Robert Cinca - Physics
Introduction
Introduction
My research question was: “How does the final current affect the surge current
My research question was: “How does the final current affect the surge current
time in a filament bulb?” The surge current time is the time interval between
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 filament
switching the bulb on and when the final steady current is first reached. A filament
bulb’s resistance varies with temperature. When a filament bulb is switched on, it has
bulb’s resistance varies with temperature. When a filament bulb is switched on, it has
a relatively low temperature, resulting in a low resistance. This phenomenon creates
a relatively low temperature, resulting in a low resistance. This phenomenon creates
a ‘surge current,’ which can be several times the size of the final current and is why
a ‘surge current,’ which can be several times the size of the final current and is why
filament bulbs sometimes blow when switched on. The filament bulb quickly heats up
filament bulbs sometimes blow when switched on. The filament bulb quickly heats up
(in a few milliseconds), increasing the resistance and lowering the current back to its
(in a few milliseconds), increasing the resistance and lowering the current back to its
normal, steady, operating level.
normal, steady, operating level.
I chose this research question due to its applications to my day-to-day life: my house
I chose this research question due to its applications to my day-to-day life: my house
uses a lot of filament bulbs and understanding surge currents could help in finding
uses a lot of filament bulbs and understanding surge currents could help in finding
solutions for avoiding the filament bulbs to blow. An NTC-thermistor, with high
solutions for avoiding the filament bulbs to blow. An NTC-thermistor, with high
resistance at low temperatures and low resistance at high temperatures, could be
resistance at low temperatures and low resistance at high 1 temperatures, could be
used in order to protect the bulb from large surge currents 1 .
used in order to protect the bulb from large surge currents .
In my research, a logical argument suggests an inverse square law relationship
In my research, a logical argument suggests an inverse square law relationship
between the surge current time and final current. This was achieved using thermal
between the surge
current time and final current. This was achieved using thermal
energy theory 2 2 . The thermal energy, , required to heat the filament is given by:
energy theory . The thermal energy, , required to heat the filament is given by:
(For a fixed mass, , where
(For a fixed mass, , where
change in temperature).
change in temperature).
is the specific heat capacity of tungsten and
is the specific heat capacity of tungsten and
is the
is the
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Figure 1: Filament Bulb Experiment Circuit 3
Figure 1: Filament Bulb Experiment Circuit
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“Thermistors Thermal Resistors”, Temperatures.com [Online][Date Accessed 15/07/13] URL:
“Thermistors Thermal Resistors”, Temperatures.com [Online][Date Accessed 15/07/13] URL:
http://www.temperatures.com/thermistors.html (Temperatures.com, Inc., 2013)
http://www.temperatures.com/thermistors.html (Temperatures.com, Inc., 2013)
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2 “Thermal
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Physics”, Higher Level Physics (Hamper, 2009) p66
“Thermal Physics”, Higher Level Physics (Hamper, 2009) p66
“I/V Characteristics of a Filament Bulb”, Nuffield Foundation/Institute of Physics [Online][Date
“I/V Characteristics of a Filament Bulb”, Nuffield Foundation/Institute of Physics [Online][Date
Accessed 15/07/13] URL: http://www.nuffieldfoundation.org/practical-physics/iv-characteristic-
Accessed 15/07/13] URL: http://www.nuffieldfoundation.org/practical-physics/iv-characteristic-
filament-lamp (Nuffield Foundation, 2011)
filament-lamp (Nuffield Foundation, 2011)
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