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Q: Current Electricity, Frequency and Light Bulbs ( Answered,   2 Comments )
Subject: Current Electricity, Frequency and Light Bulbs
Category: Science
Asked by: dravis-ga
List Price: $25.00
Posted: 12 Nov 2006 13:58 PST
Expires: 12 Dec 2006 13:58 PST
Question ID: 782175
What happens to an incandescent light bulb if it is connected to a
current with 400Hz, as opposed to 50Hz or 60Hz, or greater? What
happens to the filament?

Request for Question Clarification by mvguy-ga on 12 Nov 2006 15:10 PST
The answer is going to depend in part on what assumptions are made. So
I have (for now) two questions: What is the filament made of? Is the
filament in the form of a coil? At 400 Hz these things might not make
much difference, but at radio frequencies, for example, they might.
Subject: Re: Current Electricity, Frequency and Light Bulbs
Answered By: hedgie-ga on 11 Dec 2006 18:52 PST

          Since you did not close the question, I will  add 

'the official' answer:

  There will not be any visible difference between appearance of the bulb,
  powered by the same voltage and somewhat higher frequency.
  There will be no significant change in the lifespan of the bulb.

  Comment by  rohiththeone-ga is correct. However, it can be argued that 
  filament, usually made as a coil, has a small, but non-zero impedance.

  For that reason current flowing through the filament will be very slightly
  less, and therefore life of the bulb a lile bit longer at higher frequency. 

  Comment by chappy48-ga talks about RF component. RF, a radio frequency,
  label covers very wide range, from few to billions of Hertzs.

 In the indicated range of interest (50 to 400 Hz) two effects will be combine: 

1) with higher frequency the mechanical vibration will get smaller,
         meaning longer life 
2) the current will be less, also leading a longer life.

  The effect would be measurable, but not visible vithout instruments.

More info on effect of the frequency:

Electrical impedance

Impedance of the coil

Details on lightbulb construction: the  metal filament  coil

Example of doubly wound filament, efficiency..
".. Thus a typical 100 W bulb for 120 V systems, with a rated light
output of 1750 lumens, has an overall efficacy of 17.5 lumens per
watt, compared to an "ideal" of 242.5 lumens per watt for one type of
white light. Unfortunately, tungsten filaments radiate mostly infrared
radiation at temperatures where they remain solid (below 3683

 This is probably my last answer on GA. I hope you find it useful.
 I would like to add this (a bit of the topic) public interest message:
" Says David Goldstein, a PhD physicist, MacArthur "genius" fellow, and
senior energy scientist with the Natural Resources Defense Council:
"This could be just what the world's been waiting for, for the last 20
       Talking about CFLs:

Please, do rate the answer, and feel free to ask for clarification before you do.

Subject: Re: Current Electricity, Frequency and Light Bulbs
From: rohiththeone-ga on 17 Nov 2006 08:04 PST
If the same voltage is applied to the incandescent light bulb then it
makes no difference.There will be no difference between 50Hz 60Hz or
400Hz or even 4000Hz.
Nothing will happen to the filament.(No fireworks)
(Comment: Maybe you are confused with frequency, voltage and current)
Maybe at 400,000,000Hz there will be transmimssion line effects and
antenna effects which depend upon the length of the filament,the
resistivity etc etc.
The filament can be modelled as a resistor. The impedance of a
resistor is frequency independent.
If V is the (rmsP)voltage across the filament and R is the resistance 
then the power dissipated is (V^2/R), which is independent of
Subject: Re: Current Electricity, Frequency and Light Bulbs
From: chappy48-ga on 24 Nov 2006 08:20 PST
Rhe bulb will last considerably longer at a given voltage with a pure
DC source of current as opposed to AC due to RF, irrespective of no
imedance. The RF in AC applies mechanical forces to the filament
shrtening the lifespan.

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