Conservation of Energy Law

“Electric Motor energy transformation” by coach_robbo is licensed under CC BY-NC-SA 2.0

Many scientists and philosophers contributed to the discovery of the law of conservation of energy. People have been aware of the concept of energy conservation since ancient Greece. These ancient Greek philosophers, such as Thales of Miletus, believed that all substances on Earth originated from one material.  

Of course, that substance was not a physical one like water, air, earth, or fire, as the ancient people thought. The idea about the “substance” being energy originated around the beginning of the 17th century. Galileo Galilei conceptualized a pendulum in which energy switches back and forth between potential and kinetic energy. However, he did not have the vocabulary or existing knowledge to state his concept in such terms. 

In the later 17th century, Gottfried Wilhelm Leibniz attempted to develop a mathematical formula that described a system’s total motion as its vis viva, or “living force.” Vis viva was a precursor to the total amount of energy, as it accounted for kinetic energy and some instances of potential energy.

In 1837, Karl Friedrich Mohr described the one “agent” of the physical world as energy or work. He claimed that energy could exist as magnetism, light, cohesion, motion, electricity, and chemical affinity. He also contended that energy could transfer between these forms.

The ultimate breakthrough came from discovering that heat energy has a mechanical equivalent, which James Prescott Joule explored in one of his experiments in 1843. He developed the “Joule apparatus,” which used a falling weight to turn a paddle in a tank of water. The paddle (mechanical energy) would raise the water’s temperature. The heat was not spontaneous; mechanical energy transformed into heat energy. You can see a picture of the paddle below.

“File: Joule Apparatus.jpg” by Dr. Mirko Junge is licensed under CC BY-SA 3.0

The public still met Joule’s experiments with some resistance and criticism. However, William Robert Grove drew upon Joule’s experiment when he published theories about magnetism, mechanical energy, light, and heat as forms of a common “force” a year after Joule’s experiments. Modern acceptance of the law of energy conservation came three years after Grove’s publication. That acceptance stemmed from a book written by Hermann von Helmholtz, which he published in 1847.


The law

The conservation of energy law states that energy can neither be created nor destroyed, but it can convert from one form to another.

It may seem like we lose or waste energy every single day. Also, there are instances where it looks like energy is created from nothing. However, energy exists in various forms all around us. Most of the fossil fuels we use in our cars once belonged to decaying animals and plants, so none of the energy we use is “created” or “new.”


The basics: potential and kinetic energy

Potential and kinetic energy are the fundamental forms of energy. Potential energy is the term for stored energy (think about a dam holding water back), and kinetic energy is movement (water flowing over the dam).

In the image below, the water cannot flow downward because a dam is in the way. The trapped water is a source of potential energy.

If the dam were to fail, the water would move by pouring over the side. When the water flows over, it doesn’t create new energy; the potential energy merely converts to kinetic energy. 

Another way to visualize the change from potential to kinetic energy is by analyzing a pendulum in a grandfather clock. The pendulum slows to a stop when it goes all the way to the left or right. When it stops at its highest point, the energy in the pendulum is all potential. When the pendulum swings back into motion, that potential energy converts to kinetic energy. The pendulum moves the fastest at the lowest central point, where all the potential energy has transformed into kinetic energy.


Different types of energy and examples of them

Potential and kinetic energy are elementary concepts. They reveal the general state of energy, but they don’t tell you about where the energy comes from or what type it is. 

In both the archery and pendulum examples, those are forms of mechanical energy. Mechanical energy consists entirely of movement or motion potential. In both examples’ uses of potential and kinetic energy, both cases are forms of mechanical energy.

On the other hand, chemical energy consists entirely of chemical reactions or the potential for chemical reactions. For example, wood is a source of chemical energy in the potential state. When it combusts, the potential energy turns to kinetic energy, and the wood’s chemical energy transforms into thermal and light energy. The forest fire below is a clear example.

Fire contains lots of heat and light energy, but that energy isn’t spontaneously created; the wood merely stores it as potential energy until it meets ideal combustion conditions. Forests are chock-full of potential energy that can convert to deadly kinetic energy in the form of heat and light.

A battery is another source of potential chemical energy. When you charge your cell phone and turn it on, the potential energy within that battery converts to kinetic energy; the chemical energy becomes electrical energy.


Where do we see the law of energy conservation in HVAC/R?

In a business that’s all about heat movement, we see it quite a bit. However, we see it more in the context of thermodynamics, which is a specific branch of physics.

The first law of thermodynamics establishes that energy performs work and exists as heat. When a system obtains heat, such as when warm air flows over an A/C unit’s evaporator coil, some of that heat performs work, and some of it does not. The heat that does not perform work doesn’t disappear from the system or the earth, as it cannot be destroyed. 

We don’t eliminate heat from people’s homes and businesses. We merely move it by allowing it to change forms before our units ultimately reject it outdoors again. In the case of heat pumps, we take the heat from the outdoors and reject it inside the building. We don’t magically create heat or remove it; we rely on natural processes to convert heat as a step in the process of moving it.




Special thanks

Modern Refrigeration and Air Conditioning, 21st edition (Andrew Althouse, Carl Turnquist, Alfred Bracciano, Daniel Bracciano, and Gloria Bracciano)

Refrigeration & Air Conditioning Technology, 9th edition (Eugene Silberstein, Jason Obrzut, John Tomczyk, Bill Whitman, and Bill Johnson)

These truly are the reference guides for the industry and deserve attribution for all such articles.

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