which wieghs more a lightbulb turned on or off?

Answer 1

Neither they are essentially the same. Any tungsten that evaporates due to thermionic emission remains encapsulated in the glass of the bulb therefore the total mass remains constant.

Answer 2

It is not easy to answer. If the light bulb were in an external vacuum, it would have more mass when turned on because of its higher energy level and the equivalence of mass and energy (it may be miniscule but still exists). The same is true if the bulb were in the atmosphere, as far as the mass is concerned. However, the heating of the bulb will cause it to expand. The mass does not change due to the expansion. However, the asker said "weight" not mass, and in the atmosphere, the larger size of the bulb when heated will increase its bouyancy in the atmosphere. I believe this latter will be greater than the energy-equivalent mass and the bulb will WEIGH less, but have higher mass.

The electrons flowing make no difference, since the same number flow out as in. Evaporation of the filament makes no difference, since the material merely deposits on the inside of the glass bulb.

Edit:

I have made some calculations to estimate the effect of the photon mass, mass equivalent of the thermal energy in the filament, and the effect of bouyancy in air. This is for a 100-watt (W) bulb 6 cm in diameter (R=3cm).

The relativistic equivalent mass of the photons

m(potons) = W*R/c^3 = 10^-22 gm

The equivalent mass of the thermal energy in the filament: the filament is typically 1.5 m long and 2.5x10^4m in diameter; thus the tungsten filament would have a mass of 1.5 gm. It runs at a temperature of 2700 degK. Using its specific heat (130 J/kg*K) gives the thermal energy of 711 joule, or

m(thermal )= 8x10^-12 gm

(Note: I have my doubts about this value since 711 joule seems high. This would mean that the filament (with 100watts input power) would take 7 second to heat up. I question the numbers I got for the size of the filament; in any case, the value is less than 10^-11 gm. If the filament heats up in 1 ms, the thermal mass equivalent would be about 10^-15 gm.)

Edit again: as I suspected the filament specs were wrong. the filament diameter is 2.54x10^-5m (.001 in). With the new value, the energy is 5 joule and heating time 50ms (much more reasonable). The corrected result for equivalent mass ot thermal energy is

m(thermal) 6x10^-14gm

Finally, using the thermal coefficient of expansion of glass k = 8.5x10^-6 per deg K, assuming a glass temperature rise dT of 100deg K, and with the density of air d = 1.1 kg/m^3, the bouyancy effect is

dV=4*pi*R^3*k*dT
m(bouyancy) = d*dV
m(bouyancy) = -3.2 x 10^-4 gram

So the mass increases by about 6x10^-14 gm but the weight decreases by 3x10^-4 gram

Answer 3

The lightbulb's weight difference would be insignificant when on or off. But if you would like to be technical...

In a standard environment, the bulb would weight the most when off. This is because it is a (mostly) contained system, and the greatest factor in any weight difference would be in any convection caused by its generated heat. Since the bulb and its contents are contained, it would weigh the same as an enclosed system. However, it will also agitate and heat the air around it. This heated air would tend to rise, causing convection currents all around the bulb. These very slight but measurable currents would all attempt to rise, being replaced by cooler air from a distance. This generates a very slight upward airflow around the bulb, which would attempt to carry the bulb with it.

Again, any lesser weight from this extremely slight upward air current is almost insignificant. Somebody walking around in the same room would agigate it. However, it is still the most significant source of any weight change. Any additional weight caused by extra electrons, generated photons, sublimated tungsten, etc. etc. would be even more insignificant in comparison.

Answer 4

Good question! A lighted bulb will be slightly lighter (I suspect this is measurable with a very fine scale) due to the increased displacement caused by the heat. Larger volume means the bulb will be slightly more bouyant in the air. The mass, of course, does not change.

For ppl who are arguing the energy/electrons=more mass idea, you are wrong on two counts: the energy of a lightbulb equates to a miniscule mass through e=mc^2, and since energy/electrons going in=coming out, energy/electrons make no difference anyway.

Answer 5

Easy, a light bulb torned on weighs more. E=mc2 Extra energy is added to the light buld when you turn it on, so it isn't really a fair comparison, thusly the mass (weight) of the lightbulb will increase ever so slightly compared to a lightbulb that is turned off.

Answer 6

It is the most difficult question I have ever read.

OK. a light bulb turned on would weigh more because there are more electrons that are flowing through the tungsten.

Even though the electrons are coming out of the neutral wire, there is a pressure from the positive wire. Which is called the voltage difference.

So, when a light is turned on, it would weigh more because of the additional electrons in it.

But can you find the difference it weight???????

Answer 7

The mass does not change. Lighted bulb has electrons flowing, but there's no net increase in electron. Remember, that the A/C current in light bulb is electrons vibration. There's no real 'flow' of electron.

The energy emited by electrons are dissipated, as light and heat. The energy is not trapped within the bulb. So, net mass (even relativistic mass) remains the same.

Answer 8

A light turned on is hot, and the rising heat of the air around it would make the hot bulb somewhat lighter, though not appreciably so.

Answer 9

in case you easily need to appreciate ...turn a gentle off and take the bulb out an examine it with a circuit meter. i think of you will discover out that there is not any electric powered circulate on the criteria of touch for the gentle bulb. Which demonstrates the evidence that there is not any electricity used while the gentle is switched off.

Answer 10

neither

when the electricity is on, electrons flow through the wire. as one electron comes in one end, one flows out the other.

although... as the filiment gets hot, tungston atoms shoot off into the bulb, but they are still contained.

so im sticking with my answer - they weigh the same