detected on the scale of the Universe itself..and yet "dark energy" accounts for more than 70% of the composition of the Universe.*

SUMMARY

  1. Energy is a property or characteristic of an object (or field) which is quantifiable (measured in joules).

  2. Whenever anything happens, energy is transferred!
* [Recent astronomical discoveries have lead the realization that even empty space is itself a source of a very mysterious "dark" energy which seems to have anti-gravity properties. This energy, which works against gravity, has been discovered by noticing that the expansion of the Universe is faster than would be expected if gravity where slowing the expansion of the Universe.

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