Comparing Bulb Efficiencies

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This article is posted as a suggestion pending class trials
The efficiency of a standard bulb and a "energy efficient" light bulb can be compared to illustrate the energy saving potential of switching to compact flourescent bulbs.
Using mains powered bulbs as a class practical is potentially a risky idea and this is suggested as a demonstration or part of a more extensive circus of experiments depending on age and ability. Alternatively low voltage (e.g. 1.5V M.E.S.) bulbs and LEDs can be used. The following suggestion is put forward as a safer method than the immersion of bulbs in water and calculating the efficiency through temperature rise.

Demo (tested and recommended)

Comparing 11W Compact flourescent and standard 60W bulb.
The use of a wattmeter allows quantative comparison although the bulbs will be stamped with their wattages.
This is a simple demonstration relevant to most secondary age groups.
Suggested age 12yo,13yo,14yo
Photo to be added later...
The two bulbs are placed equidistant ~10cm from a thermochromic sheet mounted verically. When the lamps are switched on the thermochromic sheet near the standard bulb will change colour due to heat whereas the colder Compact Flourescent lamp will not change the sheet. The time taken will depend on the room temperature and air currents, but will be about 10s.
Suggested developments:
A spectroscope can be employed to show the difference between the two light sources. The teacher than compare the two apparent brightnesses and discuss this.
More advanced groups can discuss the fact that the radiation is similar to a black body and draw up a "Sankey Diagram" showing usefull light ~400-700nm and wasted IR + UV light. It is possible to use a speadsheet program such as "Excel", to examine the blackbody radiation curve equation and prove a 5% theoretical efficiency for a 2700K bulb.

Relative Brightness

The relative brightness of the bulbs can be compared with a grease spot photometer. Note this method is better than using a LDR or Photodiode based meter, as the Flourescent bulb emits only certain wavelengths of light. The Standard bulb emmits lots of unseen infra red. When the photometer is balanced the relative brighness of the bulbs is in relation to the square of the distance between the lamps and photometer. That is a light source follows an inverse square law.
P1/(d1)2 = P2/(d2)2

Class practicals (Untested)

Suggested age group 13yo,14yo,15yo ???
As a class practical the "perceived intensity" can be investigated using a grease spot photometer.
The proceedure below has not been tested in a class environment but does work comparing two lamps.
If the class is given LEDs, it is worth noting that the LEDs are directional as the case is a lens. To this end it is suggested that "pre-focussed bulbs" be used. Fortunately "white" LEDs now available will make the comparisons more rigourous.
Each light source must have a voltmeter and ammeter associated with it, although the nominal values may be used. The LED should have a proctective resistor or it will fail.
(Suggested component values to be added when trialled)
Note too, that you are not comparing an incandecent bulb with a compact flourescent
The energy consumed in the circuits is simply I x V.
Although the light efficiency is not known the relative efficiencies can be made.

External links 5% citation

--D.B.Ferguson 18:15, 6 December 2007 (GMT) Back to Physics

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