Energy can simply be described as the capacity or ability to cause physical change. It is a requirement to achieving anything physically. It does not occur in tangible state but is more of a condition possessed by naturally occurring elements. Energy is measured in joules or ergs and occurs in various forms. A fundamental rule on energy is that it can neither be created nor destroyed. Where it may seem that energy is depleted, it will simply have changed form and thus still exists in the same quantity only in a new form.
Energy is present in several forms which are kinetic, sound, thermal, potential, gravitational, elastic, electromagnetic energy and solar energy. All these forms of energy in equal quantity will give equal output when conveyed through channels that do not allow for energy losses. This means that 10KJ of solar energy is equal to 10KJ of elastic energy and will do the same amount of work when conveyed through ideal channels. The challenge for scientists has always been to get these ideal channels of conveying energy. Some energy sources are more difficult to harness than others though technology is continually improving and providing new and better ways of harnessing different energy forms.
Different materials have varying capabilities of retaining and conveying energy. This is the reason why some materials are preferred over others when designing various appliances that are to be used for storing or transferring various energy forms.
The output of energy is work. Work in scientific terms basically means a change in physical state. This could be a change in form, location, temperature or state. When this change happens it simply means that energy has taken up a new form. Sometimes this may not be very obvious especially where the change is very minimal. A rise of one degree in the temperature of an object may go undetected but it actually means that there has been work done and energy has changed form.
Another important aspect of energy is that one object can posses it in different ways. Each of these ways represents a different form of energy. It is even possible to have several energy forms in a single object. A hot metal bar thrown into the air possesses both kinetic as well as thermal energy. Should it hit another object while in the air it will transfer some of its kinetic energy to that object. When it lands it will transfer thermal energy to the object it lands on. Some of its energy will also be given out as sound energy when it hits another object.
One common misconception that must be pointed out is that energy is not the same as power. Many people think of the two as one and the same, but they actually are not. They are related and have several similarities but remain different. Energy we have seen is the capacity to cause physical change, power on the other hand is the speed at which this energy is applied.