Science Buddies: "Ask an Expert"

I am trying to help my sixth grader anaylze the results of her science project. How much more efficient are CFLs than incandescent light bulbs? She built a Joly Photometer and used it to compare the light intensities of 40, 60, and 75 watt soft white incandescents to that of comparable CFl bulbs. After using the formula for the inverse square law we are trying to determine the results for efficiency. An example date: Intensity of 40w incandescent is the standard of 1 at a distance of 43.5 inches from photometer. 9w comparable cfl is 50.75 inches from photometer. Calculating with formula gives the intensity of CFL as 1.36 (all calculations rounded to hundreds). I assume the CFl has an intensity of 36% more than the incandescent verified by the lumens output on the packaging is higher than the lumens on the incandescent. To determine efficiency Science buddies suggests dividing the Intensity by the wattage consumption of bulb, therefore the incandescent is rated at .025 and the cfl at .15. I am trying to help my daughter explain these results. All of our research says that CFL bulbs use 75% less electricity and generate 75% less heat than incandescents, so how would we best explain our results. CFl at .15 is how much more efficient than incandescent of .025 as it relates to ?? The higher the efficient number the better savings? Any help would be appreciated.

First of all, your Joly photometer doesn't measure the light intensity in a standard unit (lumens, for example), just relative to whatever is on the other side of the photometer. Let's call the units you get by computing the inverse square intensity a "Joly."

1/(43.50*43.50) = 0.000528 Jolys
1/(50.75*50.75) = 0.000388 Jolys

The ratio of intensities is 1.36.

Your result shows that the CFL emits 36% more light than the incandescent bulb. Dividing by the power consumed by the bulbs gives a measure of efficiency.

CFL 0.000528 / 9 watts = 0.000587 Jolys/watt
Incandescent 0.000388 Jolys / 40 watts = 9.70x10-6 Jolys/watt

The efficiency of the CFL is 6.05 times the efficiency of the incandescent.

One can just as sensibly compute the inverse efficiency or watts/Joly.

CFL consumes 9 watts / 0.000528 Jolys =17040 watts/Joly
Incandescent consumes 40 watts / 0.000388 Jolys = 103100 watts/Joly

The consumption of the CFL is only 16% as much as the incandescent.

So, your result shows the CFL to use 84% less electric power than the incandescent instead of 75%. Why the difference?

First of all, your result is not necessarily at odds with the 75% reduction. What CFL and what incandescent was used to get that number? How was it measured? The number doesn't necessarily apply to all bulbs.

Also, the Joly photometer is not a high-accuracy device. You might want to investigate just how accurate and precise it is. If you ask several people to choose the distance at which the paraffin blocks match intensity (without telling them what the others have measured) how much do their measurements vary? If you reverse the photometer so that the opposite side is exposed to the incandescent, do you get the same result? If you use CFLs and incandescents from other manufacturers do you get the same result? What else can you think of that might affect your results?

Sounds like you have a nice result and just need to digest it and present it clearly. Don't worry that you didn't get exactly 75% more efficient.