This was one of the questions for the repetition paper before the test. "A lit light bulb has a resistance of 1320 Ω. It uses tungsten as a filament, and when hot, has a resistivity of 0.92 µΩm. Assume the filament is 2.0 cm long, how thick is then the filament?"
I work it out as follows: Using the formula for resistance in conductors:
- ρ = Resistivity
- l = length
- A = cross section area
R = resistance
- R = ρl/A
- A = ρl/R
- A = 0.92 µΩm * 0.02m / 1320 Ω
- A = 1.39×10^-11 m^2
- A ~= 13.9pm^2
That's just ridiculous. The answer given by my professor is 4.2µm^2. What am I doing wrong here?