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Investigating the efficiency of a light bulb Key Stage 4 & 5

IntroductionEducational data logger

DrDAQ is used here as a light sensor with a temperature and voltage sensor to measure the light and temperature output of a 12 W bulb over a set voltage range so that the efficiency of the light bulb can be calculated.

Currently manufacturers are producing more efficient light bulbs, but what does this mean? Are they producing more light and less heat or are they using less electricity? This experiment provides data to calculate the efficiency of a bulb, the power output and allows you to calculate heat loss from a bulb

This experiment requires some knowledge of energy efficiency and transfer.

Equipment required

  • DrDAQ data logger connected to a PC
  • An insulated box which will contain the data logger, probe and light bulb
  • Enough tape to seal wire entries into the box
  • Power pack
  • Wires
  • 6 V (highest wattage available) bulb

Experiment setup

  • Place the DrDAQ data logger into the box with the light bulb, make sure wires come through to the outside of the box and the gaps are sealed.
  • Place the box into a larger container filled with thermally insulating material such as expanded polystyrene packaging.
  • Connect the data logger to the PC and the light bulb to the power pack.
  • Run Picolog recorder — add the correct sensors; light, temperature and voltage.
  • When sensors show a steady reading start recording - suggest 30 s sample rate with ‘as many as possible’ averaging.
  • Turn on light bulb at 2 V and record for 10 minutes.
  • Increase voltage to 3 V and repeat.
  • Increase voltage to 4 V and repeat.
  • Increase voltage to 5 V and repeat.

Carrying out the experiment

Create your table as shown below:

Voltage on sensor in volts Temperature in deg C Light level

Allow the readings to reach a constant level before increasing the voltage.

How to calculate the results

Material SHC Mass kJ/kgK HC kJ/K
bulb glass 0.81 0.03 0.024
bulb metal 0.49 0.07 0.034
air 0.71 0.001 0.001
tub 0.3 0.052 0.016
bulb holder 0.49 0.099 0.049
DAQ connector 0.4 0.068 0.027
DAQ card 0.3 0.047 0.014


  • What is the temperature increase in each case? Plot these on a graph to see if there is a pattern
  • What is the light increase in each case? Does this show the same relationship as the temperature change?
  • Calculate the power for each reading
  • Calculate the energy efficiency of the bulb. Does this correlate to the energy lost as heat?

Further study

What are the relationships between light and voltage, light and temperature, voltage and temperature?