Every machine requires energy (Capacity to do work) for performing its functions, like automobiles which require fuel to produce energy. Cell phones have batteries which store energy and utilize it for their working. Living Organisms are also like machine which require nutrients in the form of food. The special molecules of food contain energy. And living organisms use that energy in various metabolic processes such as digestion, respiration etc. They also use this energy in performing different physical activities such as walking, running, jumping etc.
The only source of energy on earth is Sun. Energy of the Sun reaches earth in the form of light (light energy). This light energy is converted into chemical energy by living organisms or in heat energy stored by non-living things.
First law of thermodynamics: It states that energy can neither be created nor be destroyed but it can change from one form to another form.
Conversion of energy from one form to another form is governed by law of conservation of energy or first law of thermodynamics.
We can see that the heat energy of light converts in K.E. energy which flows water. This K.E. of water in dams is converted into mechanical energy when falls on turbine. This mechanical energy converts into light energy in bulbs and LED lights or again in mechanical energy in our fans.
On the other hand, this light energy when falls on green parts of plant is captured and converted into chemical energy. This chemical energy is stored as food energy in plants. When these parts of plant are eaten by animals, this energy transferred into them, where the organisms buried and remain under pressure inside earth crust for millions of year their chemical energy is converted into fossil fuel.
