One 75-Watt Bulb

If you are talking about an incandescent bulb then it most certainly is true. You may remember that most of the electrical energy an incandescent light bulb produces is heat. The 25 watt bulbs do not get very hot so very little of the consumed energy gets converted to visible light.

Yes the question is surely for incandescent bulb.

Actually as far as I understand the conersion in hot wires is due to heat and it is the temperature of the WIRE not of the envelope of bulb which produces light.

Thanks for reply.

Some light bulb manufacturers give the lumens produced. From such tables you can compare the light output vs watts consumed for various combinations of bulbs. There may on average be a minor advantage for higher-wattage bulbs.

Regards.

Was replying to you when my IE browser failed.

I tried after I read this so called FACT in the link I reffered to and tried to search to confirm or otherwise but could not get any link.

So naturally I submitted in CR4 for help.

Thanks

Regards.

A good view.

Actually as far as I understand the conersion in hot wires is due to heat and it is the temperature of the WIRE not of the envelope of bulb which produces light.

The wire in low wattage bulbs is thinner than the Hi-wattage ones and design is always keeping color temperature in mind to get max efficiency of conversion.

Thanks for reply.

That's very creative. You also highlight the unspoken assumptions that all three light bulbs will be operating on the same voltage and at the designed voltage by breaking that assumption. But by not applying this assumption that all three bulbs must be operating at the same voltage you instead bump this question into the realm where nothing can be reasonably predicted or taught. If the light bulb dissipating 75 watts while operating on 120V but was designed to operate at less than 1 watt with 12V then it will produce much more light than a designed and operating 120V 25 watt bulb. However the 75 watt light will likely fail very very quickly. A whole host of other ludicrous combinations of 480V stadium bulbs operated at 12V warming the room with only 75 watts while an old 6V turn-light producing 25W at the same 12V becomes brilliant for a few moments.

Thank you for being appreciative, Actually its was early in the morning I posted the answer and thats when the brain works best.

I assume the bulb filament has some resistance. And by basic Ohm's law V = IR, and power P = (I^2)XR or (V^2)/R, there would be three pieces of resistances in series in the above circuit and voltages across each resistors would change respectively or accordingly?

By Kirchoff's law, (in Mesh Analysis) the summation of all voltages in a circuit equals to zero :)

Three filaments are assembled in three different bodies, so where is the question of series? It is never talked there,

Regards.

What I understand from your view is the DISSIPATION of light through Envelope, as this has increased the DISSIPATION of light 3 time so ultimately the lumens in space around bulbs is less. This really hits the idea.

Thanks Rakesh Semwal !

Practical acceptance of a lighting system is based on uniform illumination in the room as well as average illumination at the working surface. 3 X 25 watts will better fulfill the criteria.

Regards.

I agree with your point as more smaller bulbs will spread light more unformly than one of higher wattage as those are spread in a larger area, and I have noticed this effect in UK where this practice is common.

I just ignored it while I visited my son there and almost all electric-light fixtures were for 4/5 bulbs.

Here is an example when it is true.

Take 2 bulbs 12 volts 75 watts,

Pick any one and open it, tear its filament in three equal section, and assemble all three in three different bodies, And now you have three 12 volts 25 Watt bulbs,

on compression you find all three together less effective than of one 75watt bulb, And reason for that is total radiating surface area of filament of all three bulbs together is more than of one 75 watt bulb.

I think it is reposted.

Thanks

Haajee Its not reposted, It is example of another extreme. Please have a look again.

Thank you.

Thanks to invite my attention.

That depends on how you are separating the original filament into 3 pieces. The filament in a 12V, 75W bulb has a resistance of 1.92 ohms. If you cut it into three shorter sections of equal length, each piece will have a resistance of 0.64 ohms. If you take each of these pieces and make a new bulb, you would either have to connect them in series across a 12V source or connect them in parallel across a 4V source to get back to the original 75W power consumption.

The other way of slicing the filament would be lengthwise, but (at least for the most common type of filaments) this would be a near impossibility due to the geometry of the filament being a coiled coil. (See photo below from Wikipedia.) Nevertheless, no matter how you slice it (cross-cut or rip-cut), the filament would still obey ohm's law. In the lengthwise-cut case, the 3 new filaments would each have a resistance of 5.76 ohms. You would either have to connect them in parallel across a 12V source or in series across a 36V source to get back to the original 75W.

The filament of a lamp is simply a resistor.

Thanks a bunch for the detailed reply. I have mentioned above KVL and can see from your analysis that one could also apply KCL (nodal analysis)?

I have also read some where that:

The resistance of a conductor equation:
R = Ro (1 + alpha . delta T)

Where Alpha is a property of the material or some constant?

The brightness of the light depends on the temperature of the filament. One has to be a material engineer to go in very depth of this?

Actually, three 4 volts 25 Watt bulbs if the filament is simply chopped into 3 cross-ways. In order to create <...three 12 volts 25 Watt bulbs...> one would need to divide the filament length-ways, which is incredibly more difficult to do, as most filaments are coiled.

Perhaps to simple an answer for us engineering types, but I just went to the Phillips lighting catalog, did a few checks and voila:

A tri lamp of 30, 70 and 100 watt produces 285, 920 and 1205 lumens respectively, and so respectively 9.5, 13.1 and 12.05 Lumens/Watt.

When you compare A21 bulbs a 150 Watt bulb produces 15.4 lumens/watt and a 200 Watt bulb produces 15.5 lumens per watt. (these are both A21 bulbs)

When you compare the 40, 60, 75 and 100W natural Life Standard bulb, the efficacy is respectively 9, 11.3, 12.6 and 13.5 lumens per watt.

So, the numbers are all over the board. I know this is not a theoretical analysis, but ultimately that is what is available on the market and what we have to live with.

Last but not least, do not just look at the lumens per watt of the bulb, but of the entire fixture. Simply picking the lam p with the highest lumens per Watt does not necessarily give you the highest light output if the fixture has been manufactured to for a different lamp type/shape.

What is true is that it depends upon the efficacy of the light sources and their reflectors.

Why is it that the question gives nothing about these things? <sigh>