Watt Calculation Question

If one were to plot a graph of applied voltage versus resistance for a tungsten filament, one would find that it has a negative slope, i.e. as the applied voltage increases, so does the resistance. So it is not possible to answer the question accurately without trying the proposed arrangement out, and measuring it.

Maybe it's time to ditch those antique 100W tungsten filaments for something more up-to-date like LED cluster lamps or compact fluorescents, perhaps?

Wouldn't that be a positive slope, the way things are usually plotted?

I believe the resistance increases with temperature, hence with rising current as you say. Explains why filament bulbs have inrush current greater than running current.

Agreed.

Correct, except the total power consumed will be watts, not just a single watt as you suggest.

I think he meant "what" not watt. At any rate, Ohm's law or "wattever" they call it today, is his friend.

If you connect half the voltage, the current drawn will also be halved and so the wattage will be 25% for each(each bulb). I am considering everything linear.

Gajanan Phadte

Still grappling with Ohms Law, eh?

http://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htm

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That assumes each filament will be at the same temperature, and ignores the change in resistance with applied voltage, both of which introduce non-linearities, so one must question the validity of those assumptions.

Each lamp will have 100v across it. Because the filement resistance changes with temperature, the power consumed is not proportional to V2 but has been empirically determined to be proportional to V1.6. So the answer is 33W each.

Ref: http://en.wikipedia.org/wiki/Incandescent_light_bulb

Mine doesn't either in Opera.

My #15 seems OK. What's Opera (apart from the obvious!)?

Browser.

Well they could be 100 watt equivalent LED's, which consume only 22 watts.....

Oh...my...glob...

I cannot believe some of the answers which are getting GA's.

OP: Are you seriously asking what the wattage used by two (2) 100 Watt lamps is? Do you mean Amps?

Thanks to Lyn for moderating.

A good approximation of filament bulb attitude in varying voltage, is that of a constant current drain, i.e. it will try to keep the consumed current nearly stable. So, that specific bulb can be characterized as a .5 Ampere lamp, and connecting two in series will give a total wattage about the same (and a tad lower) as a single bulb, so the power for each bulb will be about half the original. Naturally light color temperature will drop significantly (It will become more red-ish). S.M.

When two lamps are connected in series then it becomes a series lamp and as you know in series voltage get divided so total power consumption is at 100 W and when these are connected across 400V then power consumption will be 200W.

In Parallel circuits voltage remains the same but in Series circuits voltage get divided according to resistance of load. In above case resistance of two lamps is same so power consumption will be 100W only.

Can't agree with all of that.

Your 1st pararagraph is obvious, as if total volts is 400 each bulb sees 200 volt, so behaves as design, 100W at 200 volt.

2nd paragraph is wrong. In series at 200 volt total each bulb sees 100 volt. Power = V²/R so if R stays constant power is down to 100*0.5² = 25W.

But in practice the resistance is lower as current hence filament temperature is lower, so power is > 25W. See Rixter's post #11.