# A 40 W (120 V) lightbulb has a tungsten filament of thickness 0.039 mm . The filament's operating temperature is 1500 ∘C.?

How long is the filament? What is the resistance of the filament at 20 ∘C?

Relevance
• To answer these questions, you need to know the resistivity of tungsten as a function of temperature (or at least at the temperatures 1500ºC and 20ºC).

Using the citation below, I see that at 20ºC

ρ = 5.60e-5 Ω·m

and using the second citation, at 1500ºC (= 1773 K) the resistivity is about 8.75 times that value.

I assume that you have reference tables and/or formulae at your disposal that would give you the value of ρ at both temperatures. I would be surprised if both of my values are the ones you are expected to use. Check your references.

Anyways, the solution method from this point is:

You have the power and voltage:

resistance R = V²/P = (120V)² / 40W = 360 Ω

The area of the filament (assuming it is circular) is

A = πd²/4 = π(39e-6m)² / 4 = 1.2e-9 m²

R = ρL/A

and so at 1500ºC

360 Ω = 8.75*5.60e-8Ω·m * L / 0.0012m²

which solves to

L = 0.88 m

which seems awfully long, but that's what I get.

At 20ºC,

R = 5.6e-8Ω·m * 0.88m / 1.2e-9m² = 41 Ω

which you can also get by doing

R = 360Ω / 8.75 = 41 Ω

both of which assume that the 8.75 factor I'm using is correct.

That's the method; if the numbers I'm using are right, those are the answers.

• NCS
• Login to reply the answers