Expert: Daniel Mazur - 3/12/2009

Question
Which governs the brightness of the lamp?
Power?
Voltage across it?
Current passing through it?

Other than the above concept, I have a question that is linked.

Circuit A:
A lamp is connected to an electric cell.

Circuit B:
2 lamps similar to circuit A are connected in PARALLEL to an electric cell similar to circuit A.

Will both lamps in circuit B be brighter than the lamp in circuit A?

Please enlighten me Daniel

Answer
Hi Gan,
the brightness is primarily governed by power. When you have a constant voltage source the brightness is controlled by the resistance of the lamp, P=U^2/R. Constant current source means that P=I^2*R. It is tricky to say that the brightness (power) is proportional to square of the controlling quantity, though, because higher power means higher temperature and that means higher resistivity (in the metallic filament of the bulb) and that reduces back the power a bit (in the constant voltage mode)...
The problem of lamp circuits reminds me of my really young years, when a teacher demonstrated a very similar thing in the class precisely to prove my then-opinion wrong:-D. The correct answer is that the brightnesses of all 3 lamps (1 in A and 2 in B) must be the same. In both cases the lamp resistance is the same R and the voltage across each lamp is the same U. So, that's it! Of course if you add the brightnesses of the two parallel lamps to consider total illumination of the room then the total brightness of B is double that of case A.
Take care!
Daniel