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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.

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marked as duplicate by ACuriousMind, Mark Mitchison, Kyle Kanos, JamalS, John Rennie general-relativity Feb 13 '15 at 7:15

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Energy has the capacity to do work. When you feel "full of energy", chemical energy stored in your muscles can be converted into mechanical energy, to enable you to run, or lift small cars above you head.

Different types of energy are stored in different ways. Gravitational energy is inherent in any object which the gravitational forced can move downwards (eg. moving water down a stream).

Moving water has kinetic energy simply because it is moving. Chemical energy in gunpowder can be converted into sound energy and kinetic energy, when it explodes and causes a projectile to move fast.

A closed system may inherently contain several different kinds of energy. A bucket of hot water falling from the top of a ladder includes heat energy (the average motion of the water molecules), potential gravitational energy, kinetic energy as mass of the bucket of water has motion, perhaps a little magnetic energy as the iron bucket moves through the Earth's magnetic field, perhaps generating a little electric energy as a result of the induced electric current. We could look even deeper and consider that the atoms of the bucket have nuclear energy, fortunately in a very stable form. And so on.

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