# Wondering about Energy [duplicate]

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Me and Energy

I'm trying to move along with my study of non-advanced physics but not grasping what energy really is, is driving me nuts. Whenever i see anything about energy ( Kinetic, Potential, Energy in light ) i don't precisely understand what it is talking about and I can't move forward.

Begins of Understanding

After searching in Wikipedia and reading a Feynman's article I was able to have a basic idea of what energy is about.

Given we determine a physical system ( a portion of the universe ), there is a quantity associated with that physical system at each instant of time that we call energy. If the physical system is isolated, that quantity shall be conserved as time passes ( Law of Conservation of Energy ).

Also, that specific quantity ( called energy ) is the sum of a set of other quantities, each of which related to mutually exclusive ( else, the sum of the quantities would be less or more than the total energy ) characteristics of the isolated physical system ( each one corresponds to a different energy form and we have formulas to calculate them ).

Doubts, many doubts
But i'm still far away from grasping it.

1 - Do we have a well-defined set of mutually exclusive energy forms for each physical system, such that their sum corresponds to the total energy of the system ? What are them, exactly ?

More detailed explanation about Doubt 1
I can read about many kinds of forms ( chemical, thermal, nuclear, electric, kinnetic, potential, mass, light, etc,etc ) but this tells me nothing about the well-defined set of mutually exclusive energy forms. For instance, the listed ones are certainly not mutually exclusive, afterall we could just see thermal energy as a mixture of kinnetic energy ( sensible thermal energy ) and potential energy ( latent thermal energy ). Same thing for chemical and nuclear ( a kind of potential energy at the atomic-level ). We could even extrapolate that for mass ( a kind of potential energy inside the nucleons ). In the end, almost all kinds of energy form could be viewed as Kinnetic ( related to the system's motion ) and Potential ( related to the position of the system within a force field ) , but what about light ? Are kinnetic and Potential energy forms applied to things which are always matter , so light is not included ? How does light fits in the picture of energy of physical systems, which are decomposable into mutually exclusive energy forms ?

2 - I talked so much about Physical systems but this was because i needed to refer somehow to the thing WHICH posseses energy. The problem is that i don't really know what a physical system exactly represents.Defining it as a portion of the universe ( as Wikipedia defines it ) , tells little about what it is. Is "portion" refering to specific matter in the universe ( which might occupy different regions of the space, along of time ), or is "portion" refering to specific volume in the space-time of the universe ? What about light ? Is it a physical system ? It better be, afterall it posses energy.

There it is ... all my doubts that are driving me crazy understanding energy.

I don't expect to fully solve the problem of defining what energy is, i just want to have a solid understanding of what people are talking about when they mention energy.

Some clarification, or suggestion of materials to read would be imensely helpful.