Energy

Concept of energy

Energy is defined as the ability to do work. Energy takes on many forms and we now discuss the two most basic, kinetic energy and potential energy.

SI unit for energy is joule or J and we can use others as below:
1 calorie = 4,2 joule
1 joule = 0,24 calorie
1 kJ = 1000 joule

Kinetic energy

A moving object can do work on another object it strikes. A flying cannonball does work on a brick wall it knocks down; a moving hammer does work on a nail it strikes. In either case, a moving object exerts a force on a second object and moves it through a distance. An object in motion has ability to do work and thus can be said to have energy. We call the energy of motion kinetic energy, from the Greek word “kinetic”, meaning motion.
We can say kinetic energy is the energy possessed by a moving object because of its motion. A body of mass m moving with a velocity v can do an amount of work, that is:

KE = ½ m v2
Where, KE = Kinetic Energy (Joule or J )
m = mass (kg)
v = Velocity
Kinetic energy is directly proportional to the mass of the object and it is proportional to the square of the velocity.

Potential energy

An object can have energy not only by virtue of its motion, but also by virtue of its shape or position. This is called potential energy (PE).
The most common example of PE is gravitational potential energy. A heavy brick held high in the air has potential energy because of its position. It has the ability to do work, for if it is released it will fall to the ground and can do work on a stake, driving it into the ground. We can say that potential energy is the energy possessed by an object because of its state or position. Let us determine quantitatively gravitational potential energy of an object. In order to lift an object of mass m, a force at least equal to its weight, m x g, must be exerted on it, say, by a person’s hand. In order to lift it to a height h above the ground, figure 1, the work done by the person will be the product of force m x g, and the distance h; that is W = mgh. This work is potential energy of the object. We can write potential energy as:

PE = m . g . h
Where, PE = Potential energy (Joule or J)
m = mass of the object (kg)
g = earth’s gravity (m/s2)
h = height of the object from the ground (m)

Mechanical energy

Mechanical energy is the sum of kinetic energy and potential energy. It is always the same for every position. From the formula of kinetic and potential energy we have:

ME = ½ mv2 + mgh

or

ME = KE + PE
Where ME = Mechanical energy (Joule)
m = Mass (kg)
v = Velocity (m/s)
g = Acceleration of earth gravity (m/s2)
h = Height (m)

As an example, let us consider a stone that is allowed to fall toward the ground. Because it is dropped, it has potential energy equal to m x g x h (we take the ground as the reference level). As it falls, its PE decreases but its KE increases. Just before hitting the ground, it has only kinetic energy. In fact, its KE at the bottom is exactly equal to the PE it had at the top.









The conservation of energy

Whenever energy is transformed, it is found that no energy is gained or lost in the process. We say that energy is conserved. Energy can neither be created or destroyed. Energy can only be transformed from one form into another, and the total amount of energy in a closed system always remains constant. Let us state the law of conservation of energy :
“ The total energy is neither increased nor decreased in any process. Energy can only be transformed from one kind to another, but the total amount remains constant.”

The figure above describes conservation of energy as potential energy changes to kinetic energy. Therefore, no energy lost , it was just changed from one form of energy to another. The various forms of energy include potential energy, kinetic energy, heat energy, chemical energy, solar energy, electrical energy, nuclear energy, geothermal energy and wind energy.

Here are some conversion of energy and the examples of devices:
Electric lamp changes electric energy to light
Firewood changes chemical energy to light and heat
Petrol changes chemical energy to kinetic energy and heat
Loudspeaker changes electric energy to sound
Brake pad changes kinetic energy to heat and sound
Electric fan changes electric energy to kinetic energy and wind energy
Handphone charger changes changes electric energy to chemical energy
Battery changes changes chemical energy to electric energy