How do physicists define energy?

Most of us are familiar with the energy unit of the calorie or the kilocalorie. We eat food to gain energy which is measured in terms of calories. We exchange that energy taken in as food for muscles, metabolism, and rebuilding cells, and excess energy is stored in fat.

The same kind of concepts are important in particle physics (as well as physics and science in general) but instead of using units of calories physicists use the electron volt, EV. Calories and kilocalories are units well suited to animal metabolisms, but elementary particles operate on a completely different scale of magnitude for which electron volts are more convenient. Particle physicists speak of energies and masses of electron volts that, just like calories from food do for us, allow elementary particles to move and have mass. The energy of motion is kinetic energy, and a particle at rest contains energy in its mass; these two together, motion and mass, comprise the the total energy in EV of a particle.

Einstein recognized this relationship in a famous formula that we all are familiar with:

E = mc2

The equation means that for an object at rest which has mass m, the energy of the particle is proportional to the mass by the constant factor c², the square of the speed of light in a vacuum.

There is more to the expression than simply E = mc², the energy of a particle at rest. Einstein's complete formula is this:

E2 = (pc)2 + (mc2)2

The first term, p²c², is the energy associated with the motion of the object. p is the particle's momentum, and again c is the speed of light in a vacuum, a constant. If the particle is not moving then its momentum is zero, p = 0. The energy from momentum is zero and we are left with the square of the famous E = mc². An elementary particle that has mass and is moving has an energy calculated from the mass and momentum terms of the complete equation.

Kinetic energy may be difficult to visualize for an invisible physics particle, but the same concepts apply to tangible objects. For example, you would be much more concerned if a rhinoceros were charging at you than a gnat traveling your way at the same speed. The kinetic energy of the charging rhino is much much greater than the tiny gnat's, so the rhino would do you more damage.

We can even observe energy directly, most obviously by sight. Our eyes are sensitive to a specific range of light energy our sun emits. The color of light is related to its energy: blue-violet light has greater energy than orange light, and green light has greater energy than red light. We conceive of red as a 'hot' color and blue or purple as 'cool' colors, but despite appearances, purple and blue are far more energetic than red. Because our eyes are able to detect colors, they can actually observe the energy of the light over a limited range.

Light is composed of photons, which are elementary particles. Unlike other particles, specific energy ranges of photons have their own names. Very low energy photons are known as radio waves. Microwaves have a slightly higher energy. Energies above that are infrared light, and right above infrared light is the visible spectrum of light, with innumerable names for specific colors. Higher energy photons are called ultraviolet light, and the highest are x-rays. Yet each sort of light or ray is formed of the same elementary particle, so physicists need only speak of photons measured in electron volts.

 Average energy content of different resources :

Measurement of Energy Supplies

Wood

Petroleum

Natural Gas

Coal

When tree is burnt as energy source, It is refered as  ligneous biomass energy    ( the heat and the electricity and so on ) 
New  technologies are being developed in addition to some firewood and the charcoal from the old days and are attracting.  

The biomass energy is the organic resources and the creature resources of the animals and plant origin. Contrast with fossil fuel such as petroleum and coal, it is called " living fuel biofuel ". The pellet which is used with the pellet heater is ligneous biomass energy. The pellet is the one to have firmed moisture in the decreasing to 3% - 4 %, making a tree floury.
It is ligneous biomass energy in today when replaces some firewood and charcoal from the old days.

Sawing disposing of it material such as the waste and the shavings, a by-product, a waste with ligneous systems such as the used paper and compressing. is converted into pellet form. Ligneous pellets are used as solid fuel..

Using ligneous energy  supports the activation of the forest and results in  the economic growth  in the mountain village etc.  

Solid  fuel replaces fuel oil and coal in the industries. The calorie is equal to or more than 6,000 kilocalories.It generates only carbon dioxide and a little water even if it burns. The harmful ones such as the dioxine don't come out.

Rrecycling a waste as raw materials and that it uses, reduces environmental pollution. 

The global warming prevention measure and to contribute to the waste reduction, fossil fuel such as coal and petroleum and clean new energy which can substitute are called RPF ( the fuel which is manufactured with the used paper to recycle as the recycled paper and the plastic it isn't possible recycle ). (RPF: Refuse Paper & Plastic Fuel)