I was wondering whether the physic laws or the nature in which lightbulbs are constructed would allow for lightbulb to generate electricity when subjected to strong (intensive and concentrated) light... Is the working principle of light bulb reversible? I couldn't find the answer to my silly question anywhere...
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$\begingroup$ An electric discharge is pretty much irreversible... $\endgroup$– DarioPAug 13, 2015 at 13:09
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2 Answers
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Is the working principle of light bulb reversible?
No.
Electric potential energy is converted into thermal energy in the light bulb filament. At a certain temperature range the filament will light up; that is, it will radiate with a wavelenght in the range of visible light.
That this process is non-reversible might be clear if you consider some more examples...
For a drifting car, kinetic energy is converted into heat at the rubber/asphalt interface
For burning gasoline, chemical bonds are broken and the binding energy turned into heat
When performing work like the simple example of compressing a gas, work is converted into heat (thermal energy) as the temperature of the gas increases.
My point is that many, many, MANY processes results in a release of heat just as the light bulb filament is heated to radiate visible light. By reversing this proces, the heat you apply "cannot know" where it came from. It could be any process.
What actually happens
- To make the light bulb filament light up in the first place, electrons are flowing through the filament. As they "bumb into" the filament atoms, those electrons "hand over" some of their kinetic energy resulting in heating of the filament.
- The other way around: we heat the filament and what happens is that the electrons, which are static (not flowing) at this moment, are gaining kinetic energy. The energy they receive will not make them all move in the same direction at the same time (that would be highly unlikely). Instead they will move around randomly more rapidly.
The electron density will get lower, as each electron "fills more space" as it moves more violently around. So some electrons will be pushed away from the hot filament, and since moving electrons is current, there will (very shortly) be a current, but direction away from the filament on both sides of it. This is by no means a stable current in a circuit, and it will end as soon as a new static stable equilibrium is found for the new electron arrangement.
It is in thermodynamics a general issue that heat always is a "bi-product" and at the same time a irreversable proces. Thus it is lost, and some of the energy for the intended process is wasted and cannot be converted back.
(It is possible to create electric current from heat (actually, from a temperature difference) called thermoelectricity and it is also possible to create electric current directly from light, which is the photovoltaic effect. But very specific materials with fitting properties as well as a carefully engineered circuit design are required)
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$\begingroup$ would same apply for flutescent lightbulb - I guess theres different process taking place than in the ordinary lightbulb..? $\endgroup$ Aug 11, 2015 at 20:51
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$\begingroup$ @user2820052 I can't answer about fluorescence, sorry. If this is the actual question, you could write it in the question text instead of just the headline, so the answers will be more specific. $\endgroup$– SteevenAug 11, 2015 at 20:58
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$\begingroup$ LED's are reversible (with greatly reduced efficiency), but incandescent/fluorescent/halogen are not. $\endgroup$ Aug 11, 2015 at 21:04
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The main issue with running a fluorescent light bulb backwards is that there are some irreversibility in the way that it convert electricity to light.
The Mercury vapour in the tube emits UV rays. Theses rays are absorbed by the phosphor coating on the tube which emits white light.
However, if we illuminate the phosphor with white light (reversing the process) it will not emit UV (unless you have a very large up conversion efficiency). This is the main problem is running the system backwards.
In principle if you could illuminated the Mercury plasma directly with light that it strongly absorbs then you would add additional charges to the current flow.
As mentioned above, LEDs and solar cells are very similar. In semiconductors these processes are reversible.
