Where those energy go?

  1. I have a doubt about the energy. According to law of conservation of energy, energy can neither be created nor be destroyed. But, when I light a LED with a battery untill the whole charge discharges, where those chemical energy go?
     
  2. jcsd
  3. Doc Al

    Staff: Mentor

    The chemical energy in the battery ends up as light and thermal energy.
     
  4. Are you actually asking, what happens with chemical energy? It obviously becomes light (+some other things). Or do you mean something else?
     
  5. Ignoring losses, the chemical energy in the cell went into the light emitted by the LED.

    The conversion to electricity was not 100% efficient, so some energy went into heating the cell. The transmission of electricity was not 100% efficient, so some energy went into heating the wires. The conversion of electricity was not 100% efficient, so some energy went into heating the LED.

    Some of the light emitted by the LED impacted on your retina where it caused chemical changes in your photoreceptors. Some of the light emitted by the LED caused other chemical changes when it hit the wall, the floor, the ceiling or your skin. (Perhaps you have a new melanoma). Some of the light emitted by the LED was reflected and was then absorbed by other surfaces. Most of the light absorbed on those surfaces went into heat. Some of the light emitted by the LED might have made it out through an open window and into the night sky where it may be whizzing past Neptune and Pluto now.

    Some of the heat that was deposited in various places would have been re-radiated in the infrared. Some would have been conducted to other objects. Some would have heated up the air and convected away.

    These effects seem minor and are easily ignored unless you are looking closely. But they add up to exactly match the chemical energy that was originally expended. Rest assured that the books balance. Energy is conserved.
     
  6. Thank you. But my doubt is that, at last what about the total energy? Do it store anywhere? I am asking this because somebody told me that those chemical energy is utilised to change the total entropy of the universe. Is it true?
     
  7. Drakkith

    Staff: Mentor

    Of course. As you convert the chemical energy into electrical energy, and then into light the entropy of the universe increases as a whole. However the total energy remains the same. Energy and entropy are not the same thing.
     
  8. The energy is not stored in a specific place but rather goes into the environment as heat and light and so it increases the entropy of the universe. It is not destroyed but merely the energies form is changed.
     
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