A candle burns in the dark, flickering patiently from side to side as the wind caresses the soft, bright flame. A silent wolf pads through the white snowdrifts falling, enveloping its black-furred body as its careful breaths burst out into the cold, harsh air. A grandfather clock chimes, effortless in its movements, mechanical arms bearing down as the pendulum sways soothingly. An atom, any atom, vibrates violently, dancing unceasingly to a primordial rhythm, tremors transmitted across the echoes of time, fading. All of these are propelled by a fundamental facet of the universe: energy.
Energy is a difficult concept to define. While present in all aspects of physical reality, it is not something that can be visualized nor held in a hand to squeeze, touch, or discover. Energy is something apparent but invisible and is best explained through the concepts and underlying laws of the physical world.
The physical definition of energy is that of a quantitative property transferred to an object or a system that allows work to be done upon said object or system. Embedded in the definition is a concept of work. Work is the energy transferred by a force to move an object; mathematically, it is a force multiplied by a displacement. While the definition is cyclical, a simple example clearly illustrates the concept: by transferring energy from our bodies to an object, say a brick, we are able to lift it off the ground and throw it a certain distance. The energy is seen in the form of the force applied to throw the brick and the displacement, or how far it travels. This represents the work that was done on the object. Inherent in the transfer of energy is also the transformation of energy forms; when a person throws a brick, chemical energy derived from eating food is transferred from their body into kinetic energy — the energy of motion that propels the brick through the air. So, energy is a property that allows work to be done on an object, and it changes form as it is transferred from object to object.
The different forms of energy are several and inherent in our universe.
- Chemical energy is the energy stored in bonds between molecules. We tap into it by breaking up chemical structures (food) in our digestive system.
- Nuclear energy is related to the structure of the nucleus of an atom; this is the energy released by the sun through nuclear fusion.
- Radiant energy is the energy carried by electromagnetic waves; a small amount of radiant energy is released by all objects.
- Kinetic energy is the energy of motion, inherent in all objects that move; this includes both a duck flying through the air and a comet in space.
- Potential (or gravitational) energy is the energy stored between two displaced masses; an apple hanging in a tree has potential energy due to the force of gravity between it and the earth.
- Thermal energy is the most familiar form of energy and is commonly referred to as heat. It is often a byproduct of energy transfers and transformations.
Energy Flows in the Living World
The transfer and transformation of energy creates a flow through a system. These flows are visible in the world at different scales, and all highlight the greatly intertwined relationships of natural beings with each other and their surrounding environment. The source of this living world’s energy is the sun; a highly pressurized reactor chamber where nuclear fusion turns hydrogen into helium, releasing huge amounts of nuclear energy in the form of radiant (or electromagnetic) energy. These waves travel 90 million miles to reach the earth and provide light and heat to all its systems. The sun’s energy, captured in the form of heat, causes the atmosphere and oceans to circulate through air and water currents. The energy flows through these mediums, and in turn, lays out the dynamics of weather patterns and climate across all areas of the earth. The sun’s energy also affects individual organisms; plants on land and cyanobacteria in the oceans capture and convert its radiant energy to chemical energy. These primary producers then go on to feed all of the world’s living systems through the constant capture and recapture of said energy. These natural energy flows through living and non-living systems accomplish all this while following natural laws that cannot be broken — the First and Second Laws of thermodynamics.
Laws of Thermodynamics
The First Law of thermodynamics states that energy cannot be created nor destroyed. What this means is that the universe, since its inception, has contained the same amount of energy and that this amount will never change. In any process where energy is transferred, the energy given must equal the sum of the energy received plus the energy that is dissipated. This statement can be visualized through an energy conservation equation:
Ein = Eout + Ediss
Therefore, according to the First Law of thermodynamics, neither a person nor a machine can output work without an input of energy. As in our previous example, the brick would not gain energy and be thrown any distance if there wasn’t a resulting energy loss in the person that threw the brick. Water falling down a waterfall loses all its potential energy, but that energy does not disappear; as the water falls, the potential energy is converted to kinetic energy and heat.
The Second Law of thermodynamics states that the net entropy of a closed system will always increase. Entropy is the degree of energy dispersal in a system; following the Second Law, heat in the corner of a room will spread to all areas until it is dispersed completely; stored energy in wood is scattered as the wood decays; and roads crumble unless constantly maintained. However, this concept is not entirely intuitive. There are many instances of energy accumulation in the world around us that appear to violate the Second Law. A river flowing downstream is a perfect visualization of an energy flow; potential energy flows downstream into kinetic energy, but then it is dammed and the energy flow is halted, causing water and energy to accumulate behind the dam. While this example may seem contradictory to the Second Law, it is actually perfectly in line with it. Here is how it works.
A closed system is a system through which no matter can pass; however, energy can pass between the system and its surroundings. The closed system of the river exhibits a collection of energy behind a dam, but the work of damming the river — the mechanical and chemical energy used to dig out trenches and build up walls to stop the river’s flow — represents an energy transaction between the system and its surroundings. This work represents a loss of entropy that will always be greater than the entropy increase in the river, thereby resulting in a net entropy decrease in the universe. This is how energy can still accumulate in systems; by dispersing more total energy than was accumulated. This is how stars are born, planets are created, and living beings, vessels of accumulated energy, propagate the earth. But it is also why stars die, planets crumble, and living things fade. Therefore, the Second Law shows us that no energy transfer nor transformation can be 100% efficient; nothing in the universe can last forever.
Energy in Human Systems
All systems abide by the thermodynamic laws of the universe. Human systems, while often viewed as separate from natural ones, also obey these laws. All human operations require energy sources. The most commonly used energy source in human systems is oil, which is made up of dead marine organisms that have been condensed and pressurized for millennia into a dense, low entropy energy form. When burned, the oil created by these ancient processes is released as heat to do work such as spinning the wheels of a car. This dispersal of energy from the oil to heat represents a low-entropy to high-entropy transition. The entropic properties of energy sources are of huge significance when looking at the different sources of energy. Extracting electrical energy from renewable sources such as the sun and the wind presents unique challenges as they are less dense sources of energy. Therefore, they have much lower rates of efficiency — meaning less energy is received from the available amount.
Change of energy form (or the process of energy conversion)
, is also prevalent in human systems and can be found in the electric grid which powers much of human society. Typically, a fuel such as coal is burned to heat steam that in turn , spins a turbine to generate electricity. This represents a conversion from the chemical energy of coal to heat that is transferred to water, allowing it to make the phase change to steam. The steam rises, and its kinetic energy is transferred to the turbine in the form of mechanical energy. The turbine’s mechanical energy spins a magnet around copper wires and is converted to electromagnetic energy, or an electrical current. These conversions, and the distance that energy travels from the generator to peoples’ homes, represent energy losses that are a fundamental part of the process. One of the reasons human systems tend to ignore the realities of energy expenditure and waste discussed above is that our energy sources are currently abundant and densely stored. Pumping a massive amount of energy into the system can mitigate the consequences of the energy lost, in the short term ,. But as we see in Equation 1 energy loss is an integral part of the process.
Any living system that consumes energy behaves much like a tree. A tree first begins to grow from a supply of stored, surplus energy, known as a seed, until it can start producing energy on its own. It expends energy by growing roots to reach greater sources of water and nutrients, necessary elements for its survival. It expends energy further by growing leaves outwards to reach the sun, its source of energy. In each case, the tree’s expenditure is rewarded with greater amounts of net energy, allowing it to grow and store its own surplus energy for times of darkness and drought. The only times a tree expends energy with no return is to grow flowers and bloom, but these flowers eventually become fruit and generate new seeds; an investment in the future. An understanding and refinement of energy use in accordance with thermodynamic laws leads to better energy management practices in systems, much like a tree.