Seeing the Light: The Physics and Materials Science of the Incandescent Light Bulb

This unit consists of an interlinked series of 6 multi-part experiments using inexpensive materials such as lights bulbs, heater wire, and an ohmmeter. In the first experiment, students discover that Ohm's law doesn't appear to be valid for the filament resistance of the light bulb. They then develop the understanding that this arises from the change in filament resistance with temperature. This experiment connects commonly used technology - the light bulb - with the mathematics of Ohm's law as well as with the dependence of the electrical properties of materials on their composition, length, and diameter. In a subsequent series of experiments, students investigate a 3-way bulb, a 3-way switch, and then a 3-way bulb in a 3-way switch socket. They develop the understanding - using observation, logical reasoning, and mathematical modeling - that a 3-way bulb consists of 2 filaments which are connected in parallel at the highest wattage setting. In the third experiment, students design a light bulb and describe the fabrication steps necessary to construct it; students are given some basic engineering information before attempting the experiment. They then dissect a light bulb and determine how close their earlier design resembles a real bulb. Finally, they must design and construct a light bulb that operates in air using materials that are similar to those found in a light bulb, but are oxidation resistant. These materials are available as a kit from the General Atomics Sciences Education Foundation as GASEF #013. GASEF #013 contains 10 20-cm long pieces of 0.003 inch diameter Kanthal AF wire, 2 20-cm long pieces of 0.010 inch diameter Kanthal AF wire, and 2 20-cm long pieces of 0.020 inch diameter copper wire.

This unit also consists of an extensive introduction with background information into advanced topics such as oxidation resistant materials, blackbody radiation, filament materials, filament environments, and a microscopic view of incandescence. Also explored are a brief history of the development of the light bulb and Edison's critical role in the methodology of experimental science, which set the subsequent standard for industrial research. A teacher's guide to all experiments, related mathematical problem sets, and solutions is included the module. This unit provides a natural tie to studies in economics and US history that involve the electrification of society, the industrial revolution, the rivalry between AC and DC distribution systems, and the growth of industrial laboratories. Students require a previous introduction to Ohm's Law and series and parallel circuits before beginning this unit. These experiments are aimed at grades 7-12, but would also be appropriate for an introductory university physics or materials science course.

This unit relates to the NSES physical science content standards in grades 5-8: "Energy is a property of many substances and is associated with heat, light, electricity ... Energy is transferred in many ways. Electrical circuits provide a means of transferring electrical energy when heat. light, sound, and chemical changes are produced;" and in grades 9-12: "Energy can be transferred ... in many ways. In some materials, such as metals, electrons flow easily, whereas in insulating material they can hardly flow at all."

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Section Page
Table of Contents... 2
Correspondence to the National Science Education Standards... 3
Correspondence to the Benchmarks for Science Literacy... 5
Logical Construction of Module... 8
Introduction and Basic Physics... 9
Introduction to Filament Design Parameters... 12
Experiment 1: The Room Temperature Filament Resistance of Different Wattage Bulbs... 14
Experiment 2: The Temperature Dependence of the Resistance of a 100 W Light Bulb... 17
Experiment 3: The Electrical Properties of 3-Way Bulbs... 19
Experiment 4: Light Bulb Design... 24
Experiment 5: Light Bulb Dissection... 26
Experiment 6: Light Bulb Fabrication... 28
Advanced Topic: Oxidation Resistant Materials... 30
Advanced Topic: Blackbody Radiation... 32
Advanced Topic: Filament Material... 33
Advanced Topic: Filament Environment... 35
Advanced Topic: Microscopic View of Incandescence... 36
Advanced Topic: A Brief History: The Edisonian Approach... 37
Problems... 38
Solutions... 39
Reference 44
Materials Required... 45
Appendix... 46
Response to a question about developing the cooling curve (Experiment 2, page 18)  

 

This unit was developed by Dr. Lawrence D. Woolf

 


 

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