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Name:  Kim N.
Status:  student
Age:  18
Location: N/A
Country: N/A
Date: 2000-2001

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?


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.


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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