I asked a question earlier about lamp resistance. There were some respectable answers. But.... Work it out again - lamp (bulb) 240v 100w. Resistance in ohms = ? Then get your multimeter out and set it to whatever to read the resistance - and DOES IT AGREE WITH YOUR ALGEBRAIC ANSWER?
2006-12-23 15:10:54 · 8 answers · asked by Anonymous in Science & Mathematics ➔ Physics
If you have a 240V 100 watts lamp, you can assume that the resistance of the lamp when it is under power will follow the ohms law.
BUT, one thing you must remember is, when a lamp is under load, it is glowing HOT. When metal is HOT, the molculoes of the meals are in much more active state. When this happens, the resistance will increase. Conversely, when the lamp is NOT on ON state, the filaments are cold. Moleculoes in the filaments are not as active. Thus, the resistance is lower. There is almost 10 to 1 difference in resistance from hot to cold.
Taking out a multimeter and measuring the resistance of the lamp will not help you determine the resistance of the lamp when it is actually under load (with voltage applied) Really, the only thing you can do is to measure the voltage, measure the current, then arrive at the resistance mathmatically.
Does this help you?
By the way, regular lamp under 60Hz is almost all pure resistance with little reactance. So, no, AC/DC difference is not what you are looking at....
2006-12-23 15:21:16 · answer #1 · answered by tkquestion 7 · 0⤊ 0⤋
The metal in the filament has a positive thermal resistance coefficient. When cold, it might measure as low as a few ohms - as the temperature of the filament rises, so does the resistance. At incandescence, a 240 volt, 100 W
filament has a resistance of approximately 570 ohms. Out of circuit, it's approximately 10-15 ohms.
Some materials - semiconductors, for instance - have a negative thermal resistances coefficient. Forward bias your ohm meter across a NP or PN junction and slowly heat the devise with a hair dryer - you will see the resistance decrease.
This phenomenon is just the opposite with a bulb filament.
Note: At DC or 60 Hz AC, the bulb filament is a purely resistive load.
2006-12-23 15:57:57 · answer #2 · answered by LeAnne 7 · 0⤊ 0⤋
The resistance of all metals increase with rise of temperature.
The temperature of the filament (Not the bulb) is nearly 1000 degree centigrade
A 240 v 100 w bulb has a resistance of 240 x 240 / 100 = 576 Ohm.
Set the multimeter to measure the voltage and current The current will be 0.42 A.
After switching off the high voltage if you use a small voltage to measure the resistance directly,it gives the resistance of the filament nearly at room temperature. It will not give you the resistance at 1000degree centigrade.
2006-12-23 17:47:20 · answer #3 · answered by Pearlsawme 7 · 0⤊ 0⤋
The reason the lamp is only rated at 25 watts is the close proximity of the lamp shade. Remove the shade and you can use any bulb that will fit the socket. Another choice would be to use a fluorescent bulb in the lamp. They don't put out nearly the heat of an incandescent bulb and are more energy efficient too.
2016-05-23 03:06:47 · answer #4 · answered by Anonymous · 0⤊ 0⤋
It should broadly using ohms law, but it would be silly trying to get a point decimal correct answer due to other factors like fluctuation in the mains supply and the resistance network of the connection to a meter. So an answer that ties roughly within a few points is acceptable. Also bulbs marked 60w or 100w and things so marked are really a close approximation within limits of what they really are.
2006-12-23 15:24:18 · answer #5 · answered by Anonymous · 0⤊ 1⤋
R = V/? I don't remember the equation. You'll have to wait for the light to get to full operating temperature before the reading gets fairly close (in an element like this, resistance is initially very high as the element heats). The resistance will probably increase with the age of the bulb as well (particles of tungsten leap off of the element, leading to a thinner and thinner wire, and eventual wear out).
2006-12-23 15:22:05 · answer #6 · answered by BigPappa 5 · 0⤊ 1⤋
The Resistance measured by a multimeter would be inaccurate as it is measuring DC resistance and not AC impedance.
What you want to know is the reactance of the filament.
To tired to figure it out now (never did like revision)
Hav'nt seen any other answers.
2006-12-23 15:19:32 · answer #7 · answered by Anonymous · 1⤊ 1⤋
The wattage quoted is that concerned with its power usage and not when it's in its box.
Perhaps you might want to edit the bracket content?
2006-12-24 22:09:50 · answer #8 · answered by Anonymous · 0⤊ 0⤋