Question

coal provides energy on burning, but we cannot get energy by eating it while eating an apple gives us energy. explain why is it so.

Answer 1

when we burn the coal the coal converts into CO2 and H2O, due to the formation of new bonds the energy is released, but when we eat it then their is no digestion of coal. its mean it cannot give energy to the body.
An apple contain carbohydrates (like glucose, starch etc) and lipids etc which are converted into CO2 during Kreb cycle which releases ATP (Adenosine Tri Phosphate) molecules. When ATP breaks it is convertes into ADP (Adenosine di phosphate) with the release of energy that is utilized by our body.......

Answer 2

Short answer: because they produce energy via different processes. Fossil fuel-derived energy comes from the mechanical movement of a turbine driven by the steam produced from coal (or natural gas) combustion. We harness this energy as electricity. Our bodies, instead, use enzymes to rearrange and oxidize certain molecules and we store the potential energy in the form of ATP. The hydrolysis of the "high-energy" phosphoanhydride bonds in ATP is coupled to drive forward any energetically-unfavourable reaction in our body. Since the human body isn't equipped with enzymes to catabolize hydrocarbons from coal (which is made of hydrocarbon chains and benzene), we can't derive energy from it.