2.2: Forms of Energy

There are a number of forms of energy, but most of them can be reduced to either kinetic energy (energy of motion) or potential energy (energy of relative position). These two are discussed in greater detail below.

Heat energy, more accurately called thermal energy, is a form of energy that a bulk substance can have. As the temperature of an object goes up, its thermal energy content also goes up. Ultimately, however, thermal energy is just a form of kinetic energy. It is the vibrations of the molecules that make up the substance, or, in the case of the gas, the motions of the molecules zipping about that make up this thermal energy. When you heat water up, it gets hotter because the average speed at which water molecules are vibrating goes up. Indeed, that is what it means to say that water is hotter.

Internal energy is a catch-all term sometimes used to indicate energy that you’re not keeping track of. As it sounds, this is energy that is, somehow, stored inside an object. In reality, this energy is made up of kinetic and potential energy. It may be that things inside the object are moving around, and thus the internal energy you’re talking about is in the form of kinetic energy. You can, if you insist on painting with a broad brush, treat thermal energy as a form of internal energy. As another example, it may be that inside your object there are springs or other things that, as they move around, acquire potential energy as a result of their relative positions.

Chemical energy, sometimes called chemical potential energy, is, as the latter name suggests, just a special form of potential energy. It represents the energy that you could get out of a substance by performing chemical reactions with it. The chemical energy stored in gasoline may be treated as a form of internal energy, which you can extract and turn into other forms by burning that gas. On the microscopic level, what you’re doing is rearranging the atoms into different molecules. That is, you’re putting all of the atoms into different positions relative to each other.¹ Because potential energy is the energy of relative position, this means that you’re changing the potential energy of all of these atoms.

Mass is, itself, a form of energy, leading to the term mass energy. Using Einstein’s famous equation \(\ E=m c^{2}\), you can convert from mass to other forms of energy. In chemical reactions, the amount of mass that is converted to or from energy is tiny— roughly one part in a billion. This is tiny enough that chemists will talk about the “conservation of mass”, even though this is not strictly true. In nuclear reactions, however, the amount of mass that is converted to energy can be appreciable, approaching a percent. In matter-antimatter reactions, it is possible to convert all of the mass of reactants into other forms of energy.

Light energy, or more generally radiation, is energy in particles that are moving so fast (up to as fast as possible, in the case of light!) that their kinetic energy is much higher than their mass energy, if any.

Dark energy is in fact not energy in the classic sense of the word, but is the name given to the mysterious substance that fills the Universe and is driving its expansion to accelerate. We know next to nothing about dark energy, and we certainly don’t know how to convert it to other forms of energy.

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¹Later, we’ll see that talking about the position of particles in atoms is a bit troublesome, but for now this description is reasonable.