| MadSci Network: Physics |
Michael,
A lightbulb converts electrical energy into light and heat. Each lightbulb
should produce some amount of heat and some amount light. If you added
them together, they would equal the total amount of electrical energy the
lightbulb converted. This is a demonstration of the physics idea called
conservation of energy.
So you wanted to know how best to measure the temperature a lightbulb is at. Well- you can't just stick a regular thermometer on the surface of a lightbulb- it would likely break both. What I suggest is a type of measurment called calorimetry. Here's what you do:
Clip the thermometer to the rack in the oven so that the dial is facing out, and you can see it in the window of the oven. Clip the light to the rack, with the reflector facing away from the thermometer, and away from the window. Make sure that the cord doesn't interfere with the hinges in the oven, because the cord may fray. MAKE SURE THE OVEN IS OFF, AND DOESN'T TURN ON DURING YOUR EXPERIMENT.
We are using the oven because it is intended to get hot inside, and it is also insulated. The oven also will not be heated up by the light from the light bulb, rather it will be heated because of the hot bulb in contact with the air in the oven. This is why it is important to keep the door closed; you want to keep the air you're heating inside the oven.
The oven thermometer must have a bottom temperature of 100 degrees or
lower. The lightbulbs shouldn't make a temperature rise more than 30-40
degrees. If the temperature inside the oven gets hotter than 140 degrees,
be sure to turn the light off and open the oven door to let things cool
off. The insulation of the lamp cord will melt, and we don't want to make
an electrical fire accidentally.
Plug in the light, and turn it on. Wait with the oven door closed, and
make sure the door stays shut the entire time. The temperature should
rise. Write down the temperature at 30 minute intervals. At some point,
the temperature will stop decreasing. At that time, you can say that the
lightbulb is in equilibrium with the oven, meaning that the amount of heat
leaving the lightbulb is the same as the amount of heat leaving the oven.
This gives you a reasonable measure of the temperature of the lightbulb's
surface. When you've reached equilibrium (the temperature stops moving -
maybe 1-1.5 hours) you can stop, let things cool back down, and start again
with another type of bulb.
Make graphs of the temperature vs. time for the bulbs. Then make a graph
of the final temperature vs. bulb type.
The wattage listed on the light bulb is a measure of the amount of
electrical energy the lightbulb is consuming. The light bulb will change
the energy into heat and light. If a lightbulb is producing a lot of heat,
then it is not as good as a cooler light bulb. The reason is due to
conservation of energy- the electrical energy is converted more into heat
that into light for the hotter bulb, and vise versa.
So, I hope this helps.
-Fred
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