# Resistivity of tungsten filament

1. Mar 2, 2009

### rinarez7

1. A 1.3 cm length of tungsten filament in a small lightbulb has a resistance of 0.032 ohms. Find its diameter.
Given: Length= 1.3 cm= .013m
R= 0.032 ohms
resistivity= 5.6e-8 ohms*meter

2.
A= pi (r^2)
R= resistivity (Length/ Area)

3. The attempt at a solution
R= resistivity ( Length/ pi* (r^2) )
.032 ohms= (5.6e-8 ohms*meter) * (.o13m/ (pi* (r^2)) )
.032 (pi) (r^2) = 7.28e-10
.100530965 (r^2) = 7.28e-10
r= 8.509e-5
diameter= 2r= 1.701e-4m

It asks for the answer in mm, so I divided my answer by 1000= 1.7 e-7 mm, but it's not correct = (
What am I missing?

2. Mar 2, 2009

### mgb_phys

If you have an answer in m, the answer in mm is going to a be BIGGER number

3. Dec 16, 2009

### Netspin

What makes Physics physical are the units. Units are as essential to the solution procedure as the mathematical expressions used to determine its quantities. Therefore, units must accompany their magnitudes throughout. In fact, this will often help as a conincident double check on the answer. Tables of Fundamental Units can be found in many books.