Incandescent light bulbs and Ohm's law
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Incandescent light bulbs and Ohm's law
I have a 40 watt incandescent light bulb. It measures 26 ohms DC resistance. This does not work with Ohm's law. Why? (Should measure 360R to dissipate 40 watts at 120 volts.)
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Re: Incandescent light bulbs and Ohm's law
Because: Google it.
http://www.allaboutcircuits.com/vol_1/chpt_2/6.html
It's a non-linear conductor.
http://www.allaboutcircuits.com/vol_1/chpt_2/6.html
It's a non-linear conductor.
I build and repair tube amps. http://amps.monkeymatic.com
Re: Incandescent light bulbs and Ohm's law
The electrical resistance of metals typically increase with temperature.
In don't think that an order of magnitude increase is unreasonable to expect given the temperature of the filament.
Cheers,
Dave O.
In don't think that an order of magnitude increase is unreasonable to expect given the temperature of the filament.
Cheers,
Dave O.
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Re: Incandescent light bulbs and Ohm's law
Why the light-bulb "limiter" works so well. If current stays low, the lamp doesn't light .xtian wrote:Because: Google it.
http://www.allaboutcircuits.com/vol_1/chpt_2/6.html
It's a non-linear conductor.
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Re: Incandescent light bulbs and Ohm's law
But ... that much difference?xtian wrote:I have a 40 watt incandescent light bulb. It measures 26 ohms DC resistance[COLD]. This does not work with Ohm's law. Why? (Should measure 360R[HOT] to dissipate 40 watts at 120 volts.)
The temperature difference is huge.
Lamp filaments are the few things we are in daily contact with which work white hot (not orange or red hot) which ,means they are as hot as they can go without melting.
And they are not made of standard metals such as iron/copper/aluminum/lead but tungsten, one of the highest meltingn point metals in current use.
The higher the temperature increase, the higher the resistance variation.
Re: Incandescent light bulbs and Ohm's law
Also they are so hot that if they were not in a vacuum, but had oxygen they would burn up quickly.
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Re: Incandescent light bulbs and Ohm's law
This is also the reason that a light bulb will blow when you first turn the switch on, rather than when you turn the switch off.
Re: Incandescent light bulbs and Ohm's law
I think the reason the two numbers don't match up is due to the energy loss in the heat of the filament.
Tom
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Re: Incandescent light bulbs and Ohm's law
When you energize current to the light bulb, the filament is shocked by the sudden current flow (induction), putting stress on the filament which can break due to the mechanical shock that takes place.sluckey wrote:This is also the reason that a light bulb will blow when you first turn the switch on, rather than when you turn the switch off.
Tom
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Re: Incandescent light bulbs and Ohm's law
Thanks Roberto
From one table in your link, Tungsten at 2700 degrees Kelvin (about what lamp filaments rise to) resistance is over 20X that of ambient temperature.
From one table in your link, Tungsten at 2700 degrees Kelvin (about what lamp filaments rise to) resistance is over 20X that of ambient temperature.
Re: Incandescent light bulbs and Ohm's law
I thought they used Argon in light bulbs?pops wrote:Also they are so hot that if they were not in a vacuum, but had oxygen they would burn up quickly.
Tom
Don't let that smoke out!
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Re: Incandescent light bulbs and Ohm's law
Leslie 122 reverb amp uses incandescent to limit drive into the tank, paralleled with a 16 ohm speaker voice coil. Light leaks from under the chassis when cranking it!
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