MadSci Network: Physics

Re: What is the difference between a 20 and 100 watt lightbulb?

Date: Fri Dec 22 11:36:35 2000
Posted By: Todd Jamison, Staff, Image Science, Observera, Inc.
Area of science: Physics
ID: 975297141.Ph


Voltage is not the key here, but power is.  Power is measured in Watts or 
sometimes Volt-Amps (VA).  Power, in electrical terms, is calculated as the 
voltage times the current.  All of our US power systems have a voltage of 
nominally 110 volts.  A 100 watt bulb is designed to use about 100/110 
amperes (amps) of current or about 0.9 amps.  The 20 watt bulb only uses 
about 0.2 amps.  As the current flows through the bulb, it is really many, 
many electrons moving through the wires.  As these electrons move through 
the bulb, they heat up a coil of wire that has a resistance to the flow of 
the electrons.  As the wire heats up, it gives off photons of light in the 
visible part of the spectrum, that we can see, and also in the infrared part 
of the spectrum, that we can't see but that can heat other things up, like 
the glass of the bulb.  The resistance of the 20 watt bulb to the flow of 
the electrons is higher, so fewer of the electrons can go through the coil.  
The wire is generally smaller, so as it heats up and glows, but fewer 
photons are produced - that is why the light is dimmer and the bulb is 

I hope this helped.
Todd Jamison, Chief Scientist, Observera, Inc.

[note added by MadSci Admin:  The power rating of bulbs (in your
case 20 and 100 Watts) is the *electrical* power that the bulb
operates with.  The amount of this energy that actually goes
into visible light is only about 2.5% to 5% of the total
(electrical) power.  They are not efficient!]

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