Energy is everywhere and comes in many forms, with the two most common forms known as potential energy and kinetic energy.
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Though they’re very different in terms of how they interact with the physical world, they have certain aspects that make them complementary to one another. But to understand how they work, you first need to understand what they are — and the definition of energy itself.
What Is Potential and Kinetic Energy?
Before understanding either form of energy, it’s vital to understand what energy really is. In the simplest terms, energy is the ability to do work, which is when a force is applied to an object and it moves<1>.
Potential energy is one of the two main types of energy in the universe. It’s fairly straightforward, although slightly difficult to grasp intuitively: It is a form of energy that has the potential to do work but is not actively doing work or applying any force on any other objects. Potential energy of an object is found in its position, not its motion. It is the energy of position.
When objects are displaced from positions of equilibrium, they gain energy that was stored in the objects before being knocked out of equilibrium by elastic rebound, gravity, or chemical reactions. This is best demonstrated in an object like an archer’s bow, which stores the energy that is created from pulling back the bowstring. The potential energy stored in the pullback is responsible for the energy that occurs upon release, which is known as kinetic energy.
Understanding kinetic energy is intuitively easier because it’s more obvious that moving things have energy.
Kinetic energy is created when potential energy is released, spurred into motion by gravity or elastic forces, among other catalysts.
Kinetic energy is the energy of motion. When work is done on an object and it accelerates, it increases the kinetic energy of an object. The most important factors that determine kinetic energy is the motion (measured as velocity) and the mass of the object in question.
While mass is a universal measurement, the motion of an object can occur in many different ways, including rotation about an axis, vibration, translation, or any combination of these and other motions<2>.
There are three subcategories of kinetic energy: vibrational, rotational, and translational.
Vibrational kinetic energy is, unsurprisingly, caused by objects vibrating. Rotational kinetic energy is created by moving objects, while translational kinetic energy is caused by objects colliding with one another.
These three subcategories of kinetic energy comprise nearly all of the energy in motion throughout the known universe.
What Are the Differences Between Potential and Kinetic Energy?
The main difference between potential and kinetic energy is that one is the energy of what can be and one is the energy of what is. In other words, potential energy is stationary, with stored energy to be released; kinetic energy is energy in motion, actively using energy for movement.
Another important difference is velocity. This measurement is the basis of kinetic energy, yet has nothing to do with potential energy. In fact, velocity is the most important part of the equation when determining the amount of kinetic energy for any given object, and it is nonexistent in the potential energy equation<3>.
What Is the Relationship Between Potential and Kinetic Energy?
Although these primary forms of energy are very different, they are complementary to one another.
Potential energy always leads to kinetic energy when it is released<4>, and kinetic energy is needed to allow an object to store energy as potential, in one way or another. For example, a rock on the edge of a cliff does not directly need kinetic energy to store the potential energy that will send it down the eroding cliff face. But the act of erosion to get the rock to the edge requires kinetic energy. Therefore, the rock requires it for its potential energy.
Given that these are the two main forms of energy in the world, especially on the human scale, there’s a constant push and pull between potential energy and kinetic energy in everyday life.
What Are Examples of Potential and Kinetic Energy?
Although the definition of both potential and kinetic energy may seem pretty straightforward and simple, it’s still not always easy to tell which form of energy is present for certain objects or processes.
The movement of planets around the sun and other stars in the galaxy is kinetic energy at work. As they are drawn towards the large objects at the center of their respective orbits, due to the strong gravitational pull, they fall towards the center of the mass. This results in an orbiting motion — and all motion is a form of kinetic energy. <5>
2) Rubber Bands
Rubber bands can be classified as both potential and kinetic energy, depending on the state of the band. When an elastic is stretched, it’s loaded with potential energy; when released there is a shift to kinetic energy. This is especially true if the elastic is transporting another object, such as with a rock being flung from a slingshot. <6>
Rivers are strictly kinetic energy at work. The water is constantly moving, and all that motion is constantly creating kinetic energy. The only time a river could have potential energy is if it is dammed, with the artificial reservoir storing energy to be used when needed along a hydroelectric dam. <7>
4) Specific Variations
There are specific variations of energy within both kinetic and potential energy classifications. While some variations, such as the potential energy stored in batteries, are readily apparent, others are not as easy to identify.
What Is the Potential Energy of an Electron?
Electrons are in motion, so they contain kinetic energy. This is one of the best examples of a nonphysical object carrying kinetic energy.
However, it’s still possible to discern the potential energy that an electron has stored. To find this potential energy, it requires a complicated formula<8>:
U(r) = -qeV(r) = -keqe2/r
The total energy is the sum of the electron’s kinetic energy and its potential energy, which is represented by an even more complicated formula<8>:
KE(r)+PE(r)=-½keqe2/r=(-½)(9*109)(1.60*10-19)/(5.29*10-11) J = -2.18*10-18 J
Is a Battery Kinetic or Potential Energy?
Batteries are a form of chemical energy, with energy stored in the bonds of molecules contained within the battery acid at their core. The key word here is “stored” — meaning batteries are a form of potential energy, which is how all chemical energy is classified <9>. Biofuels and fossil fuels are other examples of stored chemical energy.
Is Electrical Energy Potential or Kinetic?
Electrical energy is classified as potential energy before it’s released and used in the form of power, which is most commonly harnessed and used as electricity<10>. However, once it is converted from its potential state, electrical energy can become one of the subtypes of kinetic energy, including motion or sound, among others.
Is Sound Energy Potential or Kinetic?
Sound can be considered both forms of energy at the same time, though the primary way we experience it is in kinetic form. Sound energy in air, which is produced from longitudinal waves that create motion in gas molecules, is kinetic. In solids and liquids, which carry sound much farther than through air, this is both kinetic and potential energy at work.<11>
Potential energy can be broken down into two subforms of energy. Each of these subforms are types of stored potential energy. But the method in which they are stored — and released — are vastly different.
What Is Chemical Potential Energy?
Chemical potential energy is stored in molecular bonds, which are also known as chemical bonds. When these bonds are broken, the stored potential energy is released and gives off varying degrees of kinetic energy, depending on the strength of the bonds.<14>
What Is Gravitational Potential Energy?
Gravitational potential energy is stored in the object due to the potential for the force of gravity to move it and pull it towards the Earth. The amount of stored potential gravitational energy depends directly on the mass of the object and, more importantly, its height above the ground.
The two subforms of potential energy covered above function primarily through the two most basic fields of science. However, the mechanism that governs potential energy in the realm of physics is much different than the mechanism that governs in the realm of chemistry.
What Is Potential Energy in Physics?
Potential energy in the realm of physics is in the form of gravitational potential energy. Gravity is perhaps the most important element of physics, as it is the basis of general relativity, which the entire modern world is based upon. It is this force that created potential energy in physics terms.<16>
What Is Potential Energy in Chemistry?
Potential energy in the realm of chemistry is in the form of chemical potential energy. This is energy stored in molecular bonds, which is the basis of chemistry and chemical reactions. Stored potential energy is released through these chemical reactions.
What Are the Formulas for Kinetic Energy and Potential Energy?
The formulas for potential and kinetic energy are fairly straightforward, but they are by no means simple.
Kinetic energy can be found using the formula: KE=12mv2
m = mass (kg) v = velocity (m/s)
Gravitational potential energy can be found using the formula: W = m×g×h = mgh
m = mass (kg) g = acceleration due to gravitational field (9.8 m/s2) h = height (m)
Elastic potential energy can be found using the formula: U=12kx2
k = spring force constant x = stretch length (m)
The units used to measure each of these forms of energy is the same as all the other forms of energy: the Joule (J), which is equal to 1 kg.m2.s-2.<17>
You have learned about several forms of energy — along with some pertinent examples — but there is much more that needs to be covered to fully understand the concept of energy.
However, understanding the two main forms of energy that permeate not only everyday life, but also the core of physics throughout the universe, is a good starting point and sets the foundation for any deeper dives into the nature of energy itself.
There are only two things that absolutely must be remembered: stationary objects with stored energy have potential and moving objects are kinetic.
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<2> Priddy B. What Is the Meaning of Kinetic Energy? https://sciencing.com/meaning-kinetic-energy-6646801.html. Published March 2, 2019. Accessed November 9, 2020.
<3> Work, Energy, and Power. The Physics Classroom. https://www.physicsclassroom.com/Class/energy/u5l1c.cfm. Accessed November 9, 2020.
<4> Energy: Potential and Kinetic Energy. Infoplease. https://www.infoplease.com/encyclopedia/science/physics/concepts/energy/potential-and-kinetic-energy. Accessed November 9, 2020.
<5> Fenio, B. Rolling Race: A Spinning Science Activity. Published March 23, 2017. https://www.scientificamerican.com/article/rolling-race/. Accessed November 15, 2020
<6> Science World. Elastic Energy. https://www.scienceworld.ca/resource/elastic-energy/. Accessed November 16, 2020.
<7> University of Calgary Energy Education. Energy from Water. https://energyeducation.ca/encyclopedia/Energy_from_water. Accessed November 15, 2020.
<8> University of Tennessee, Knoxville. Electric Potential. http://labman.phys.utk.edu/phys222core/modules/m2/Electric%20potential.html#:~:text=The%20potential%20energy%20of%20the,2.18*10%2D18%20J. Accessed November 15, 2020
<9> Dusto A. Potential Energy: What Is It & Why It Matters (w/ Formula & Examples). Sciencing. https://sciencing.com/potential-energy-what-is-it-why-it-matters-w-formula-examples-13720804.html. Published December 5, 2019. Accessed November 10, 2020
<10> U.S. Energy Information Agency. Forms of Energy. https://www.eia.gov/energyexplained/what-is-energy/forms-of-energy.php. Accessed November 15, 2020.
<11> Sound Waves and Music – Lesson 1 – The Nature of a Sound Wave: Sound as a Longitudinal Wave https://www.physicsclassroom.com/class/sound/Lesson-1/Sound-as-a-Longitudinal-Wave Accessed November 15, 2020.
<12 University of Illinois Department of Physics. Q&A: Kinetic and Potential. https://van.physics.illinois.edu/qa/listing.php?id=1768&t=kinetic-and-potential#:~:text=Sound%3A%20In%20a%20solid%2C%20this,microscopic%20scale%20of%20moving%20molecules.. Accessed November 15, 2020.
<13> Solar Schools. Radiant Energy. https://www.solarschools.net/knowledge-bank/energy/types/radiant. Accessed November 15, 2020.
<14> CK-12. Chemical Potential Energy. https://www.ck12.org/chemistry/chemical-potential-energy/lesson/Chemical-Potential-Energy-CHEM/. Accessed November 15, 2020.
<15> Solar Schools. Gravitational Energy. https://www.solarschools.net/knowledge-bank/energy/types/gravitational. Accessed November 15, 2020.
<16> Hyperphysics. Gravitational Potential Energy. http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html. Accessed November 16, 2020.
<17> Kinetic & Potential Energy. Kinetic & Potential Energy – Web Formulas. https://www.web-formulas.com/Physics_Formulas/Kinetic_Potential_Energy.aspx. Accessed November 9, 2020.