Lightbulb Power: not Homework

An electric bulb is designed to draw X power at V voltage. If the voltage is B, it draws P power. Then how can we express P in terms of X, V and B?
- rikun (age 14)
bbsr

This looks like one of the many homework problems that we routinely delete. I'll answer this one, however, because there's an important physical point to be made. Also, I'm not sure the teacher would accept our answer. Lightbulbs  are not resistors, for which P=V²R. The effective R of the bulb changes a lot when you change V. The bulb becomes hotter at higher V, of course. In the hot bulb, the electrons moving through the filament bounce of many more little thermal sound waves- random wiggles of the atoms. That increases R. How much depends on details of the wire, etc. So we can't give a formulaic answer to your question. The formulaic answer that you're probably expected to give will be wrong in real life.

Mike W.

posted without checking while Lee travels

(published on 05/19/2013)