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Q:
I have a light globe rated 60 Watts and it runs on a 240 Volt AC supply. I believe it then uses 0.25 Amps,ie watts = Amps x Volts so Amps = Watts/Volts.
However, I measured the resistance of the globe as 80 Ohms so using the formula Volts = Amps x Resistance I get Amps equal to 3 this way, ie 240/80. Where am I in error?
- Michael Sobb (age 70)
Rydalmere New South Wales Australia
A:
Dear Michael,
Your result is about right. The resistance of tungsten increases by about a factor of 20 between room temperature, where you measured it, and about 3000
o Kelvin, when the bulb is lit. Take a look at:
http://hypertextbook.com/facts/2004/DeannaStewart.shtml for a table of values.
This is why you get a brief current surge when you turn on the lights, before the filament gets hot.
LeeH
(published on 12/12/2011)
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