# Homework Help: Temperature of Bulb Filament

1. Feb 1, 2009

### JJones_86

1. The problem statement, all variables and given/known data

A common flashlight bulb is rated at 0.3 A and 3.3 V (the values of current and voltage under operating conditions). If the resistance of the bulb's tungsten filament at room temperature (20.0 degrees Celsius) is 1.10 Ohms , estimate the temperature of the tungsten filament when the bulb is turned on.

Take the temperature coefficient of resitivity for tungsten to be 4.5e10-3

2. Relevant equations

R = V/I

3. The attempt at a solution
R = 3.3V / 0.3A = 11 Ohms when turned on

Little lost on how to find the temperature when turned on.

2. Feb 1, 2009

### Delphi51

What are the units on that coefficient of resistivity? Perhaps we can make up a formula relating it to temperature and resistance.

3. Feb 1, 2009

### skeptic2

I'm curious about a custom I see universally in this forum. It's been quite a few years since I was in school and I wonder if this custom indicates a change in the way EE is being taught nowadays. Traditionally E instead of V has been used to represent electromotive force just as I represents current instead of A. In the past a distinction has been made between the unit of electromotive force and its designator.

Are instructors and textbooks now using V for electromotive force?

4. Feb 1, 2009

### LowlyPion

How much does the resistance of the filament change?

With that Δ in resistance perhaps you can figure the Δ in temp needed to develop that much resistance?

5. Feb 1, 2009

### Delphi51

I remember that, Skeptic! I've been in high school for 30 years and all our books and government exams just used V for electric potential, which is a bit confusing leading inevitably to saying "Voltage" instead of "potential". But P and E are busy already so V is a relief.

6. Feb 1, 2009