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Hi, I have a question related to an already posted topic:"Why is tungsten used for filaments in light bulbs when nichrome's resistivity is so much higher?

"http://scienceline.ucsb.edu/getkey.php?key=2548

I understood, after reading your answers, the importance of the high melting point of the material of the filament. In order to get visible electromagnetic waves (i.e. light), we need the material to be at a certain temperature and it is tungsten that can reach that temperature without melting. However I do\'t understand another thing. The power supply is given by a constant VOLTAGE supply, say 120 V (not constant current). Ohm's Law states V=I*R And Joule's effect states Q=I2R By combining these two equations, we get that Q=V2/R. Then, why we would want to have a material with a high resistivity? If V is fixed, then Q actually DECREASES with the higher R. The former equation, I2.R, might be confusing but the thing is that, as said before, I is not fixed, but V is.

So by increasing R, we are decreasing I and that's why I2.R will decrease. In conclusion, I understand that tungsten is a good material because of many of its properties, as the high melting point. But, why to increase the resistance of the filament by increasing its length and decreasing its cross-section since it seems to me that it should be the other way round? Were am I making the mistake? Sincerely

Answer 1:

First of all, I want to applaud you for thinking about the problem critically and asking this question. It's a great question and you did everything right (mathematically)! If all that you want to do is maximize Q, then you're correct - since Q scales with the square of the current, then it's most potent to maximize current.

Remember, though, that your goal is not simply to heat up wire indiscriminately but to specifically heat up the filament. There is copper wire with fairly low resistance connecting the power lines through your house to your sockets, then from the sockets to your light bulb via some kind of cord. If your light bulb has a filament with low resistance, then the current running through the whole circuit would be very high, not just the light bulb, since there's not resistance contrast between the various wires and the filament. You'd end up almost immediately tripping a circuit breaker and losing power to that part of your house!

This is why we run filament at high resistance - we want to heat up the filament (maximize Q) given the constraint that the filament is the only thing that gets very hot in the circuit.

Hope this helps!


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