Resource Lesson
Filaments
Printer Friendly Version
The rating on a bulb specifies its "room temperature" resistance. Thus a 100-watt bulb rated for 120 volts would have an ideal resistance of
P = IV
P = (V/R)V
P = V
^{2}
/R
rearranging for R
R = V
^{2}
/P
R = 120
^{2}
/100
R = 144 ohms
while a 60-watt bulb rated for 120 volts would have an ideal resistance of
R = V
^{2}
/P
R = 120
^{2}
/60
R = 240 ohms.
The resistance of a wire, or
filament
, is proportional to its length and inversely proportional to its cross-sectional area. Longer and/or thinner wires have greater electrical resistance. A wire's resistance is also based on the type of substance out of which it is made: copper, zinc, tungsten, iron. This property is called the wire's
resistivity
, or ρ which is measured in ohm-meters (Ωm).
Putting these properties together gives us the mathematical relationship:
R = ρ(L/A)
Since electrical power is directly proportional to the square of the current running through a device, a high wattage light bulb will draw a larger current. Therefore it has to have a smaller resistance filament, which implies that it has a filament with a larger cross-sectional area. Conversely, low wattage bulbs, which are dimmer, have thinner filaments that present a higher resistance, and for a common voltage, would draw a smaller current.
When a 100-watt bulb and a 60-watt bulb are wired in series which one will glow brighter when the current is "turned on?"
When a 100-watt bulb and a 60-watt bulb are wired in parallel which one will glow brighter when the current is "turned on?"
Summary
If two bulbs
share the same current
, the bulb which experiences the greater voltage drop will glow more brightly.
If two bulbs
share the same potential
, the bulb which draws the greater amount of current will glow more brightly.
If circuits use combinations of the same identical bulb; that is, a circuit with several 60-watt bulbs all having the same resistance, the bulbs experiencing the greater voltage drop or those carrying the greater amount of current will glow more brightly.
In all cases, the greater amount of power used in a bulb's operation, the brighter it will glow.
Related Documents
Lab:
CP -
Series and Parallel Circuits
Labs -
Parallel and Series Circuits
Labs -
RC Time Constants
Labs -
Resistance and Resistivity
Labs -
Resistance, Gauge, and Resistivity of Copper Wires
Labs -
Telegraph Project
Labs -
Terminal Voltage of a Lantern Battery
Labs -
Wheatstone Bridge
Resource Lesson:
RL -
A Comparison of RC and RL Circuits
RL -
Ampere's Law
RL -
An Introduction to DC Circuits
RL -
Capacitors and Dielectrics
RL -
Dielectrics: Beyond the Fundamentals
RL -
Electricity and Magnetism Background
RL -
Kirchhoff's Laws: Analyzing Circuits with Two or More Batteries
RL -
Kirchhoff's Laws: Analyzing DC Circuits with Capacitors
RL -
Magnetic Field Along the Axis of a Current Loop
RL -
Magnetism: Current-Carrying Wires
RL -
Meters: Current-Carrying Coils
RL -
Parallel Plate Capacitors
RL -
RC Time Constants
RL -
Torque on a Current-Carrying Loop
Worksheet:
APP -
The Circuit Rider
APP -
The Cycle Shop
CP -
DC Currents
CP -
Electric Power
CP -
Ohm's Law
CP -
Parallel Circuits
CP -
Power Production
CP -
Power Transmission
CP -
RIVP Charts #1
CP -
RIVP Charts #2
CP -
Series Circuits
NT -
Brightness
NT -
Light and Heat
NT -
Parallel Circuit
NT -
Series Circuits
NT -
Shock!
WS -
Capacitors - Connected/Disconnected Batteries
WS -
Combinations of Capacitors
WS -
Introduction to R | I | V | P Charts
WS -
Kirchhoff's Laws: DC Circuits with Capacitors
WS -
Kirchhoff's Laws: Sample Circuit
WS -
Resistance, Wattage, and Brightness
TB -
34A: Electric Current
TB -
35A: Series and Parallel
TB -
Advanced Capacitors
TB -
Basic Capacitors
TB -
Basic DC Circuits
TB -
Multiple-Battery Circuits
TB -
Textbook Set #6: Circuits with Multiple Batteries
PhysicsLAB
Copyright © 1997-2018
Catharine H. Colwell
All rights reserved.
Application Programmer
Mark Acton