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Consider a dam, in that first the hydraulic energy due to gravitational force gets converted into mechanical energy by using turbine. Then those mechanical energy is converted into electrical energy by generator which gives the electricity to the whole area. Then we use fans , coolers, TV etc,. which gives mechanical energy to the fan. As we know energy can neither be created nor destroyed but just keep changing its form, my question is now after mechanical energy what does it convert into?

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    $\begingroup$ Heat ${}{}{}{}{}$ $\endgroup$ – John Rennie Jul 20 '15 at 17:02
  • $\begingroup$ kinetic energy of air molecules, light from a tv, heat from a heater. But invariably, as John pointed out, almost all the energy we consume is turned into heat energy (some of it stays as light forever) $\endgroup$ – Jim Jul 20 '15 at 17:04
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Fans make air molecules move, and the energy is in a such case converted to kinetic energy. TV:s, and everything else with screens, are also giving off photons, which carries energy. All electronics also produce heat, which is a form of energy.

In the end almost all energy is in some way or an other converted into heat, due to Thermodynamics – or more exactly the second Law of Thermodynamics which states that the entropy in a closed system increases over time. This means that the "quality" or "usefulness" of the energy lowers over time. The photons from the TV will eventually hit you, the walls or anything else and heat that thing slightly up. The increased movement of air particles caused by a fan will also become heat.

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The fan increases the kinetic energy of the air molecules in the room. From there the energy usually dissipates uselessly (molecules bouncing off of each other, different objects, and you) but if you tried holding up a pinwheel, it would rotate proving that the energy has just been converted to a new type. The net result of the fan is a rise in temperature, (like John said) because increasing the KE of molecules = increasing heat!

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When thinking about converting into another form, it does not have to be usable energy again for example when fan spins it makes sound due to it hitting other air molecules and thus giving them energy and so causes noise and heats up air every so slightly(after all noise is air vibrating).

Next, the energy is converted into heat due to the friction between the fan and the holder of the fan, thus the energy is absorbed into the environment in form of heat. Next, it will turn into light which will simply heat surroundings and the electrical energy as they travel from generators heat up the wires ever so slightly and so it is lost to surroundings, due to electrons in the wire hitting into the atoms and so forth.

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