# Different kinds of energy Are they relative?

I'm not sure if this is a physics-based question. There are many kinds of energy in physics.

For example, kinetic energy, potential energy, thermal energy, electrical energy Magnetic energy, radiant energy, nuclear energy, gravitational energy, etc.. are defined physically.

We can 'measure' and 'observe' those energies.

Newton and the theory of relativity are easy to understand that in the case of kinetic and potential energies, the total amount of energy is measured differently depending on the observer.

What I'm curious about is that there are other forms of energy such as electrical energy, light energy, and thermal energy, but is it also measured with different values depending on the reference frame?

I wonder if some energies can have absolute values independent of the observer.

There are many kinds of energy in physics.

There are only two basic forms of energy: kinetic (energy of motion) and potential ( stored energy of position). All "kinds" of energy boil down to being either kinetic or potential.

For example, kinetic energy, potential energy, thermal energy, electrical energy, Magnetic energy, radiant energy, nuclear energy, gravitational energy, etc.. are defined physically.

Magnetic, nuclear, and chemical energy are stored potential energy at the molecular or atomic level. Gravitational energy is potential energy associated with the relative position of two or more bodies. Electrical energy may be potential or kinetic energy. Radiant energy, the energy of the motion of electromagnetic waves (or motion of photons), is kinetic energy.

We can 'measure' and 'observe' those energies.

Energy is not a thing in and of itself to be measured or observed. Generally we cannot say this object contains so much energy. But we can measure or observe the consequences of the usage or transfer of energy in terms of work or heat under a given set of circumstances, based on changes in measurable physical properties of substances such as temperature, pressure, volume, etc.. Such measurements or observations show us that energy is always conserved when it changes from one kind or form of energy to another.

...in the case of kinetic and potential energies, the total amount of energy is measured differently depending on the observer.

That is only true in the case of mechanical kinetic or potential energy. An example is the kinetic and gravitational potential energy of a ball in motion and at a certain elevation where the kinetic and potential energy depend on the frame of reference of the observer. These are the macroscopic (observable) or external kinetic and potential energy of the ball.

On the other hand, the molecular kinetic energy (associated with the temperature of the ball due to molecular motion) and molecular potential energy (associated with intermolecular forces, chemical bonds, etc.) of the ball generally do not depend on the reference frame of the observer. These comprise the internal (microscopic) molecular kinetic and potential energy of the ball.

What I'm curious about is that there are other forms of energy such as electrical energy, light energy, and thermal energy, but is it also measured with different values depending on the reference frame?

Thermal energy is an ambiguous term that should be avoided as it sometimes used to describe heat (which is energy transfer due to temperature difference), molecular kinetic energy (which is properly called internal energy), and temperature. The other energy kinds of energy are reference frame independent as previously discussed.

I wonder if some energies can have absolute values independent of the observer.

The kinds of energy that are independent of the observer were discussed above. In general, we do not consider absolute values of energy but rather changes in energy.

Hope this helps.

• This is a very clear answer. My doubts seem to have been resolved at once. Thank you for your reply. Mar 18 at 3:03
• You use the phrasing 'transfer of energy'. I prefer the phrasing 'conversion of energy'. The reason for bringing this up: recently (on another forum) someone insisted on defining 'work done' as transfer of energy. As a consequence, it was insisted to me, when a self-propelled vehicle accelerates no work is done. If the vehicle is an electric car: the potential energy comes out of the internal battery pack, therefore, it was insisted to me, there is no net transfer of energy. That illustrates: some people can latch on to a word (here: 'transfer') in a way that defies understanding. Mar 18 at 13:03
• @Cleonis Energy transfer and energy conversion are two separate things. When I say "energy transfer" I am strictly referring to energy in transit due to work and heat, not the conversion of energy from one form to another (e.g., chemical potential to electrical potential in a battery). Mar 18 at 15:03

The names 'kinetic energy' and 'potential energy' are names of categories. As pointed out in the earlier answer by contributor 'Bob D', it is standard practice to assign a particular form of energy to one of those two categories.

Kinetic energy is mainly known in the form of motion of objects with inertial mass.

Example of a form of potential energy: a charged capacitor. When a capacitor is charged then that state of charge is categorized as a form of potential energy.

A very common form of electric circuit is one that includes both inductors and capacitors. The simplest case is an LC circuit with a single capacitor and a single inductor.

The way an inductor is wired gives it the following property. When you try to ramp up a current through that wire the change of current strength induces a magnetic field that acts in opposition to that change of current strength. However, once a current is going then a decrease in current strength induces a magnetic field that tends to sustain the current strength. This property of an inductor is referred to as 'inductance'.

Inductance is analogous to inertia. In accordance with that we can see the energy associated with the current in a circuit that includes a inductor as a form of kinetic energy.

Energy stored in the form of elastic deformation of a material is categorized as a form of potential energy. Example: in compressing a coil spring work is done; compressing a coil spring stores potential energy in the form of deformation of the coil spring.

By analogy with that it is natural to think of a gas spring as storing elastic potential energy.

Looking closer at the physics of a gas spring:
As we know: compressing the gas inside the gas spring does not compress the gas molecules in the gas spring. As the piston is pushed in the piston is doing work upon the gas, increasing the kinetic energy of the gas molecules. The pressure that the gas exerts upon the walls increases for two reasons: the molecules have higher kinetic energy, so the collisions with the walls are harder collisions, and the compression makes the density of the gas higher and because of that the collisions occur more frequently.

So:
Sometimes - on closer examination - a form of storing energy that looks like potential energy turns out to be conversion to a form of kinetic energy.

That said, when from a distance it looks like potential energy, while it's really kinetic energy, it is always a case of confined kinetic energy. The kinetic energy is confined to a finite volume of space.

Another example of confined kinetic energy is a flywheel energy storage.

A flywheel energy storage device can be charged up rapidly, and it can discharge its energy rapidly. As far as the end user is concerned, it's an energy storage device.