Efficiency of an incandescent lamp EXPERIMENTS
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E-508
Purpose Start with the voltage a little below 6 V. Switch the cable
In this experiment you will determine the amount of
between lamp and resistor and adjust the voltage so that
energy that is radiated away from the filament of an
the currents are the same. Leave the voltage setting on
incandescent lamp. This energy is partly visible and part-
the power supply in this position. Just turn the power off.
ly infrared light.
1
st
measurement: Fill the apparatus with cold water.
Determine the temperature of the water. Hook up the
cables to make the current go through the resistor.
Switch on for 120 seconds. Switch off. Shake the
apparatus gently to let it reach thermal equilibrium, and
measure the temperature again. Calculate the rise in
temperature ΔT1
2
nd
measurement: Refill the apparatus with cold water
and find the start temperature. This time the current
should go through the bulb. Leave the current on for 120
seconds, switch off, shake gently, and measure the
temperature. This time we call the rise in temperature ΔT2
3
rd
measurement: Cover the bulb with a small piece of
aluminum foil. A thin rubber band may be used around
the neck of the bulb. Repeat the procedure again – call
the rise in temperature ΔT3
Calculations
1 – In the second measurement, some of the energy
leaves the system as radiation, and the temperature
does not rise as far as in the first. From the equations
above you can calculate the percentage of the energy
that is converted into radiation. The result is (remember
to do the calculations yourself!):
Theory
For a resistor, almost all the electric energy applied is
Try to find a figure of the efficiency of an incandescent
turned into thermal energy. This is noticed as a rise in
light bulb on the Internet and compare that to the value
temperature.
that you obtained. Explain any observed difference.
2 – Comparing situation 1 and 3, you will notice that
although the physical devices are different, the energy
When lighting an incandescent lamp, the electric energy
flows are the same. (The light does not leave the system
applied to the bulb is converted to radiation as well as to
and is ultimately converted into thermal energy.) One
thermal energy.
should expect the two rises in temperature to be equal.
Is this what you observed?
In these measurements we will keep current, voltage and
time constant, which means that the amount of supplied
electric energy will be the same for each measurement.
Required Equipment
The thermal energy transferred to the system is propor-
tional to the rise in temperature.
3207.00 Apparatus for the study of
light energy 1.00 Pcs
Power Supply
Procedure
Thermometer, Cables
You will perform an initial adjustment followed by three
measurements.
Every time, fill the apparatus with the same amount of
cold water; enough to cover the light bulb.
®
Science Equipment for Education Physics
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