Conservation of energy

The law of conservation of energy is a fundamental principle of physics. It states that the total energy of an isolated system remains constant over time. In a closed system, the total amount of energy can only change if energy enters or leaves that system. A key part of this law is that energy can neither be created nor destroyed. Instead, energy is only transformed from one form to another or transferred between objects.

This principle explains why the universe operates in a predictable, balanced way.

To understand how this works, we can look at a chemical reaction, such as an explosion. When dynamite explodes, its stored chemical energy is converted into other forms. These forms include kinetic energy, which is the energy of motion, and potential energy in the flying pieces. The explosion also releases energy as heat and sound. If you add up all these different forms of energy, the total will exactly equal the amount of chemical energy that was lost during the combustion. This shows how energy moves through a process without ever disappearing.

Modern science has expanded this idea through special relativity. While classical physics treated mass and energy as separate things, special relativity shows they are related. The equation for mass-energy equivalence demonstrates that mass can be converted into energy and vice versa.

We observe this in the nuclear binding energy found in atomic nuclei, where a mass defect occurs. This relationship becomes very important in extreme environments. Examples include the universe shortly after the Big Bang or when black holes emit Hawking radiation.

Historically, humans have wondered about the permanence of the world for thousands of years. Around 550 BCE, Thales of Miletus suggested an underlying substance made everything. Empedocles, writing between 490 and 430 BCE, argued that the four elements—earth, air, water, and fire—only undergo continual rearrangement.

In 350 BCE, Epicurus believed the universe was made of indivisible units of matter. He stated that the sum total of things would always remain as it is now. These early ideas were precursors to our modern understanding of conservation.

Scientific progress accelerated during the 17th century. In 1639, Galileo analyzed the motion of pendulums. He observed that a moving body rises to the same height from which it fell. This showed how energy converts between potential and kinetic forms. In 1669, Christiaan Huygens studied collisions and noted that kinetic energy was invariant in certain cases.

Later, between 1676 and 1689, Gottfried Leibniz developed the concept of "vis viva," or living force. He used this to describe the energy associated with motion in mechanical systems.

In the 18th century, Émilie du Châtelet provided crucial evidence for these theories. She tested how kinetic energy relates to velocity by dropping balls into soft clay. Her experiments showed that the deformation of the clay was proportional to the height of the drop.

This helped prove that energy was distinct from momentum. Daniel Bernoulli also contributed significantly in 1738. He linked the kinetic energy of gas molecules to temperature and studied how energy behaves in flowing water. These studies helped move science toward understanding how energy is conserved in fluids and gases.

By the 19th century, scientists began to understand the connection between motion and heat. For a long time, people believed in the caloric theory, which said heat was a substance that could not be destroyed. However, observations showed that mechanical motion could be converted into heat through friction.

This led to the concept of the mechanical equivalent of heat. Engineers like Gaspard-Gustave Coriolis and Jean-Victor Poncelet began using "mechanical work" in their calculations. They helped bridge the gap between abstract physics and practical engineering. Today, we recognize that energy can transform into light, electricity, magnetism, and many other forms.