We are used to using the term energy in our everyday language. We talk about electrical energy, thermal energy, solar, nuclear, or wind energy. We are even familiar with terms such as kinetic energy, heat, potential energy, and radiation.
What is energy really?
It is often more difficult to define “simple” things than others that apparently seem more complex. In general, we can say that, traditionally, classical physics considers the universe to be made up of mass and energy. Nevertheless, with relativistic physics, the barriers between mass and energy dissolved. The definition of them became more complex than what, a century ago, one could imagine. For this reason, we will make a classical treatment of energy here, which is the one that we usually use in the field of engineering and in everyday life.
Thus, we can define energy, as a property of a system that has the capacity to perform work. It is important to highlight that the concept of work in engineering is defined as,
As we know, according to Newton’s law
It is deduced that energy causes the modification of the kinetic state of a system. That is, energy and work are related to the modification of the velocity of the particles of a system.
For example, kinetic energy is the movement of the masses of a system. Temperature is the random movement of the particles of a body. Radiation is the movement of electromagnetic waves. Systems can interact by transferring energy from one to another, as occurs in the collision of two bodies, or by bringing two systems with different temperatures into contact.
Similarly, we assume that this energy, under certain circumstances, can be stored in what we call potential energy. This potential energy can be gravitational, electrical, magnetic, hydraulic, or chemical. This stored energy can be converted back into work. This process occurs, for example, in electric batteries, or when we pump water into an elevated tank to turn it into energy when needed.
Finally, it is worth mentioning that the unit of energy is the Joule (J), which corresponds to the amount of energy needed to exert a force of one Newton over a displacement of one meter. The multiples are the kilojoule (1 kJ = 1,000 J) and the megajoule (1 MJ = 1,000,000 J)