# Homework Help: Light bulb temperature variation

1. May 12, 2004

### BananaMan

as part of a physics practical write up i have to measure the temperature of a light bulb envelopes temperature as the voltage increases.

first .... am i right in thinking a type J exposed thermocouple would do the trick for this?

if so, i heard something about one end being cold, to maintain a constant pd ?

second (most importantly)....am i right in thinking that a graph of V against Temp would be a straight line not through the origin?

thank you :)

p.s. already looked on the net for this and rly cant find n e thing on either point :S

2. May 13, 2004

### Chi Meson

A thermocouple would work, as would a thermistor (thermistors don't need a constant cold side).

Since the filament of a bulb will increase in resistance with temperature, and temperature will be proportional to the square of the current which will be proportional to the voltage and inversely proportional to the resistance which is proportional to the temperature... well, let's say that it might not be a straight line.

EDIT: correction, the power is proportional to the square of the current; the temperature depends on other factors including the rate of heat radiation which is proportional to the surface area of the filament as well as the difference in temperature between the filament and its surroundings. It's looking even less like a straight line graph now.

Antother point to consider, is that the "heat" energy that is carried away from the filament wil be increasingly in the form of infrared radiation. For things that are glowing, a more accurate reading of temperature is found by determining the peak wavelength of the output light. If you have not ecountered the Stefan-Boltzmann Law or "Blackbody radiation" then this is not the way to go.

Last edited: May 13, 2004