There are many definitions of energy, from its Greek root “force in motion” to its physical definition: “the property of a material system capable of doing work.” It is characterized as a physical quantity with the ability to modify the environment. It takes various forms: radiation, chemical energy, nuclear, thermal, or mechanical energy. Energy can modify temperature, speed, shape, chemical composition, atomic composition, or even position in a field, whether magnetic, gravitational, or electric. Not only do we use it daily, but it is also an integral part of our bodies, which can be likened to a machine converting nutrients (from the food we eat) into heat and motion.

Its primary characteristic is its ability to be easily converted into another form of energy through machines or organisms. We can distinguish between primary energy, the energy entering the system, and final energy, the energy used. Let’s take a daily example: the combustion engine of a car. The primary energy is gasoline, a form of chemical energy, which is converted into heat energy through combustion, then into mechanical energy by the crankshaft system, and finally into motion. Here, we see that energy takes on four successive forms, with each transformation resulting in some loss. This is called energy efficiency, the ratio between final energy and primary energy, and energy vectors, which refer to the intermediate steps between the initial and final forms of energy. Energy is governed by two physical laws: the law of conservation (it can neither be created nor destroyed, only transformed; the energy entering equals the energy exiting) and the law of increasing entropy (energy always tends to disperse). In other words, as Lavoisier’s famous saying goes, “nothing is lost, nothing is created, everything is transformed.”

Another example is nuclear electricity. The primary energy comes from the fission of an atom, which is converted into heat in the reactor. This heat is collected by a fluid (often pressurized water) in motion (mechanical energy), which drives a turbine that converts mechanical energy into electricity. Electricity, in particular, is never the final energy but is the most universal and transportable vector discovered to date. However, it is very difficult to store, especially through batteries, which convert electricity into chemical energy with lower efficiency, as it will be reconverted into electricity for final use.

We also distinguish between the notion of energy and power. These concepts are related and complementary: power characterizes the capacity to transform energy in a given time. This is, however, the crux of human energy use. Machines allow us to multiply human power. Jean-Marc Jancovici aptly describes it as follows: the mastery of energy transformations has turned us into “Iron Man,” and we now use it daily without even realizing it. To illustrate, please refer to the below video:

https://youtu.be/S4O5voOCqAQ

 The energy required to toast a slice of bread is the same, whether the toaster is powered by the electric grid or a stationary bicycle. However, the effort required by a human to toast a slice of bread is considerable. The same goes for moving a cubic meter of earth: a bulldozer is much more efficient than a shovel and a pair of hands.

This leads us to the next
concept: the origin of primary energy and energy density.