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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