Lightbulb Brightness Homework: Series & Parallel Circuits

[JWSiow]

Homework Statement

Two light bulbs of resistance R1=10 ohms and R2 = 15ohms are connected in series with a 25V battery. Which bulb glows more brightly?

Now the bulbs are connected in parallel with the battery. Which bulb now glows more brightly?

Homework Equations

V=IR

The Attempt at a Solution

...This seems like an easy question, but I just don't understand it! I know if they were the same resistance the brightness of each bulb would be the same, but don't really know why. I thought the amount of current passing through each bulb determined brightness, but then I read that from the Junction rule, the current through each component is the same!

 

[n.karthick]

Hey, the brightness depends on the power consumed by the bulb. When bulbs are in series,yes, as you said current flow is same. You calculate power consumed by each bulb now using relation
$$P= I^2 R$$
from which you can find which bulb is more brighter.
When they are in parallel, surely, bulb with higher current through it (or which has lower resistance) will be brighter, since voltage here is same on two bulbs.

 

[rl.bhat]

In series combination the same current flows through both the resistances. Brightness depends on the heating effect. And it is given by either I^2*R, or V^2/R or V*I. So in series combination larger resistance is brighter.

In parallel combination...?

 

[JWSiow]

Ok, thanks heaps!

 

[rwisz]

Dear student,

You are on the right track. The brightness of a light bulb is determined by the amount of current passing through it. In a series circuit, the current is the same through each component, so the bulbs will have the same brightness. This is because the current is limited by the total resistance of the circuit, which in this case is the sum of R1 and R2.

In a parallel circuit, the current is split between the components. This means that the bulb with the lower resistance (R1 in this case) will have a higher current passing through it, and therefore will be brighter. This is because the current follows the path of least resistance, and R1 provides less resistance than R2.

I hope this helps to clarify the concept for you. Keep up the good work in your studies of circuits and electricity!

Best,