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Thread: The weight of a light bulb.

  1. #1
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    The weight of a light bulb.

    This is a not-very-serious debate that's been going for some time now in my household, and I think it's a good example of "a little knowledge is a dangerous thing". The question is, if a light bulb were attached to a sensitive enough scale, would its weight when turned on be different at all from its weight when turned off?

    Here are some of the arguments that have been made:

    1. The bulb expands slightly as it heats up, displacing air. This causes a slight buoyancy that reduces its apparent weight.
    2. The energy in the bulb has mass (m = E / c^2), so the bulb gets heavier.
    3. The bulb heats the air around it, causing that air to rise. This creates friction that pulls the bulb upward slightly (reducing its apparent weight).
    4. The heated air around the bulb rising actually creates a mild rocket-like effect that pushes the bulb downward.
    5. It depends on the bulb's orientation -- the socket connection causes the light emitted to be asymmetric, and the photon momentum from the opposite side will create a very slight thrust.

  2. #2
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    My guess is, the mass increases, therefore in a vacuum the bulb would weigh more. In normal atmospheric conditions then the "apparent" weight would vary very slightly either way, depending on some of the factors you mention.

  3. #3
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    Quote Originally Posted by cosmocrazy View Post
    My guess is, the mass increases, therefore in a vacuum the bulb would weigh more. In normal atmospheric conditions then the "apparent" weight would vary very slightly either way, depending on some of the factors you mention.
    It seems to me from the language in the OP that the question is specifically about weight, i.e. how it would be weighed by a scale on earth in normal atmosphere. And on that question, I sort of agree with you, i.e. "don't know". It seems that all the effects are going to be very, very small.

    But it sounds like a cool experiment.
    As above, so below

  4. #4
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    Quote Originally Posted by Jens View Post
    It seems to me from the language in the OP that the question is specifically about weight, i.e. how it would be weighed by a scale on earth in normal atmosphere. And on that question, I sort of agree with you, i.e. "don't know". It seems that all the effects are going to be very, very small.
    I agree with all of them, who knows what the magnitude actually is, except #4. I don't think that'd work. Also, how do you account for the socket or the wires leading to the bulb?

    2a The energy of light and heat released reduces the weight mumble mumble
    But it sounds like a cool experiment.

  5. #5
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    Quote Originally Posted by grapes View Post
    I agree with all of them, who knows what the magnitude actually is, except #4. I don't think that'd work. Also, how do you account for the socket or the wires leading to the bulb?

    .2a The energy of light and heat released reduces the weight mumble mumble
    Good point and an obvious one that I did not consider. I know the filament incandescent type bulbs degenerate over time during use. Is any of the mass of the filament turned into heat & light energy?

  6. #6
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    I'd agree that these are all plausible, except for number 4; I'm not sure about that one. I'm pretty certain that the relativistic energy and the photon momentum will both be extremely small (the relativistic energy should be the smaller of the two, I think), so it would be unsurprising if other effects dominated.
    Conserve energy. Commute with the Hamiltonian.

  7. #7
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    Insufficient information. Experiment requires further definition before a meaningful answer can be forthcoming.

    Where is the power supply for the bulb? Is it

    a] part of the mass being weighed?, or

    b] supplied from an external source? (and if so, how is the energy delivered to the bulb?)
    Last edited by DaveC426913; 2018-May-07 at 02:40 PM.

  8. #8
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    Arguing is useless. Do the experiment.

    Let us know your results.

  9. #9
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    The biggest of the small volume effects would be the expansion and glass does not expand much so if its weight and buoyancy, it will weigh less given very fine coiled wires to power it. The updraft of air, if allowed, would probably be a larger force than the change in buoyancy. The hot glass causes a hot boundary layer too, so less dense and that complicates the picture. If perfectly insulated everything would heat up, the filament eventually melting, or if run cooler, the glass would melt as well as the base parts. So the experiment needs to state the environment carefully
    sicut vis videre esto
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  10. #10
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    You guys are all making assumptions about where the energy is that eventually heats the bulb.

    That energy is stored somewhere, and it can't be discounted. If you assume it's already part of the system, you will get a different outcome than if you assume it starts off external to the system.

    The original question does not specify. Thus it is insufficiently defined to provide a meaningful answer.

  11. #11
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    Once electricity is applied, the bulb will become lighter ;- )

  12. #12
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    Quote Originally Posted by Spacedude View Post
    Once electricity is applied, the bulb will become lighter ;- )
    Spacedude wins the Internet for today.


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  13. #13
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    I think when we start to talk about buoyancy and similar effects then an analogy of this would be - does a balloon weigh more or less when filled with helium? Which is why in this case the initial state and the environment is crucial to the experiment.

  14. #14
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    There's "mass", "weight", "weight in air" and "measured weight in the situation".

    You have to pick which one you are interested in.

    We seem to be discussing the "measured weight in the situation".

    I feel the overriding effect is the convection current induced by the rise in temperature. The weight of the energy and the thrust of the photons are negligible compared to this. Expansion of the bulb due to heat is likely small in comparison also.

    You can clearly see temperature effects when you weigh items on a 4-decimal place balance in a laboratory. To get a stable indication the item needs to be very close to ambient temperature. If it is hot you don't stand a chance.

    Somewhere I have a document explaining which way this convection effect works, I can't remember offhand.

  15. #15
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    Another thing to be aware of in this experiment is magnetic fields.
    The electric current may affect the weight measurement via the magnetic field.
    Electronic balances are sensitive to this.

  16. #16
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    1. The bulb expands slightly as it heats up, displacing air. This causes a slight buoyancy that reduces its apparent weight.
    2. The energy in the bulb has mass (m = E / c^2), so the bulb gets heavier.
    3. The bulb heats the air around it, causing that air to rise. This creates friction that pulls the bulb upward slightly (reducing its apparent weight).
    4. The heated air around the bulb rising actually creates a mild rocket-like effect that pushes the bulb downward.
    5. It depends on the bulb's orientation -- the socket connection causes the light emitted to be asymmetric, and the photon momentum from the opposite side will create a very slight thrust.



    Something else has occurred to me:

    Only point (2) actually affects the weight.

    All the other points are factors which influence the measurement of that weight.

    To obtain the actual weight, those other factors have to be removed or corrected for.

    Some of the other factors will be far larger than the weight of the energy as in point (2), so the true weight gain will be extremely difficult to measure.

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