Robert Cinca - Physics
Contents
Introduction 22
Methodology 24
Terminology Clarification 24
Part 1: Logical Argument 25
Part 2: Testing the Filament Bulb
27
Part 3: Excel Modeling 35
Conclusion 42
Appendix 43
Bibliography 54
Abstract
When a filament bulb is first switched on, its metal filament is cold and
so has a relatively low resistance. This leads to a large surge current until
the filament heats sufficiently to glow white hot with a steady electrical
current. This essay investigates the research question: “How does the
final current affect the surge current time in a filament bulb?”
The surge current time is the time interval between switching the bulb on
and when the final steady current is first reached.
A logical argument suggests the surge current time is inversely
proportional to the final current squared. This hypothesis is then tested
using two different small filament bulbs. An excel spreadsheet is then
used to see if the filament bulb can be effectively modeled as a hot body
radiator. The experiments agree with the inverse square relationship
predicted. The model also agrees. There are however some significant
differences between the experimental results and the excel model. The
surge current times with the spreadsheet are about a factor of three
shorter than the experimental results. The maximum current is reached
instantly in the model rather than after a short but constant delay with the
bulb. These differences possibly arise as the model does not include any
self-induction of the coil or thermal expansion of the filament.
21