We are used to using the term energy frequently in our everyday language. We talk about electrical energy, thermal energy, solar, nuclear, or wind energy. We are even familiar with terms like kinetic energy, heat, potential energy, radiation.
But…
What is energy really?
Often, it is more difficult to define “simple” things than others that seem more complex. In general, we can say that, traditionally, classical physics considers the universe to be made up of mass and energy. However, with relativistic physics, the barriers between mass and energy dissolved. Their definition became more complex than one could have imagined a century ago. Therefore, we will give a classical treatment of energy here, which is the one we commonly use in engineering and everyday life.
Thus, we can define energy as a property of a system that is the capacity to perform work. It is important to note that the concept of work in engineering is defined as,
As we know, according to Newton’s law
It follows that energy causes a change in the kinetic state of a system. That is, energy and work have to do with the modification of the speed of the particles in a system.
For example, kinetic energy is the movement of the masses in a system. Temperature is the random motion of the particles in a body. Radiation is the movement of electromagnetic waves. Systems can interact by transferring energy from one to another, as happens in the collision of two bodies, or when 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 to an elevated reservoir to turbine it when we need energy.
Finally, let’s mention 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. Its multiples are the kilojoule (1kJ = 1,000J) and the megajoule (1MJ = 1,000,000J).
