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Efficiency of a Light Bulb
Name: Kim N.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 2000-2001
Question:
How can I measure the heat/light produced by a lightbulb,
so that I can measure the power input of a solar cell to find its efficiency?
Replies:
Kim,
Make a calorimeter from a nested pair of large polystyrene foam cups. Put in a
known mass
of water, Immerse the light bulb upside down into the water until it is almost
completely covered with water -- not so deep so as to allow the socket to
get wet.
Use a thermometer to measure the temperature of the water with the bulb in
place.
Record this value as the initial temperature.
Wire the bulb circuit so that you can measure the volts and amps when the
bulb is
in operation. Turn on the light -- the bulb won't break because the heat
will be
absorbed by the water. After a measured time interval, shut off the bulb and
immediately measure the temperature of the water. Record this value as the
final
temperature. It would be helpful if you could design your experiment so
that the
water could be gently stirred during the heating cycle. Subtract the
initial from
the final temperature to get the temperature change.
Use the relationship:
Heat absorbed by the water =
(specific heat of water)(mass of the water)(the temperature change in
degrees C)
From this relationship you can calculate the calories liberated by the
bulb during
the heating interval.
You can calculate the watts of energy used by Power = (volts)(amps)
Now you have an expression for the approximate energy released from the
bulb, and
the energy consumed during its operation. Its just a matter of a little unit
conversion and you are on your way to what you sought. I think this is
enough to
get you started. Good luck with your experiment.
Regards,
ProfHoff
Measuring the heat/light output over the entire range of wavelengths to
get the total energy would be difficult. You might consider a couple of
alternative ways to get the same info:
1. Put the power source in series with an ammeter and a calibrated variable
resistor. You can use an electrical test set to measure the current. Both
are available at a reasonable price from Radio Shack or some other place
that handles electronic equipment. The energy, E, produced by the power
source
[in joules] is the square of the current, I, and the resistance R, i.e.
E=I^2*R.
2. If you cannot put your hands on a calibrated variable resistor, you can
use any resistor having the proper ohms [You will have to determine what
"proper" is by some trial and error.] and measure the current with an
ammeter in series with the power supply and the resistor, and the voltage
drop across the resistor. Then the energy produced by the power supply is
the current, I, times the voltage drop across the resistor, V, that is E =
I*V.
Vince Calder
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