The brightness of the lamp (power) depends on two factors:
The voltage and the current which flow through it.
In Figure(A) as the variable reaistance increases its reaistance, the current which flow through the lamp will decrease and therefore its brightness decreases.
My question is what about the voltage across the lamp,does it decrease or increase?
In figure (B) what the factor which depend on the brightness?
I read a lot of forums concerning this topic and they always state that the brightness depend on the Resistance and as the resistance increases the voltage increases
Why don't they refer the current which flow through the lamp?
(I KNOW the current is constant in series connection but the lamps are connected in parallel to the resistance so I think the current must be mentioned when we talk about brightness)
Best Answer
It decreases. The source voltage is split between the resistor and the lamp. The higher the resistor, the higher the voltage across the resistor and the lower the voltage across the lamp, since the sum is constant
Let's assume a very simple model of the lamp (real impedance, constant with current, temperature and voltage). This case depends on the ratio of the resistors to the resistance of the lamp. If the resistors are very large, they have no impact and the lamps will be the same brightness. If the resistors are very small, the brightness will be determined by the ratio of the resistors (bigger one -> brighter lamp). Anything else will be something in between.