# Brightness of lamps 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)

• The complication here is that the resistance of a lamp increases with its temperature, and temperature increases with current. Your question is a little vague. But generally the brightness depends on the power dissipated in the lamp. So the brightness depends on voltage and current. Brightness depends on resistance in a sense: if you know the resistance you can calculate power from voltage or current. But that is just a shortcut to calculating $P=IV$, and besides, there's no way to measure the resistance of an operating lamp without measuring voltage and current. – garyp Jan 17 '19 at 18:13
• Can you please be specific about what happens in each situation which I represented in figure (A) and (B)? – Rami ki Jan 17 '19 at 18:29

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?


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

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?

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.

• I understand what happens i figure A, but in figure(B) do you mean that the lamp will be the same of the voltmeter(which has high resistance) and therefore negligible current flows through lamp? – Rami ki Jan 17 '19 at 18:45
• What voltmeter? I don't see one. – garyp Jan 17 '19 at 22:04
• I wonder if it works with the same idea of voltmeter(that very small current flows through the lamp and therefore we can neglect it) – Rami ki Jan 18 '19 at 7:05