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I understand that any light bulb, whether incandescent or LED, transforms a part of the energy it consumes into heat and the rest into radiation (mainly visible light).

Assuming a 10W LED light bulb with a 90% efficiency:

  • 1W of electricity will be converted into heat that the light bulb itself will dissipate.
  • 9W of electricity will be converted into light energy, but (assuming that the room has no windows) all this light will be eventually absorbed by the objects in the room and transformed into heat.

Because of that, would it make sense to say that 100% of the energy consumed by a light bulb will eventually be converted into heat?

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    $\begingroup$ By the latter logic, would you say that all energy is eventually converted into heat? $\endgroup$
    – Quantum Mechanic
    Commented Dec 15, 2022 at 1:53
  • $\begingroup$ If I stick a solar cell next to the bulb and use it to charge a battery, does that mean the light is not eventually heat, or do I have to see what happens next? $\endgroup$
    – Matt
    Commented Dec 15, 2022 at 2:45
  • $\begingroup$ Focus only on LED $\endgroup$
    – SAKhan
    Commented Dec 15, 2022 at 5:31
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    $\begingroup$ @Matt, if you then use that battery to run a motor that lifts a weight 10 ft, then that portion was converted to Work, not Heat. Probably safe to say converting LED light to any appreciable work is rare from a practical standpoint. $\endgroup$
    – RC_23
    Commented Dec 15, 2022 at 7:02
  • $\begingroup$ Related: How is light emitted by an incandescent lamp? $\endgroup$
    – Roger V.
    Commented Dec 15, 2022 at 19:12

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Your two bullets are treating the light emitted by the LED, which is electromagnetic radiation, as separate from heat. But the distinction is meaningless since electromagnetic radiation in the form of light is one of the three forms of heat transfer, the others being conduction and convection. So essentially 100% of the electrical energy dissipated in the LED will eventually be transferred in the form of heat to, and absorbed by, the materials in the room (including the LED materials).

But technically the energy is not "converted" to heat. Things do not "contain" heat. Heat is strictly a means of transferring energy (the other being work, which does not apply here) and is due solely to temperature difference. The transferred energy is converted to internal molecular kinetic energy of the materials resulting in an increase in temperature. Once the transfer occurs, the energy is no longer identifiable as "heat".

Hope this helps.

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It is almost true that all the light is eventually converted to heat, but there are tiny exceptions if the light bulb is outside where "eventually" could be the future age of our universe. If you look down on the earth from space on clear night you can see all the glowing lights of our civilization. Many of these photons that escape the earth's atmosphere will keep on travelling almost forever. Space is pretty transparent to visible light, otherwise we wouldn't be able to see galaxies billions of light years away.

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  • $\begingroup$ That was a beautiful image. :) $\endgroup$
    – Daniel
    Commented Dec 16, 2022 at 7:53
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Yes, it is correct to say that 100% of the energy consumed by a light bulb is eventually converted into heat. Although a light bulb produces light, the process of generating light also generates heat as a byproduct. Over time, the heat produced by the light bulb will cause the filament to weaken and eventually break, which is why light bulbs have a limited lifespan. Additionally, the heat generated by the light bulb can also cause the bulb itself to become hot to the touch. Therefore, it is accurate to say that all of the energy consumed by a light bulb is eventually converted into heat.

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When talking about the source one has to concentrate on the source. The enviro effects are usually not taken into account.

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