Energy touches upon everything we do. From the lights we turn on in the morning, to the car we drive to work or school, to our ability to read this page. Energy is both a constant in the human existence while also representing the process of change. All of our daily activities are possible because of the conversion of one form of energy to another. As such, scientists and even philosophers through the ages have tried to understand and come to terms with the concept. Yet until recently, a clear understanding of energy has escaped us. So how did we come to understand it?
The four-element theory was championed by Aristotle and other influential philosophers of Ancient Greece, but in practice this theory provided a very poor framework by which we could come to understand the universe, especially with respect to energy. Yet due to Aristotle’s influence, the philosophy held strong against all challengers for almost 2,000 years. A true understanding of energy eluded Western science throughout most of the Middle Ages.
In 1687, the legendary scientist Isaac Newton (see Figure 2) published his Mathematical Principles of Natural Philosophy (casually known as the Principia), in which he described the laws governing bodies in motion. The Principia gave a thorough, mathematical description of the laws of motion, providing the first accurate description of the energy associated with moving objects: mechanical energy. Since Newton’s time, scientists have described in great detail other major forms of energy, including thermal, chemical, electrical, electromagnetic, and nuclear.
What is energy?
At its foundation, energy is a very simple concept. The term was first invented by Thomas Young in 1807 after the Greek word energeia, which roughly translates to “activity.” Energy is the ability for an object to perform work on another object. At its core, this is a simple definition; however, it can manifest itself in many ways.
Mechanical energy is the energy possessed by an object because of its movement or position. We often describe mechanical energy in two different forms, potential and kinetic. Potential energy is so-called “stored” energy because it is the energy an object possesses as a result of its position in relation to a field of force, such as gravity. For example, when we lift a ball off the ground into the air, the object gains potential energy as it is moving to a position in which gravity will cause it to drop to the ground if we release it. If we then drop the ball, the potential energy that is present is converted into kinetic energy, which is the energy associated with the motion of an object. The concepts of potential and kinetic energy do not apply only to mechanical energy. A battery that is not connected to a circuit is considered to have potential energy that can be converted to electrical energy if the circuit is closed. Potential energy is found in other energy forms, as well.
The conversion of potential into kinetic energy is demonstrated in the following example using a weight and pulley (see concept simulation below). As the individual lifts the weight against the force of gravity, the potential energy rises. Dropping the weight will convert this potential energy into kinetic energy momentarily as the object is falling. The higher we lift the object, the more potential energy we impart as it is further from the surface of the Earth, and so the more kinetic energy it releases as it falls. You can try this experiment yourself; the link below allows you to lift and drop a 100-kg weight. You can observe how potential and kinetic energy are related to each other by “freezing” the weight at different heights during its fall.
Thermal energy is also associated with motion, but in this case it is the motion of objects at the atomic level. Thermal energy is derived from the kinetic energy of atoms or molecules within a system. In other words, the atoms and molecules of all substances are in constant motion at any temperature above absolute zero, and this is true even for solids. The thermal energy of a material, as measured by its temperature, is related to the motion of the atoms and molecules within that material – hot materials contain molecules moving very rapidly, while colder materials have molecules moving more slowly.
So what’s the difference between thermal and mechanical energy? Put simply, it’s the scale of the problem. For example, if you touch a hot pan, the atoms of metal are moving very quickly, and they can transfer the thermal energy on that atomic level, causing pain and even burns. Mechanical energy is the energy released when you drop the pan because it is hot.
Chemical energy is also related to atoms and molecules, but it is a function of their structure and interaction as opposed to their motion. As described in our Chemical Bonding module, atoms and molecules can bond with one another as a result of their electron structure. Similar to when we lift or drop an object within a gravitational field of force, the bonding of two atoms represents an interaction within an electromagnetic field of force. And this interaction results in a change of energy. In some cases chemical changes require the input of energy, while in other cases chemical changes give off energy in the form of heat, mechanical energy (such as during an explosion), or electrical energy (such as in a battery).
Atoms also contain energy stored as a function of their internal structures. Within the nucleus of the atom, the strong nuclear force keeps protons and neutrons bound together. The breaking or forming of these nuclear interactions can take up or release nuclear energy. These processes can occur naturally, as when a radioactive element like uranium decays. In fact, the heat from naturally-occurring radioactive elements in the Earth’s crust contributes substantially to the production of heat at the Earth’s core. They can also be stimulated to occur artificially, and the nuclear energy released by these stimulated radioactive processes is what powers nuclear power plants.
Another form of energy is associated with particles even smaller than atoms. Electromagnetic energy is caused by the motion of photons, which are packets of energy that behave both like particles and like waves. Photons make up all the forms of electromagnetic radiation that we are familiar with, like visible, infrared, and ultraviolet light, radio waves, and microwaves, as well as those we might be less familiar with, like gamma radiation. Electromagnetic energy often causes changes in the energy level of electrons within atoms, or the motion of atoms and molecules. For example, water molecules can absorb microwave radiation, which causes them to vibrate, increasing their thermal energy, and heating your food in the process. And ultraviolet waves can cause damage to the molecules in your skin itself, causing a sunburn even in the absence of thermal energy.
Electrical energy also is based in the movement of particles associated with atoms, but in this case it is the flow of electrons within a system. Electrical energy can be generated in a number of ways. For example, certain chemical reactions, such as those that take place in batteries, cause electrons to flow. In addition, physically moving a conductor like a metal in a magnetic field can generate electrical current. And light can also stimulate the flow of electrons in certain materials, such as photovoltaic cells. Once electrical current is generated, it can flow through materials that have “loosely” attached electrons, specifically metals, making them good conductors of electricity.
Other forms of energy
While we are most likely to encounter these six forms of energy in daily life, they are not the only ways that energy can be seen. In general, however, other forms of energy are really special descriptions of the six forms we've discussed. Sound energy, for example, can create concussive forces through vibration of air particles. This is really a specific form of mechanical energy. The key point is that you will encounter energy in a large variety of ways.
James Joule and the conversion of energy
Energy manifests in so many different forms that at first glance it may seem impossible to relate them. In fact, for a long time scientists thought each form of energy represented a unique property about the universe. For example, it was thought that work (mechanical energy) and heat (thermal energy) were two completely separate entities!
This concept of separate forms of energy was finally successfully challenged by James Prescott Joule (1818–1889), a Scottish chemist who contributed several important experimental findings to our understanding of energy (Figure 3). His most famous experiment, published in 1845, used a paddle wheel to show that different forms of energy are interchangeable. This concept was not entirely discovered by Joule; several researchers attempted to demonstrate that heat and energy were interconvertible before Joule, but their experiments were poorly designed, leading to ambiguous results that were challenged. Joule, through diligent planning and careful measurement, was the first scientist to back this concept with solid data. To do this, Joule built a special pulley system and a sealed water vessel that was insulated from the environment. He used a system of weights to perform a precise amount of work on the paddle wheel, which, in turn, caused the water in the vessel to move. The friction associated with stirring the water inside a vessel raised the temperature of the water, which could be measured using a thermometer. His experimental data showed that a weight of 772 pounds falling one foot would raise the temperature of one pound of water by one degree Fahrenheit. In doing this, Joule was the first to clearly demonstrate beyond the shadow of a doubt that mechanical energy could be changed into thermal energy.
In many ways, this concept of transforming motion into energy is very obvious to us in modern times. If you rub your hands together, they get hotter due to friction. It’s important to note, however, that scientists of James Joule’s day did not find this concept intuitive. They thought that heat was caused by a substance stored within hot objects called “caloric,” and as this substance moved, you could get work (for more about the caloric theory of heat, refer to our Thermodynamics I module). By understanding that thermal energy and mechanical energy were two different forms of the same thing, Joule demonstrated a critical concept and added greatly to our understanding of energy.
Interestingly, the idea that forms of energy could not be converted was not limited to the interplay between mechanical and thermal energy. Prior to the 1800s, the relationship between electricity, magnetism, and light eluded scientists. They were each said to require a different “ether” and had unique properties. This changed, however, when Michael Faraday (shown in Figure 4) invented the electric generator in 1821. Faraday had been experimenting with devices to maintain a constant electric current. He found that he could achieve this by rotating a copper disk within a magnetic field. Remarkably, he found that he could cause the opposite effect as well: Running a current through the disk triggered the disc to rotate. Faraday was able to demonstrate that, with the right design, this rotation could be used to power a shaft. Faraday had invented a precursor to the electric motor and in doing so was one of the first scientists to show that electrical and mechanical energy could be interconverted.
Certainly, there are many different ways to convert energy. Describing each could fill volumes of books: Heat released by nuclear energy drives the formation of steam and is used to turn giant turbines to generate electricity, our body uses chemical energy to power our muscles, and the list goes on. It is possible to transform any form of energy into any other.
As discussed already, all of the different manifestations of energy are related. They all describe a system’s ability to perform some kind of work. As such, they can all be measured using the same unit. Because James Joule was among the first scientists to document the phenomenon of energy conversion, and because his observations were so carefully detailed in his writings, we now call the unit used to measure energy the joule. This unit formally describes the energy it takes to produce one newton of force over a distance of one meter or the electrical energy it takes to pass one ampere of current through a one-ohm resistor for one second.
While it is important to recognize that we use only one unit to convey the amount of energy used or given off in a process, it is also important to recognize that we rarely, if ever, can directly measure the energy output itself. Measuring the amount of energy converted in a process is usually done by analyzing changes in other parameters, like temperature, and then calculating backwards to joules.
For example, if you needed to know how much chemical energy is stored in a block of wood – and if you don’t have access to tables with that information – there is no tool that we can use to say “that wood contains this much chemical energy.” In order to determine the amount of energy contained in the substance, we must release that energy by burning the block of wood and then measuring the temperature change in the surroundings (this is usually measured using calorimetry, a technique you can read more about in the Thermodynamics I module).
Without experiment, it would not be possible to know the quantity of energy in an object. As such, measuring energy is perhaps one of the more elusive concepts encountered when we discuss energy. Energy is the potential to do work; however, until it gets used up, we cannot observe how much energy we had!
The importance of energy to science
From astronomy to zoology, all forms of natural science rely on an understanding of energy to some degree. In the physical sciences, our understanding of energy flow helps to predict chemical reactions, determine the trajectory of objects, and many other processes. In the life sciences, energy is used to study how enzymes work and why different biomolecules interact in certain ways. Energy is a fundamental concept for all students of science, and it is a cornerstone for existence at large.
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