Conservation of energy

What is energy? The standard definition of energy is the capacity to do work, which describes what energy can do but not really what it is. We know that it comes in different forms, that there are different sources of energy and that there are scientific laws that tell us how it works.

Energy is a complex concept

It can be difficult to explain things that we cannot see. We can see the results of energy when we turn on the lights or run up the stairs, but it is hard to imagine that the apple sitting on our desk has energy – it’s just not using it. The apple has potential or stored energy as it sits on the desk. This can turn to kinetic energy (if it falls off the desk) and chemical energy (if you take a few bites).

We often turn to analogies or stories to explain complex things like energy. Richard Feynman, a Nobel Prize-winning physicist, used wooden blocks to explain the conservation of energy. This is a law that states that energy is neither created nor destroyed. It does not disappear when we use it – it changes from one form of energy to another.

An energy story

Richard Feynman’s story goes like this:

A child has a set of 28 wooden blocks that are absolutely indestructible. Each block is the same as the other, and they cannot be divided or broken into pieces. At the end of each day, the child’s mother counts the blocks. No matter what the child does to the blocks, there are always 28 blocks.

One day, the mother discovers there are only 27 blocks but finds a block under the rug. She has to look everywhere to be sure the number of blocks did not change. Another time, she finds 30 blocks but realises that a friend had been around to play and left two blocks behind. Mum returns the blocks to the friend, and the set is back to 28 blocks.

One day, the mother can only find 25 blocks but notices a box in the room. She’s not able to open the box, but by knowing the weight of each block and the box, she develops an equation for the blocks.

The story goes on with other places where blocks are hidden and unable to be seen, such as a bath with dirty water. The mother adds more information to her equation. The equation becomes more complex, but the quantity (28 blocks) stays the same.

Conservation of energy

Feynman’s story of the mother and the blocks is silly, but the way each new hiding place for the blocks creates a new term in the equation is similar to the way physicists deal with energy. We don’t really know what energy is, but we have learned to measure the various places where it can be found. Energy is in heat, light, chemical bonds, compressed springs, heavy objects (which can fall) and a number of other places.

A remarkable fact we have learned about energy is that it appears to be conserved. What this means is that, much like the child’s blocks, if we carefully look in all of the places energy can be found and add it all up, we always have exactly the same amount of energy. When one carefully accounts for all of the energy before something happens and after, the total amount of energy is always exactly the same.

There is an exception to this rule – the direct conversion of matter to energy governed by Einstein’s equation E = mc₂, which involves nuclear reactions.