What the heck is Energy?

“My kids have been sick for over a week now so I am feeling a little low on energy”.  This is a comment I made to my students this morning and it got me thinking about this word “energy” and how often we use it in our lives but have you ever asked yourself what is energy?

In my forthcoming book I speak about energy all the time and now I am realizing how I may need to do some editing to try and define this frequently used but ill-defined word.  Think about how many times you use the word in your own life.  You likely use the word to describe:

  • your own energy levels – may be related to sleep or food
  • you might say things about other people such as,  “that person has good energy”
  • how much energy certain activities take – such as shoveling snow
  • the energy efficiency of your home – related to R values in windows, doors, and insulation
  • the energy efficiency of your car – is it hybrid, electric, diesel, Ecoboost
  • The energy used by lights or appliances in your home
  • The energy exhibited by “energetic people”

You can see that you probably use the word regularly in your day but what does it mean?  When I used to teach Physiology, I would talk about how the body uses oxygen and sugar (glucose) inside our cells to produce energy.  This process in your cells where energy is produced using sugar and oxygen produces “energy” by breaking apart a molecule of something called ATP.  ATP are like little battery packs that produce energy.  We have, at any given time, 1 BILLION molecules of ATP in our cells!  With oxygen present, each molecule of sugar (glucose) from our food, produces 32 ATP.  Energy is produced when ATP is broken apart.  

BUT still… what is ENERGY?

As it turns out, there is no simple and easy answer!  Even renowned physicists such as the late Richard Feynman – a Nobel laureate have been quoted on his answer to the question,  “It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount.”

We often say that  energy is “ability to do work”.  Energy is often divided into forms such as: thermal, chemical, radiant, nuclear, electromagnetic, mechanical.  Work is defined as a change in position, speed, state, or form of matter.  

One thing we know for certain about energy is while it may change in its type: mechanical into thermal, for example, the amount of energy (measured in Joules or kilojoules) is never lost.  This is known as the law of conservation of energy.  Feynman said this of the law, “There is a fact, or if you wish, a law, governing all natural phenomena that are known to date. There is no known exception to this law—it is exact so far as we know. The law is called the conservation of energy. It states that there is a certain quantity, which we call energy, that does not change in manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.”

Not only is energy conserved – the amount of energy present in the universe has not changed since the beginning of time.  Never more; never less – just changed in form.

So, we can measure energy, it can exist in different forms and if it is converted to other forms it is still not lost but….still…. what is it?!

We really do not have a good answer.  

In addition, there are 2 broad categories of energy:

1) Potential – stored energy

2) Kinetic – energy possessed by an object by its movement

We know that energy is required to do work and this is measured by things like heat, change in distance, change in tension, change in mass but this invisible entity we call energy is not so easily defined – yet easily measured.  Quite a conundrum.

So, the next time you feel low in “energy” you can rest easy that while we can’t easily define energy, we can measure it and we can change it.  The most recognizable calculation of energy is Einstein’s famous calculation is E=mc2.  This famous calculation says that Energy = mass times the speed of light (squared).  So mass effects energy.  The more mass; the more energy.  

To summarize and address my opening comment about my low energy – I guess it represents a decreased ability to do work.  So, to recharge my batteries, I permit myself to lie on the couch and do less work!




  1. It is interesting that even the etymology of the word is not well known either. The Greek root “energaia” was used around 500 BC to mean “the source of activity”. Circa 350 BC Aristotle used the Latin form “energia” and defined it as a “realized state of potentialities” (presumably having to do with that which has the ability to bring about something else).

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