# Physics (resistance and lightbulbs

Im pretty new to this forum so Im not really sure if i should ask the question here. Anyhow here goes...

1) what determines the brightness of a bulb

so...i have this question ->

X and Y are lamps with filaments made of the same material. The filament of lamp X is thicker(hmm...) and shorter(hmmm...) than that of lamp Y. Each lamp is connected to the mains and switched on. Which is the brighter lamp and has the larger resistance?

2) Also, what does the gradient of a V/I graph give you? I read up in one of my textbooks that it is a misconception to think the gradient of a V/I graph gives resistance, instead we are suppose to use a value of voltage/current then use the corresponding voltage/current from a graph. Followed by just using R = V/I instead of dV/dI.

Anyone can help me understand this?

-x-x-x-

ideasrule
Homework Helper
X and Y are lamps with filaments made of the same material. The filament of lamp X is thicker(hmm...) and shorter(hmmm...) than that of lamp Y. Each lamp is connected to the mains and switched on. Which is the brighter lamp and has the larger resistance?

The brighter lamp is the one that has less resistance, because with less resistance, more current flows through it. What does shortening the filament due to resistance? How about increasing cross-sectional area?

2) Also, what does the gradient of a V/I graph give you? I read up in one of my textbooks that it is a misconception to think the gradient of a V/I graph gives resistance, instead we are suppose to use a value of voltage/current then use the corresponding voltage/current from a graph. Followed by just using R = V/I instead of dV/dI.

The slope of a V/I graph is resistance. That's no misconception; it's how resistance is defined.