# Difference in Brightness between Two Bulbs

## Homework Statement

Two lightbulbs of resistance R1 and R2 (R2 > R1) are connected in series. Which is brighter? What if they are connected in parallel? Explain.

P=V^2/R
P=I^2R

## The Attempt at a Solution

I know the solution.

In series, P1=I^2R1 < P2=I^2R2, so R2 is brighter.

In parallel, P1=V^2/R1 > P2=V^2/R2; since R2 > R1, R1 is brighter here.

But my question is this: How can I know which equation to use for series and which for parallel? (The text says nothing.)

Why couldn't I have argued in the following way?

In paralle, P1=I^2R1 < P2=I^2R2, so, even in parallel, R2 is brighter. Why does this line of reasoning fail?

I is different in each case because they are connected in parallel. So, we can't use P=I^2R for this part; we have to find an equation for which another variable is the same. Best bet is P=V^2/R. Even if that's right, I still don't think I really have grasped the relevant issues here.

Thanks.

Related Introductory Physics Homework Help News on Phys.org
Buzz Bloom
Gold Member
How can I know which equation to use for series and which for parallel?
Hi crastinus:

In series what is the same for both R1 and R2?
In parallel what is the same for both R1 and R2?

Hope this helps.

Regards,
Buzz

Thanks, Buzz! That helped!

Buzz Bloom