That information was wrong.

 

Which brings us back to the original message. OP was informed that 40 W bulb will be brighter.

The TS needs to ask for the reason why the 40 W bulb is supposed to be brighter. It's pretty sad that it wasn't given to begin with -- just stating the answer without providing the reasoning behind it is almost the same as providing no answer at all. The TS is in a good position to justify why the 5 W should be brighter and to probably refute the rationale given for the other.

 

To add a little fuel to the fire, another question that could be asked would be, "what if we applied 24 volts as the source for the 5W and 50W, 12 volt rated bulbs?" (Hint... How long do you think they would produce light?)

 

To add a little fuel to the fire, another question that could be asked would be, "what if we applied 24 volts as the source for the 5W and 50W, 12 volt rated bulbs?" (Hint... How long do you think they would produce light?)

An answer to that POPS into my mind

 

When I was conducting technical interviews for potential maintenance technicians, the questions were focused on basic electronics, relay logic and some basic digital thrown in, but also sought out the reasoning power of the applicant. The question of un-equal wattage bulbs in series was one of the questions that was a part of every interview. Even if I had to make up a question, I would always take to applicant to the point that he/she didn't know the answer. The way they responded to not knowing told a lot about the person. Some would try to BS their way through, while others would admit that they didn't know. The ones that I really liked were the ones that admitted not knowing, but asked for the answer.

 

To add a little fuel to the fire, another question that could be asked would be, "what if we applied 24 volts as the source for the 5W and 50W, 12 volt rated bulbs?" (Hint... How long do you think they would produce light?)

Likely not as long as it took you to write the question.

 

To add a little fuel to the fire, another question that could be asked would be, "what if we applied 24 volts as the source for the 5W and 50W, 12 volt rated bulbs?" (Hint... How long do you think they would produce light?)

I would expect the 5W bulb to last a couple of milliseconds before its filament vaporizes/breaks, stopping the flow of current. The 50W bulb will not even have a chance to get warm.

 

Are you sure about that? It doesn't match up what other places selling bulbs have to say -

• 40 Watt incandescent lamp produces only 380-460 lumens and uses 40 Watts of energy per hour.

• 100 Watt incandescent lamp produces 1700 – 1800 lumens and uses 100 Watts of energy per hour. From https://www.sepco-solarlighting.com/blog/wattage-vs-lumens-know-the-difference-for-better-lighting

Hi,

In short, the Watt is not a unit of energy.

 

I have been asked the question: If we put a 5W and a 40W lamp in series and connect them to a 12 V car battery, what will happen?

I stated that the 5W bulb would be brighter than the 40W, but was told it was the other way around...
Could someone explain to me why this is? Cheers!

Hi,

Incandescent filament bulbs are usually taken to be very simple but in reality they are very dynamic devices. Their resistance depends on several things that include voltage and time, and are considered very non linear.
So to solve this problem, you have to use appropriate models for both bulbs. The most amazing result could be that one bulb gets brighter for some milliseconds and then dims down a little after that. That would be my guess going by a rough thought experiment of how bulbs react when first turned on due to filament size. The smaller bulb (5w) gets hot faster than the larger one (40w) so it will get bright first, but then after the larger one gets hot the smaller one would dim a little as the resistance rises in the larger one. In the end though the smaller one will be the brightest because the 40 watter will not get much voltage.

Of course we can imagine that the 40 watter is inherently 8 times brighter than the 5 watter and so even with 1/8 the voltage it will be just as bright as the 5 watter, but that's not how it works because the voltage/brightness relationship is very nonlinear and so we see quite a reduction in brightness as the 40 watter voltage decreases, not 1/8 but much less than that.

We could look at this in more detail by using appropriate dynamic models, or we could simply look at some simple static models and determine the ending brightness of both bulbs using simpler formulas. An electrical engineering book would have these formulas or maybe look them up on the web. The relationships are all exponential in nature not linear as mentioned above. If i get a chance a little later i'll do this and see how it works and maybe try to calculate two bulb sizes that would produce equal light even though they are different wattages, if that is possible.

 

In short, the Watt is not a unit of energy.

Sorry. I don’t understand your comment in context; I DO understand the statement by itself. The Watt is NOT s unit of energy.

There is a simple linear relationship between Watts and units of energy - Joules. In simple value comparisons, both units can be used interchangably.

There is one way that I could understand a modified statement in context. I’d state that Watts are not a unit of luminosity - Lumens.

 

Sorry. I don’t understand your comment in context; I DO understand the statement by itself. The Watt is NOT s unit of energy.

There is a simple linear relationship between Watts and units of energy - Joules. In simple value comparisons, both units can be used interchangably.

There is one way that I could understand a modified statement in context. I’d state that Watts are not a unit of luminosity - Lumens.

The context was given in the post segment he was referring to, namely the quote from a website saying things like, "...uses 40 Watts of energy per hour."

 

Sorry. I don’t understand your comment in context; I DO understand the statement by itself. The Watt is NOT s unit of energy.

There is a simple linear relationship between Watts and units of energy - Joules. In simple value comparisons, both units can be used interchangably.

There is one way that I could understand a modified statement in context. I’d state that Watts are not a unit of luminosity - Lumens.

Hello again,

This kind of misunderstanding comes up now and then. The reason for this is because time as we know it is always present and always progressing so sometimes when we say something that involves time we leave time out but more about this in a minute.

First i was intending to show that the website being quoted may not be reliable because they dont seem to understand basic units, or else they just didnt word it right, and either case i would question the reliability at least briefly.

The context is as WBahn pointed out:
"... uses 40 watts of energy per hour".

The reason why this statement fails is because energy is not measured in watts. Watts is a unit of power, Joules is a unit of energy. We can look at a few statements that are true to see the difference.
"...uses 40 watts of power."
This is correct.

"...uses 144000 Joules of energy per hour".
This might be correct, but it's a poor way to state it because Joules of energy is Watts times Seconds, so we just made it more complicated to comprehend the true meaning because we multiplied by time and then have to divide by time. It would be better to just state the result in Watts as above and once we know the time, if we ever do, we can always calculate the energy later.
A Joule is a Watt Second, so 144000 Joules per hour is 144000 divided by 3600 comes out to 40 and that is 40 watts assuming a constant load.

The 40 watts is a specification that is void of time so that we have a way to classify devices without having to know how long they are running even though we know time never stops, at least for us normally.
So to say that something uses N Joules of energy per hour means we are including a redundancy that just complicates the whole meaning, and besides that, in the original statement they quote "Watts of energy" which is just plain wrong

 

Another thread has dealt with a similar problem, ending with experimental results: https://forum.allaboutcircuits.com/threads/series-circuit.107587/

 

Hi again,

Here is a simple solution for the current through two bulbs in series that have different watts where the normal operating voltage is the same for both and the ratio of currents is 4:1 so the larger wattage bulb has 4 times the normal current of the smaller bulb.

i=(4*I)/(4^(1/x)+1)^x

where
x=29/50, and
I is the normal operating current.

[an example would be a 48 watt bulb and 12 watt bulb both normally operate at 12v]

So i think this generalizes to:
i=(N*I)/(N^(1/x)+1)^x

where N is the ratio of normal operating currents of the larger bulb to the smaller bulb N:1.
For example if the normal operating current of the larger bulb is 5 amps and the smaller is 2.5 amps, then N=2.

That allows us to approximate the actual operating current i.

[in the following upper case means normal values, lower case actual values]

Once we have that, we can use:
watts/WATTS=(amps/AMPS)^(x+1/x)

to calculate the wattage of each bulb, and then the lumens per watt from:
LPW=lpw*AMPS^(1/3)/amps^(1/3)

and finally the lumens from:
L=LPW*watts

Thus we can calculate the lumen output for each bulb.

These calculations are based on approximations given in an engineering handbook in the section on filament bulbs.
There are models on the web that could be used in a simulator.

 

Yesterday I wanted to build a rig and test the theory. Unfortunately all the spare bulbs I've kept have two filaments with one burned out. So testing on automotive bulbs for the moment seems to be on hold. I DID find two bulbs, one with the high intensity filament and one with a low intensity filament, not knowing their wattages, and powered them with 12 VDC. To my surprise the higher intensity bulb lit near full intensity while the lower intensity bulb barely glowed. I was going to film it but I can't find another bulb holder. When I get that part sorted out I'll see if I can shoot a video and either prove or disprove that a dual filament bulb predicts that the higher intensity lights, not the lower. I may have to go to the auto parts store and buy a bulb just for chits & giggles.

 

I just did an experiment with a 40W 120V bulb wired in series with a 4W 120V bulb. With 120V across the pair, the 4W bulb glowed brightly and had 119V across it; the 40W bulb had only 1.0V across it and was dark, as expected.

 

So, once again Ohm's law has been vindicated.

 

Now you have gone and made Watt’s law all jealous.

 

So, once again Ohm's law has been vindicated.

Thank goodness. I can breathe again.

 

Yesterday I wanted to build a rig and test the theory. Unfortunately all the spare bulbs I've kept have two filaments with one burned out. So testing on automotive bulbs for the moment seems to be on hold. I DID find two bulbs, one with the high intensity filament and one with a low intensity filament, not knowing their wattages, and powered them with 12 VDC. To my surprise the higher intensity bulb lit near full intensity while the lower intensity bulb barely glowed. I was going to film it but I can't find another bulb holder. When I get that part sorted out I'll see if I can shoot a video and either prove or disprove that a dual filament bulb predicts that the higher intensity lights, not the lower. I may have to go to the auto parts store and buy a bulb just for chits & giggles.

Connect each filament to the 12v source separately and measure the current it draws. From this you can calculate the wattage drawn by each bulb when connected individually to the 12v. Then with the two connected in series across the 12v source, which glows more brightly--the higher wattage or the lower wattage filament?