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Measuring the Efficiency of a Light Bulb



Know that a lightbulb converts electrical energy into light energy and heat energy.

Carry out a practical task to measure the electrical energy supplied to the bulb and the heat energy produced.

Use the data collected to calculate the light energy produced and hence the efficiency.





Task 1 - Recap

Remind the person next to you:

What is meant by an efficient device.

What is meant by an inefficient device.

The units used to measure efficiency.




Task 2

We are going to carry out a practical task to measure the electrical energy supplied to the bulb and the heat energy produced, in order to calculate the efficiency.  Your teacher will show you how to set up the apparatus.



Foundation tier: stick this sheet into your exercise book and follow the instructions.  Record your results on the table provided.  Fill in the relevant numbers and complete the calculations on the second page to calculate the efficiency.


Higher tier: follow these instructions to carry out the practical task.

You will need to:

Write a brief explanation of how you will carry out the experiment

Stick in a copy of this diagram

Design a table to record your result.


When you have finished the experiment, carry out your calculations according to the hints on the last slide.  Present your calculations methodically, showing full working.





Task 3

Draw a Sankey diagram of your results, showing the total energy input, useful energy output and wasted energy output.




Task 4

Write an evaluation of your experiment.  You will need to discuss the following points with the other members of your group.


Do you think all the heat produced by the bulb went into the water? If not, where else might it have gone?

Do you think the temperature of the water was the same throughout the beaker?  If not, where would it have been highest? Where did you put the bulb of the thermometer?  Did you stir the water? If not, why not?

Do you think there was any infrared radiation produced by the bulb? Would this have been absorbed by the water?


Overall, was your estimate of the amount of heat produced too high or too low? Does this mean that your answer for the efficiency was too high or too low?



Task 5 - Extension

Plan a follow up experiment.  Follow the instructions on this sheet.