There is a school of thought that classifies all energy as either potential energy or kinetic energy. Other scientists dislike this division, arguing that it is artificial; however, the terms ‘potential energy’ and ‘kinetic energy’ are frequently used, so it is important to understand what they mean.
Of the two, kinetic energy is easier to define. The popular definition is that ‘kinetic energy is the energy of motion’. Every moving object has kinetic energy, and the faster it moves, the greater its kinetic energy. When it stops moving, it no longer has kinetic energy.
(In fact, this is not strictly true, because there are two kinds of kinetic energy: external and internal. Even when an object it stationary, it may contain moving atoms and molecules, or thermal energy, and therefore a small amount of internal kinetic energy. However, internal kinetic energy will not be mentioned in this brief explanation of potential energy.)
When a car is brought to a rapid halt, large amounts of kinetic energy must be converted into other forms of energy. Much of this can be converted into heat energy, which is then dispersed by the braking system. However, the automotive industry is very interested in recovering kinetic energy, because one way in which recovered kinetic energy can be used is to re-charge a battery. This has been an important factor in the development of hybrid vehicles.
As explained above, when a moving object stops moving, it no longer has kinetic energy; instead, it has potential energy. Potential energy is stored within a substance, waiting to be released and converted into another form of energy. Electrical potential energy is released when an appliance is switched on and starts to work. Chemical potential energy is released by a chemical reaction when certain substances are mixed together; unstable compounds such as nitroglycerine have a lot of potential energy that can be released very easily. Similarly, nuclear energy is potential energy stored in the nuclei of atoms, and this can be released by fission or fusion.
Two other types of potential energy are ‘elastic’ and ‘gravitational’. Elastic potential energy is created when mechanical energy is stored in an object – for instance, by winding up a spring or stretching a rubber band. The tighter it is wound or the further it is stretched, the more potential energy it will have, because more energy has gone into the winding or the stretching. When the spring or the rubber band is released, the elastic potential energy will be released and converted into movement.
Gravitational potential energy is similar. Imagine lifting a picture off the floor and hanging it on the wall. The higher you lift it, the more energy is being transferred from you to the picture – in this case, gravitational energy. If, after you have hung it on the wall, the hook breaks and it falls off, then gravitational potential energy has been released and it falls to the ground because of the force of gravity. No supplementary energy source has been involved in making it fall.
Illustrations of Potential Energy
Fossil fuels contain potential energy. This can be viewed as stored solar energy, since it came originally from sunlight; or it can be viewed as chemical energy, since the solar energy has been transformed by the process of photosynthesis. When released through combustion, this potential energy is transformed into heat and light.
An important application of gravitational potential energy, sometimes described as ‘the energy of position or place’, is in hydroelectric installations, where electricity is generated by the force of water rushing down from a high level to a lower one.
Spring-wound pendulum clocks provide an illustration of gravitational potential energy linked with elastic potential energy. It is the release of gravitational potential energy that causes the pendulum to swing; but with each swing a small amount of energy is lost through heat, and the pendulum would soon stop if it was not connected to a spring. The spring contains elastic potential energy – provided by the mechanical action of winding – which is released gradually, so that the clock keeps ticking until all the potential energy has been converted back into movement.
Energy Cannot be Destroyed
Potential energy has applications in day-to-day life; but perhaps the main reason for exploring this concept is because it provides an explanation of how energy continues to exist when it is not actually doing any work. It is essential to understand this in order to fully understand the first law of thermodynamics, which that states that energy cannot be destroyed, and which is one of the most commonly-cited laws relating to the study of energy.