What is Energy?
Energy is an elusive driving force for all physical and chemical changes, and it exists in many forms such as heat, mechanical work (including potential and kinetic energy), light (radiation), sound, electric, and chemical energy. This is a simple description of energy, but not a complete one. A lot more need to be said to do justice to explain what is energy. Please keep the question in mind and identify the effect of energy around you.
In the development of civilization, a long time passes before humans recognize what energy is. Its recognition is relatively a recent event in terms of the history. Heat was recognized very early, but its quantity was only defined after the invention of the temperature scale in 1714 by Fahrenheit (1686-1736), and in 1742 by Celsius. The quantity of potential and kinetic work was defined by Newtons theory of motion. James Joule (1818-1889). work led to the recognition of heat being equivalent to mechanical work.
Since energy is equivalent to mechanical work, it has the same units, J. The Joule (= 1 Newton-meter) is a very small unit, however it is equivalent to the amount of work of lifting a small apple (98 grams) vertically up one meter. In terms of heat,
Rather recently, Albert Einstein (1879-1955) derived an equation, which suggested that energy and matter are inter-convertible. Energy can be converted into matter and matter can be converted into energy. Experiments have confirmed Einstein's theory to be true. Does this mean a fast-moving baseball has more mass than one that is resting? Well, this question is left for you to ponder.
Energy causes all the changes in the material world, but energy does not disappear (destroyed), nor is it ever created. This is known as the principle of conservation of energy.
Energy is transmitted in the form of heat from one place to another or in the form of mechanical work (potential and kinetic energy). Both type of transmission need a medium. At the atomic and molecular level, transmission of heat is also a result of transferring of kinetic energy among atoms, molecules or ions in the medium.
Transmission of energy via no medium is a phenomena known as electromagnetic radiation, in which bundles of energy are emitted as photons of light according to Max Planck. We shall discuss this aspect of energy after we have introduced the quantum theory. on the page of Electromagnetic Radiation.
Energy can be used to perform mechanical work. Energy is required to cause any change, physical or chemical. Energy is really the driving force for all changes. During the transformation from one form to another, amounts of energy remain the same. Energy cannot be destroyed or created. This is the principle of conservation of energy.
There are demonstrations to illustrate the principle of conservation of energy, but there is no proof for this theory. However, so far, those who claimed to have invented a machine that will perform work without the input of energy have been shown to be wrong.
To understand the principle of conservation of energy in the energy transfer processes, we have to isolate a system from its surroundings.
A system is some thing we isolate either by imagination or by physical means. For example, a closed container with gas or liquid in it is a system, so is a machine. Anything comes into contact with the system is called the surrounding or environment. After the isolation, we can identify material or energy to be transferred from the system into the surrounding or from the surrounding into the system. A system is isolated for the convenience of discussion, and it can be as small as a subatomic particle or as big as the entire universe.
With a defined system, we can identify energy transferred from the system to the surrounding and vice versa.
Since energy is conserved, the amount of energy contained in a system will not change, unless energy is transferred into or out of the system. To measure the energy contained in a system is very difficult, and usually a net change of energy content is more meaningful. For the purpose of evaluating the net change of energy in a system, the concept of internal energy is devised.
Internal energy, represented by E (or U a matter of preference), is essentially the thermal energy contained in a system (or particles making up the system). Unless change takes place, we usually have no way of evaluating it. A change in internal energy dE is due to the transfer of energy into or out of a system, but the volume stays constant. For example, energy transferred into the system, usually heat (q) and work (w), represents an increase of internal energy, dE, of the system. Thus,
In the case when heat or work is transferred from the system to its surroundings, the heat and work will be treated as negative quantities, resulting in a decrease in internal energy E.
The internal energy, E, does not depend on how energy is transferred and at what rate. It is purely an accounting of energy content of the system, and as such, the internal energy, E, is called a state function. The difference of a state function depends on the final and initial states, and we represent the change by,
The loss of internal energy is,
The radiator receives the same amount of energy from the engine per hour, and in reality the radiator is at a steady state. Thus the temperature is constant.
It is well known that 4.2 J is required to raise the temperature of 1.0 gram of water by 1oC. The density of water is 1.0 g per mL. Thus, the internal energy increase is
The heat can be provided by electric work. In the heating process, not all energy is absorbed by the cup of water. The wasted energy does not contribute to the increase in internal energy of the cup of water.